Until now, we have mentioned only one application for the diode-rectification, but there are many more applications that we have not yet discussed. Variations in doping agents, semiconductor materials, and manufacturing techniques have made it possible to produce diodes that can be used in many different applications. Examples of these types of diodes are signal diodes, rectifying diodes, Zener diodes (voltage protection diodes for power supplies), varactors (amplifying and switching diodes), and many more. Only applications for two of the most commonly used diodes, the signal diode and rectifier diode, will be presented in this chapter. The other diodes will be explained later on in this module.
1. Half-Wave Rectifier
One of the most important uses of a diode is rectification. The normal PN junction diode is well-suited for this purpose as it conducts very heavily when forward biased (low-resistance direction) and only slightly when reverse biased (high-resistance direction). If we place this diode in series with a source of ac power, the diode will be forward and reverse biased every cycle. Since in this situation current flows more easily in one direction than the other, rectification is accomplished. The simplest rectifier circuit is a half-wave rectifier (Figure 1 and Figure 2) which consists of a diode, an ac power source, and a load resister.
The transformer (T1) in the figure provides the ac input to the circuit; the diode (CR1) provides the rectification; and the load resistor (RL) serves two purposes: it limits the amount of current flow in the circuit to a safe level, and it also develops the output signal because of the current flow through it.
Before describing how this circuit operates, the definition of the word "load" as it applies to power supplies must be understood. Load is defined as any device that draws current. A device that draws little current is considered a light load, whereas a device that draws a large amount of current is a heavy load. Remember that when we speak of "load," we are speaking about the device that draws current from the power source. This device may be a simple resistor, or one or more complicated electronic circuits.
During the positive half-cycle of the input signal (solid line) in Figure 1, the top of the transformer is positive with respect to ground. The dots on the transformer indicate points of the same polarity. With this condition the diode is forward biased, the depletion region is narrow, the resistance of the diode is low, and current flows through the circuit in the direction of the solid lines. When this current flows through the load resistor, it develops a negative to positive voltage drop across it, which appears as a positive voltage at the output terminal.
When the ac input goes in a negative direction (Figure 1), the top of the transformer becomes negative and the diode becomes reverse biased. With reverse bias applied to the diode, the depletion region increases, the resistance of the diode is high, and minimum current flows through the diode. For all practical purposes, there is no output developed across the load resistor during the negative alternation of the input signal as indicated by the broken lines in the figure. Although only one cycle of input is shown, it should be realized that the action described above continually repeats itself, as long as there is an input. Therefore, since only the positive half-cycles appear at the output this circuit converted the ac input into a positive pulsating dc voltage. The frequency of the output voltage is equal to the frequency of the applied ac signal since there is one pulse out for each cycle of the ac input. For example, if the input frequency is 60 hertz (60 cycles per second), the output frequency is 60 pulses per second (pps).
However, if the diode is reversed as shown in Figure 2, a negative output voltage would be obtained. This is because the current would be flowing from the top of RL toward the bottom, making the output at the top of RL negative with respect to the bottom or ground. Because current flows in this circuit only during half of the input cycle, it is called a half-wave rectifier.
The semiconductor diode shown in the figure can be replaced by a metallic rectifier and still achieve the same results. The metallic rectifier, sometimes referred to as a dry-disc rectifier, is a metal-to- semiconductor, large-area contact device. Its construction is distinctive; a semiconductor is sandwiched between two metal plates, or electrodes, as shown in Figure 3. Note in the figure that a barrier, with a resistance many times greater than that of the semiconductor material, is constructed on one of the metal electrodes. The contact having the barrier is a rectifying contact; the other contact is nonrectifying. Metallic rectifiers act just like the diodes previously discussed in that they permit current to flow more readily in one direction than the other. However, the metallic rectifier is fairly large compared to the crystal diode as can be seen in Figure 4. The reason for this is: metallic rectifier units are stacked (to prevent inverse voltage breakdown), have large area plates (to handle high currents), and usually have cooling fins (to prevent overheating).
There are many known metal-semiconductor combinations that can be used for contact rectification. Copper oxide and selenium devices are by far the most popular. Copper oxide and selenium are frequently used over other types of metallic rectifiers because they have a large forward current per unit contact area, low forward voltage drop, good stability, and a lower aging rate. In practical application, the selenium rectifier is used where a relatively large amount of power is required. On the other hand, copper-oxide rectifiers are generally used in small-current applications such as ac meter movements or for delivering direct current to circuits requiring not more than 10 amperes.
Since metallic rectifiers are affected by temperature, atmospheric conditions, and aging (in the case of copper oxide and selenium), they are being replaced by the improved silicon crystal rectifier. The silicon rectifier replaces the bulky selenium rectifier as to current and voltage rating, and can operate at higher ambient (surrounding) temperatures.
2. The Rectifier
From previous discussions, you should know that rectification is the conversion of an alternating current to a pulsating direct current. Now let’s see how the process of RECTIFICATION occurs in both a half-wave and a full-wave rectifier.
2.1. The Half-Wave Rectifier
Since a silicon diode will pass current in only one direction, it is ideally suited for converting alternating current (ac) to direct current (dc). When ac voltage is applied to a diode, the diode conducts ONLY ON THE POSITIVE ALTERNATION OF VOLTAGE; that is, when the anode of the diode is positive with respect to the cathode. This simplest type of rectifier is the half-wave rectifier. As shown in Figure 5, the half-wave rectifier uses only one diode. During the positive alternation of input voltage, the sine wave applied to the diode makes the anode positive with respect to the cathode. The diode then conducts, and current (I) flows from the negative supply lead (the secondary of the transformer), through the milliammeter, through the diode, and to the positive supply lead. As indicated by the shaded area of the output waveform in Figure 6, this current exists during the entire period of time that the anode is positive with respect to the cathode (in other words, for the first 180 degrees of the input sine wave).
During the negative alternation of input voltage (dotted polarity signs), the anode is driven negative and the diode cannot conduct. When conditions such as these exist, the diode is in cutoff and remains in cutoff for 180 degrees, during which time no current flows in the circuit. The circuit current therefore has the appearance of a series of positive pulses, as illustrated by the shaded areas on the waveform in view B. Notice that although the current is in the form of pulses, the current always flows in the same direction. Current that flows in pulses in the same direction is called PULSATING DC. The diode has thus RECTIFIED the ac input voltage.
2.1.1. RMS, Peak, and Average Values
Figure 7 is a comparison of the rms, peak, and average values of the types of waveforms associated with the half-wave rectifier. AC voltages are normally specified in terms of their rms values. Thus, when a 115-volt ac power source is mentioned in this chapter, it is specifying the rms value of 115 volts ac. In terms of peak values,
Erms = Epeak × .707
The peak value is always higher than the rms value. In fact,
Epeak = Erms × 1.414
therefore, if the rms value is 115 volts ac, then the peak value must be:
Epeak = Erms × 1.414
Epeak = 115 volts ac × 1.414
Epeak = 162.6 volts
The average value of a sine wave is 0 volts. Figure 8 shows how the average voltage changes when the negative portion of the sine wave is clipped off. Since the wave form swings positive but never negative (past the "zero-volt" reference line), the average voltage is positive. The average voltage (Eavg) is determined by the equation:
where
Eavg = Epeak × .318
thus
Eavg = 162.6 × .318
Eavg = 51.7 volts
2.1.2. Ripple Frequency
The half-wave rectifier gets its name from the fact that it conducts during only half the input cycle. Its output is a series of pulses with a frequency that is the same as the input frequency. Thus when operated from a 60-hertz line, the frequency of the pulses is 60 hertz. This is called RIPPLE FREQUENCY.
| Q5: |
What is the name of the simplest type of rectifier which uses one diode? |
| Q6: |
If the output of a half-wave rectifier is 50-volts peak, what is the average voltage? |
| Q7: |
In addition to stepping up or stepping down the input line voltage, what additional purpose does the transformer serve? |
2.2. The Conventional Full-Wave Rectifier
A full-wave rectifier is a device that has two or more diodes arranged so that load current flows in the same direction during each half cycle of the ac supply.
A diagram of a simple full-wave rectifier is shown in Figure 9 and Figure 9. The transformer supplies the source voltage for two diode rectifiers, D1 and D2. This power transformer has a center-tapped, high-voltage secondary winding that is divided into two equal parts (W1 and W2). W1 provides the source voltage for D1, and W2 provides the source voltage for D2. The connections to the diodes are arranged so that the diodes conduct on alternate half cycles.
During one alternation of the secondary voltage, the polarities are as shown in Figure 9. The source for D2 is the voltage induced into the lower half of the secondary winding of the transformer (W2). At the specific instant of time shown in the figure, the anode voltage on D2 is negative, and D2 cannot conduct. Throughout the period of time during which the anode of D2 is negative, the anode of D1 is positive. Since the anode of D1 is positive, it conducts, causing current to flow through the load resistor in the direction shown by the arrow.
Figure 10 shows the next half cycle of secondary voltage. Now the polarities across W1 and W2 are reversed. During this alternation, the anode of D1 is driven negative and D1 cannot conduct. For the period of time that the anode of D1 is negative, the anode of D2 is positive, permitting D2 to conduct. Notice that the anode current of D2 passes through the load resistor in the same direction as the current of D1 did. In this circuit arrangement, a pulse of load current flows during each alternation of the input cycle. Since both alternations of the input voltage cycle are used, the circuit is called a FULL-WAVE RECTIFIER.
Now that you have a basic understanding of how a full-wave rectifier works, let’s cover in detail a practical full-wave rectifier and its waveforms.
2.3. A Practical Full-Wave Rectifier
A practical full-wave rectifier circuit is shown in view A of Figure 11. It uses two diodes (D1 and D2) and a center-tapped transformer (T1). When the center tap is grounded, the voltages at the opposite ends of the secondary windings are 180 degrees out of phase with each other. Thus, when the voltage at point A is positive with respect to ground, the voltage at point B is negative with respect to ground. Let’s examine the operation of the circuit during one complete cycle.
During the first half cycle (indicated by the solid arrows), the anode of D1 is positive with respect to ground and the anode of D2 is negative. As shown, current flows from ground (center tap), up through the load resistor (RL), through diode D1 to point A. In the transformer, current flows from point A, through the upper winding, and back to ground (center tap). When D1 conducts, it acts like a closed switch so that the positive half cycle is felt across the load (RL).
During the second half cycle (indicated by the dotted lines), the polarity of the applied voltage has reversed. Now the anode of D2 is positive with respect to ground and the anode of D1 is negative. Now only D2 can conduct. Current now flows, as shown, from ground (center tap), up through the load resistor (RL), through diode D2 to point B of T1. In the transformer, current flows from point B up through the lower windings and back to ground (center tap). Notice that the current flows across the load resistor (RL) in the same direction for both halves of the input cycle.
View B represents the output waveform from the full-wave rectifier. The waveform consists of two pulses of current (or voltage) for each cycle of input voltage. The ripple frequency at the output of the full-wave rectifier is therefore twice the line frequency.
The higher frequency at the output of a full-wave rectifier offers a distinct advantage: Because of the higher ripple frequency, the output is closely approximate to pure dc. The higher frequency also makes filtering much easier than it is for the output of the half-wave rectifier.
In terms of peak value, the average value of current and voltage at the output of the full-wave rectifier is twice as great as that at the output of the half-wave rectifier. The relationship between the peak value and the average value is illustrated in Figure 12. Since the output waveform is essentially a sine wave with both alternations at the same polarity, the average current or voltage is 63.7 percent (or 0.637) of the peak current or voltage.
As an equation:
where
Emax = The peak value of the load voltage pulse
Eavg = 0.637 × Emax (the average load voltage)
Imax = The peak value of the load current pulse
Iavg = 0.637 × Imax (the average load current)
Example: The total voltage across the high-voltage secondary of a transformer used to supply a full-wave rectifier is 300 volts. Find the average load voltage (ignore the drop across the diode).
Solution: Since the total secondary voltage (ES) is 300 volts, each diode is supplied one-half of this value, or 150 volts. Because the secondary voltage is an rms value, the peak load voltage is:
Emax = 1.414 × ES
Emax = 1.414 × 150
Emax = 212 volts
The average load voltage is:
Eavg = 0.637 × Emax
Eavg = 0.637 × 212
Eavg = 135 volts
|
Note
|
If you have problems with this equation, review the portion of NEETS, module 2, that pertain to this subject. |
As you may recall from your past studies in electricity, every circuit has advantages and disadvantages. The full-wave rectifier is no exception. In studying the full-wave rectifier, you may have found that by doubling the output frequency, the average voltage has doubled, and the resulting signal is much easier to filter because of the high ripple frequency. The only disadvantage is that the peak voltage in the full-wave rectifier is only half the peak voltage in the half-wave rectifier. This is because the secondary of the power transformer in the full-wave rectifier is center tapped; therefore, only half the source voltage goes to each diode.
Fortunately, there is a rectifier which produces the same peak voltage as a half-wave rectifier and the same ripple frequency as a full-wave rectifier. This circuit, known as the BRIDGE RECTIFIER, will be the subject of our next discussion.
| Q8: |
What was the major factor that led to the development of the full-wave rectifier? |
| Q9: |
What is the ripple frequency of a full-wave rectifier with an input frequency of 60 Hz? |
| Q10: |
What is the average voltage (Eavg) Output of a full-wave rectifier with an output of 100 volts peak? |
2.4. The Bridge Rectifier
When four diodes are connected as shown in Figure 13, the circuit is called a BRIDGE RECTIFIER. The input to the circuit is applied to the diagonally opposite corners of the network, and the output is taken from the remaining two corners.
One complete cycle of operation will be discussed to help you understand how this circuit works. We have discussed transformers in previous modules in the NEETS series and will not go into their characteristics at this time. Let us assume the transformer is working properly and there is a positive potential at point A and a negative potential at point B. The positive potential at point A will forward bias D3 and reverse bias D4. The negative potential at point B will forward bias D1 and reverse bias D2. At this time D3 and D1 are forward biased and will allow current flow to pass through them; D4 and D2 are reverse biased and will block current flow. The path for current flow is from point B through D1, up through RL, through D3, through the secondary of the transformer back to point B. This path is indicated by the solid arrows. Waveforms (1) and (2) can be observed across D1 and D3.
One-half cycle later the polarity across the secondary of the transformer reverses, forward biasing D2 and D4 and reverse biasing D1 and D3. Current flow will now be from point A through D4, up through RL, through D2, through the secondary of T1, and back to point A. This path is indicated by the broken arrows. Waveforms (3) and (4) can be observed across D2 and D4. You should have noted that the current flow through R L is always in the same direction. In flowing through R L this current develops a voltage corresponding to that shown in waveform (5). Since current flows through the load (RL) during both half cycles of the applied voltage, this bridge rectifier is a full-wave rectifier.
One advantage of a bridge rectifier over a conventional full-wave rectifier is that with a given transformer the bridge rectifier produces a voltage output that is nearly twice that of the conventional full- wave circuit. This may be shown by assigning values to some of the components shown in Figure 14 and Figure 15. Assume that the same transformer is used in both circuits. The peak voltage developed between points X and Y is 1000 volts in both circuits. In the conventional full-wave circuit shown in Figure 14, the peak voltage from the center tap to either X or Y is 500 volts. Since only one diode can conduct at any instant, the maximum voltage that can be rectified at any instant is 500 volts. Therefore, the maximum voltage that appears across the load resistor is nearly — but never exceeds — 500 volts, as a result of the small voltage drop across the diode. In the bridge rectifier shown in Figure 15, the maximum voltage that can be rectified is the full secondary voltage, which is 1000 volts. Therefore, the peak output voltage across the load resistor is nearly 1000 volts. With both circuits using the same transformer, the bridge rectifier circuit produces a higher output voltage than the conventional full-wave rectifier circuit.
| Q11: |
What is the main disadvantage of a conventional full-wave rectifier? |
| Q12: |
What main advantage does a bridge rectifier have over a conventional full-wave rectifier? |
3. Limiters (Clippers)
As a technician, you will be confronted with many different types of LIMITING circuits. A LIMITER is defined as a device which limits some part of a waveform from exceeding a specified value. Limiting circuits are used primarily for wave shaping and circuit-protection applications.
A limiter is little more than the half-wave rectifier you studied in NEETS, Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies. By using a diode, a resistor, and sometimes a dc bias voltage, you can build a limiter that will eliminate the positive or negative alternations of an input waveform. Such a circuit can also limit a portion of the alternations to a specific voltage level. In this chapter you will be introduced to five types of limiters: SERIES-POSITIVE, SERIES-NEGATIVE, PARALLEL-POSITIVE, PARALLEL-NEGATIVE, and DUAL-DIODE LIMITERS. Both series- and parallel-positive and negative limiters use biasing to obtain certain wave shapes. They will be discussed in this chapter.
The diode in these circuits is the voltage-limiting component. Its polarity and location, with respect to ground, are the factors that determine circuit action. In series limiters, the diode is in series with the output. In parallel limiters, the diode is in parallel with the output.
3.1. Series Limiters
You should remember, from NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, that a diode will conduct when the anode voltage is positive with respect to the cathode voltage. The diode will not conduct when the anode is negative in respect to the cathode. Keeping these two simple facts in mind as you study limiters will help you understand their operation. Your knowledge of voltage divider action from NEETS, Module 1, Introduction to Matter, Energy, and Direct Current will also help you understand limiters.
In a SERIES LIMITER, a diode is connected in series with the output, as shown in Figure 16. The input signal is applied across the diode and resistor and the output is taken across the resistor. The series-limiter circuit can limit either the positive or negative alternation, depending on the polarity of the diode connection with respect to ground. The circuit shown in Figure 17 is a SERIES-POSITIVE LIMITER. Reversing D1 would change the circuit to a SERIES-NEGATIVE LIMITER.
3.1.1. Series-Positive Limiter
Let’s look at the series-positive limiter and its outputs in Figure 16 and in Figure 17. Diode D1 is in series with the output and the output is taken across resistor R1. The input must be negative with respect to the anode of the diode to make the diode conduct. When the positive alternation of the input signal (T0 to T1) is applied to the circuit, the cathode is positive with respect to the anode. The diode is reverse biased and will not conduct. Since no current can flow, no output is developed across the resistor during the positive alternation of the input signal.
During the negative half cycle of the input signal (T1 to T2), the cathode is negative with respect to the anode. This causes D1 to be forward biased. Current flows through R1 and an output is developed.
The output during each negative alternation of the input is approximately the same as the input (−10 volts) because most of the voltage is developed across the resistor.
Ideally, the output wave shape should be exactly the same as the input wave shape with only the limited portion removed. When the diode is reverse biased, the circuit has a small amount of reverse current flow, as shown just above the 0-volt reference line in Figure 18. During the limiting portion of the input signal, the diode resistance should be high compared to the resistor. During the time the diode is conducting, the resistance of the diode should be small as compared to that of the resistor. In other words, the diode should have a very high front-to-back ratio (forward resistance compared to reverse resistance). This relationship can be better understood if you study the effects that a front-to-back resistance ratio has on circuit output.
The following formula can be used to determine the output amplitude of the signal:
Let’s use the formula to compare the front-to-back ratio of the diode in the forward- and reverse- biased conditions. Given:
You can readily see that the formula comparison of the forward- and reverse-bias resistance conditions shows that a small amount of reverse current will flow during the limited portion of the input waveform. This small amount of reverse current will develop as the small positive voltage (0.09 volt) shown in Figure 18 (T0 to T1 and T2 to T3). The actual amount of voltage developed will depend on the type of diode used. For the remainder of this chapter, we will use only idealized waveforms and disregard this small voltage.
SERIES-POSITIVE LIMITER WITH BIAS.—In the series-positive limiter (Figure 16), the reference point at the bottom of resistor R1 is ground, or 0 volts. By placing a dc potential at point (1) in Figure 19 and in Figure 20, you can change the reference point. The reference point changes by the amount of dc potential that is supplied by the battery. The battery can either aid or oppose the flow of current in the series-limiter circuit. POSITIVE BIAS (aiding) is shown in Figure 19 and NEGATIVE BIAS (opposing) is shown in Figure 20.
When the dc aids forward bias, as in Figure 19, the diode conducts even with no signal applied. An input signal sufficiently positive to overcome the dc bias potential is required to reverse bias and cut off the diode.
Let’s look at a series-positive limiter with positive bias as shown in Figure 21 and in Figure 22. The diode will conduct until the input signal exceeds +5 at T1 on the positive alternation of the input signal. When the positive alternation exceeds +5 volts, the diode becomes reverse biased and limits the positive alternation of the output signal to +5 volts. This is because there is no current flow through resistor R1 and battery voltage is felt at point (B). The diode will remain reverse biased until the positive alternation of the input signal decreases to just under +5 volts at T2. At this time, the diode again becomes forward biased and conducts. The diode will remain forward biased from T2 to T3. During this period the negative alternation of the input is passed through the diode without being limited. From T3 to T4 the diode is again reverse biased and the output is again limited.
Now let’s look at what takes place when reverse bias is aided, as shown in Figure 23. The diode is negatively biased with -5 volts from the battery. In Figure 24, compare the output to the input signal applied. From T0 to T1 the diode is reverse biased and limiting takes place. The output is at -5 volts (battery voltage) during this period. As the negative alternation increases toward -10 volts (T1), the cathode of the diode becomes more negative than the anode and is forward biased. From T1 to T2 the input signal is passed to the output. The diode remains forward biased until the negative alternation has decreased to −5 volts at T2. At T2 the cathode of the diode becomes more positive than the anode, and the diode is again reverse biased and remains so until T3.
3.1.2. Series-Negative Limiter
In Figure 25, the SERIES-NEGATIVE LIMITER limits the negative portion of the waveform, as shown in view (B). Let’s consider the input signal and determine how the output is produced. During T0 to T1 (Figure 26), the anode is more positive than the cathode and the diode conducts. Current flows up through the resistor and the diode, and a positive voltage is developed at the output. The voltage across the resistor is essentially the same as the voltage applied to the circuit.
During T1 to T2 the anode is negative with respect to the cathode and the diode does not conduct. This portion of the output is limited because no current flows through the resistor.
As you can see, the only difference between series-positive and series-negative limiters is that the diode is reversed in the negative limiters.
SERIES-NEGATIVE LIMITER WITH BIAS.—Figure 27 shows a series-negative limiter with negative bias. The diode is forward biased and conducts with no input signal. In Figure 28 it will continue to conduct as the input signal swings first positive and then negative (but only to −5 volts) from T0 through T1. At T1 the input becomes negative with respect to the −5 volt battery bias. The diode becomes reverse biased and is cutoff until T2 when the anode again becomes positive with respect to the battery voltage (−5 volts) on the cathode. No voltage is developed in the output by R1 (no current flow) and the output is held at −5 volts from T1 to T2. With negative bias applied to a series-negative limiter, only a portion of the negative signal is limited.
Now let’s look at a series-negative limiter with positive bias, as shown in Figure 29. Here we will remove all of the negative alternation and part of the positive alternation of the input signal. We have given a full explanation of the series-positive limiter, series-positive limiter with bias, series- negative limiter, and series-negative limiter with negative bias; therefore, you should have little difficulty understanding what is happening in the circuit in the figure.
The series-negative limiter with positive bias is different in only one aspect from the series-positive limiter with bias (Figure 23 and Figure 24) discussed earlier. The difference is that the diode is reversed and the output is of the opposite polarity.
signal?
3.2. Parallel Limiters
A PARALLEL-LIMITER circuit uses the same diode theory and voltage divider action as series limiters. A resistor and diode are connected in series with the input signal and the output signal is developed across the diode. The output is in parallel with the diode, hence the circuit name, parallel limiter. The parallel limiter can limit either the positive or negative alternation of the input signal.
Recall that in the series limiter the output was developed while the diode was conducting. In the parallel limiter the output will develop when the diode is cut off. You should not try to memorize the outputs of these circuits; rather, you should study their actions and be able to figure them out.
3.2.1. Parallel-Positive Limiter.
The schematic diagram shown in Figure 31 is a PARALLEL-POSITIVE LIMITER. The diode is in parallel with the output and only the positive half cycle of the input is limited. When the positive alternation of the input signal is applied to the circuit (T0 to T1), the diode is forward biased and conducts. This action may be seen in Figure 32. As current flows up through the diode and the resistor, a voltage is dropped across each. Since R1 is much larger than the forward resistance of D1, most of the input signal is developed across R1. This leaves only a very small voltage across the diode (output). The positive alternation of the input signal has been limited.
From T1 to T2 the diode is reverse biased and acts as an extremely high resistance. The negative alternation of the input signal appears across the diode at approximately the same amplitude as the input. The negative alternation of the input is not limited.
As with the series limiter, the parallel limiter should provide maximum output voltage for the unlimited part of the signal. The reverse-bias resistance of the diode must be very large compared to the series resistor. To determine the output amplitude, use the following formula:
PARALLEL-POSITIVE LIMITER WITH BIAS. — Figure 33 shows the schematic diagram of a PARALLEL-POSITIVE LIMITER WITH NEGATIVE BIAS. The diode is forward biased and conducts without an input signal. D1 is essentially a short circuit. The voltage at the output terminals is -4 volts.
As the positive alternation of the input signal is applied to the circuit, the diode remains forward biased and limits the entire positive alternation, as shown in Figure 34. As the signal goes in a negative direction Oust before T1), the diode remains forward biased (limiting is still present) until the input signal exceeds −4 volts (T1). D1 becomes reverse biased as the anode becomes more negative than the cathode. While the input signal is more negative than the −4 volts of the bias battery (T1 to T2), the diode is reverse biased and remains cut off. The output follows the input signal from T1 to T2. At all other times during that cycle, the diode is forward biased and limiting occurs. This circuit is called a parallel-positive limiter with negative bias because the positive output is limited and the bias in the circuit is negative with reference to ground. Limiting takes place at all points more positive than −4 volts.
The circuit shown in Figure 35 is a PARALLEL-POSITIVE LIMITER WITH POSITIVE BIAS. The positive terminal of the battery is connected to the cathode of the diode. This causes the diode to be reverse biased at all times except when the input signal is more positive than the bias voltage (T1 to T2), as shown in Figure 36.
As the positive alternation of the input signal is applied (T0), the output voltage follows the input signal. From T1 to T2 the input signal is more positive than + 4 volts. The diode is forward biased and conducts. At this time the output voltage equals the bias voltage and limiting takes place. From T2 to T4 of the input signal, the diode is reverse biased and does not conduct. The output signal follows the input signal and no limiting takes place.
This circuit is called a parallel-positive limiter with positive bias because limiting takes place in the positive alternation and positive bias is used on the diode.
3.2.2. Parallel-Negative Limiter
A PARALLEL-NEGATIVE LIMITER is shown in Figure 37. Notice the similarity of the parallel-negative limiter and the parallel-positive limiter shown in Figure 31. From T0 to T1 of the input signal, the diode is reverse biased and does not conduct, as shown in Figure 38. The output signal follows the input signal and the positive alternation is not limited.
During the negative alternation of the input signal (T1 to T2), the diode is forward biased and conducts. The relatively low forward bias of D1 develops a very small voltage and, therefore, limits the output to nearly 0 volts. A voltage is developed across the resistor as current flows through the resistor and diode.
PARALLEL-NEGATIVE LIMITER WITH BIAS. — The circuit shown in Figure 39 is a parallel-negative limiter with negative bias. With no input, the battery maintains D1 in a reverse-bias condition. D1 cannot conduct until its cathode is more negative than its anode. D1 acts as an open until the input signal dips below −4 volts at T2 in Figure 40. At T2 the input signal becomes negative enough to forward bias the diode, D1 conducts and acts like a short, and the output is limited to the −4 volts from the battery from T2 to T3. Between T3 and T4 the diode is again reverse biased. The output signal follows the input signal and no limiting occurs.
Figure 41 shows a parallel-negative limiter with positive bias. The operation is similar to those circuits already explained. Limiting occurs when the diode conducts. No limiting occurs when the diode is reverse biased. In this circuit, the bias battery provides forward bias to the diode without an input signal. The output is at +4 volts, except where the input goes above +4 volts (T1 to T2), as shown in Figure 42. The parts of the signal more negative than +4 volts are limited.
limiter?
3.3. Dual-Diode Limiter
The last type of limiter to be discussed in this chapter is the DUAL-DIODE LIMITER, shown in Figure 43. This limiter combines a parallel-negative limiter with negative bias (D1 and B1) and a parallel-positive limiter with positive bias (D2 and B2). Parts of both the positive and negative alternations are removed in this circuit. Each battery aids the reverse bias of the diode in its circuit; the circuit has no current flow with no input signal. When the input signal is below the value of the biasing batteries, both D1 and D2 are reverse biased. With D1 and D2 reverse biased, the output follows the input. When the input signal becomes more positive than +20 volts (Figure 44), D2 conducts and limits the output to +20 volts. When the input signal becomes more negative than −20 volts, D1 conducts and limits the output to this, value. When neither diode conducts, the output follows the input waveform.
4. Clampers
Certain applications in electronics require that the upper or lower extremity of a wave be fixed at a specific value. In such applications, a CLAMPING (or CLAMPER) circuit is used. A clamping circuit clamps or restrains either the upper or lower extremity of a waveform to a fixed dc potential. This circuit is also known as a DIRECT-CURRENT RESTORER or a BASE-LINE STABILIZER. Such circuits are used in test equipment, radar systems, electronic countermeasure systems, and sonar systems. Depending upon the equipment, you could find negative or positive clampers with or without bias. Figure 45 through Figure 49 illustrate some examples of waveforms created by clampers. However, before we discuss clampers, we will review some relevant points about series RC circuits.
4.1. Series RC Circuits
Series RC circuits are widely used for coupling signals from one stage to another. If the time constant of the coupling circuit is comparatively long, the shape of the output waveform will be almost identical to that of the input. However, the output dc reference level may be different from that of the input. Figure 50 shows a typical RC coupling circuit in which the output reference level has been changed to 0 volts. In this circuit, the values of R1 and C1 are chosen so that the capacitor will charge (during T0 to T1) to 20 percent of the applied voltage, as shown in Figure 51. With this in mind, let’s consider the operation of the circuit.
At T0 the input voltage is −50 volts and the capacitor begins charging. At the first instant the voltage across C is 0 and the voltage across R is −50 volts. As C charges, its voltage increases. The voltage across R, which is the output voltage, begins to drop as the voltage across C increases. At T1 the capacitor has charged to 20 percent of the −50 volts input, or −10 volts. Because the input voltage is now 0 volts, the capacitor must discharge. It discharges through the low impedance of the signal source and through R, developing +10 volts across R at the first instant. C discharges 20 percent of the original 10-volt charge from T1 to T2. Thus, C discharges to +8 volts and the output voltage also drops to 8 volts.
At T2 the input signal becomes −50 volts again. This −50 volts is in series opposition to the 8-volt charge on the capacitor. Thus, the voltage across R totals −42 volts (−50 plus +8 volts). Notice that this value of voltage (−42 volts) is smaller in amplitude than the amplitude of the output voltage which occurred at TO (−50 volts). Capacitor C now charges from +8 to +16 volts. If we were to continue to follow the operation of the circuit, we would find that the output wave shape would become exactly distributed around the 0-volt reference point. At that time the circuit operation would have reached a stable operating point. Note that the output wave shape has the same amplitude and approximately the same shape as the input wave shape, but now "rides" equally above and below 0 volts. Clampers use this RC time so that the input and output waveforms will be almost identical, as shown from T11 to T12.
4.2. Positive-Diode Clampers
Figure 52 illustrates the circuit of a positive-diode clamper. Resistor R1 provides a discharge path for C1. This resistance is large in value so that the discharge time of C1 will be long compared to the input pulse width. The diode provides a fast charge path for C1. After C1 becomes charged it acts as a voltage source. The input wave shape shown in Figure 53 is a square wave and varies between +25 volts and −25 volts. Compare each portion of the input wave shape with the corresponding output wave shape. Keep Kirchhoff’s law in mind: The algebraic sum of the voltage drops around a closed loop is 0 at any instant.
At T0 the −25 volt input signal appears across R1 and D1 (the capacitor is a short at the first instant). The initial voltage across R1 and D1 causes a voltage spike in the output. Because the charge time of C1 through D1 is almost instantaneous, the duration of the pulse is so short that it has only a negligible effect on the output. The −25 volts across D1 makes the cathode negative with respect to the anode and the diode conducts heavily. C1 quickly charges through the small resistance of D1. As the voltage across C1 increases, the output voltage decreases at the same rate. The voltage across C1 reaches -25 volts and the output is at 0 volts.
At T1 the +25 volts already across the capacitor and the +25 volts from the input signal are in series and aid each other (SERIES AIDING). Thus, +50 volts appears across R1 and D1. At this time, the cathode of D1 is positive with respect to the anode, and the diode does not conduct. From T1 to T2, C1 discharges to approximately +23 volts (because of the large values of R and C) and the output voltage drops from +50 volts to +48 volts.
At T2 the input signal changes from +25 volts to −25 volts. The input is now SERIES OPPOSING with the +23 volts across C1. This leaves an output voltage of −2 volts (−25 plus +23 volts). The cathode of D1 is negative with respect to the anode and D1 conducts. From T2 to T3, C1 quickly charges through D1 from +23 volts to +25 volts; the output voltage changes from −2 volts to 0 volts.
At T3 the input signal and capacitor voltage are again series aiding. Thus, the output voltage felt across R1 and D1 is again +50 volts. During T3 and T4, C1 discharges 2 volts through R1. Notice that circuit operation from T3 to T4 is the same as it was from T1 to T2. The circuit operation for each square- wave cycle repeats the operation which occurred from T2 to T4.
Compare the input wave shape of Figure 53 with the output wave shape. Note the following important points: (1) The peak-to-peak amplitude of the input wave shape has not been changed by the clamper circuit; (2) the shape of the output wave shape has not been significantly changed from that of the input by the action of the clamper circuit; and (3) the output wave shape is now all above 0 volts whereas the input wave shape is both above and below 0 volts. Thus, the lower part of the input wave shape has been clamped to a dc potential of 0 volts in the output. This circuit is referred to as a positive clamper since all of the output wave shape is above 0 volts and the bottom is clamped at 0 volts.
The positive clamper circuit is self-adjusting. This means that the bottom of the output waveform remains clamped at 0 volts during changes in input signal amplitude. Also, the output wave shape retains the form and peak-to-peak amplitude (50 volts in this case) of the input wave shape. When the input amplitude becomes greater, the charge of the capacitor becomes greater and the output amplitude becomes larger. When the input amplitude decreases, the capacitor does not charge as high as before and clamping occurs at a lower output voltage. The capacitor charge, therefore, changes with signal strength.
The size of R1 and C1 has a direct effect upon the operation of the clamper. Because of the small resistance of the diode, the capacitor charge time is short. If either R1 or C1 is made smaller, the capacitor discharges faster (TC = R · C).
The ability of a smaller value capacitor to quickly discharge to a lower voltage is an advantage when the amplitude of the input wave shape is suddenly reduced. However, for normal clamper operation, quick discharge time is a disadvantage. This is because one objective of clamping is to keep the output wave shape the same as the input wave shape. If the small capacitor allows a relatively large amount of the voltage to discharge with each cycle, then distortion occurs in the output wave shape. A larger portion of the wave shape then appears on the wrong side of the reference line.
Increasing the value of the resistor increases the discharge time (again, TC = R · C). This increased value causes the capacitor to discharge more slowly and produces an output wave shape which is a better reproduction of the input wave shape. A disadvantage of increasing the resistance value is that the larger resistance increases the discharge time of the capacitor and slows the self-adjustment rate of the circuit, particularly in case a sudden decrease in input amplitude should occur. The larger resistance has no effect on self-adjustment with a sudden rise in input amplitude. This is because the capacitor charges through the small resistance of the conducting diode.
Circuits often incorporate a compromise between a short RC time constant (for self-adjustment purposes) and a long RC time constant for less distortion. A point to observe is that the reverse resistance of the diode sometimes replaces the, physical resistor in the discharge path of the capacitor.
4.2.1. Positive-Diode Clamper With Bias
Biased clamping circuits operate in exactly the same manner as unbiased clampers, with one exception. That exception is the addition of a dc bias voltage in series with the diode and resistor. The size and polarity of this bias voltage determines the output clamping reference.
Figure 54 illustrates the circuit of a positive clamper with positive bias. It can be identified as a positive clamper because the cathode of the diode is connected to the capacitor. Positive bias can be observed by noting that the negative side of the battery is connected to ground. The purposes and actions of the capacitor, resistor, and diode are the same as in the unbiased clamper circuit just discussed.
With no input, D1 is forward biased and the +10 V battery is the output. C1 will charge to +10 V and hold this charge until the first pulse is applied. The battery establishes the dc reference level at +10 volts. The input wave shape at the top of Figure 54 is a square wave which alternates between +25 and −25 volts. The output wave shape is shown at the bottom half of view (B).
Here, as with previous circuits, let’s apply Kirchhoff’s voltage law to determine circuit operation. With no input signal, the output is just the +10 volts supplied by the battery.
At time T0 the −25 volt signal applied to the circuit is instantly felt across R1 and D1. The −25 volt input signal forward biases D1, and C1 quickly charges to 35 volts. This leaves +10 volts across the output terminals for much of the period from T0 to T1. The polarity of the charged capacitor is, from the left to the right, minus to plus.
At T1 the 35 volts across the capacitor is series aiding with the +25 volt input signal. At this point (T1) the output voltage becomes +60 volts; the voltage across R1 and D1 is +50 volts, and the battery is +10 volts. The cathode of D1 is positive with respect to the anode and the diode does not conduct. From T1 to T2, C1 discharges only slightly through the large resistance of R1. Assume that, because of the size of R1 and C1, the capacitor discharges just 2 volts (from +35 volts to +33 volts) during this period. Thus, the output voltage drops from +60 volts to +58 volts.
At T2 the −25 volt input signal and the +33 volts across C1 are series opposing. This makes the voltage across the output terminals +8 volts. The cathode of the diode is 2 volts negative with respect to its anode and D1 conducts. Again, since the forward-biased diode is essentially a short, C1 quickly charges from +33 volts to +35 volts. During most of the time from T2 to T3, then, we find the output voltage is +10 volts.
At T3 the +25 volts of the input signal is series aiding with the +35 volts across C1. Again the output voltage is +60 volts. Observe that at T3 the conditions in the circuit are the same as they were at T1. Therefore, the circuit operation from T3 to T4 is the same as it was from T1 to T2. Circuit operation continues as a duplication of the operations which occurred from T1 to T3.
By comparing the input and output wave shapes, you should note the following: (1) The peak-to- peak amplitude of the input wave shape has not been changed in the output (for all practical purposes) by the action of the clamper circuit; (2) the shape of the input wave has not been changed; (3) the output wave shape is now clamped above +10 volts. Remember that this clamping level (+10 volts) is determined by the bias battery.
4.2.2. Positive-Diode Clamper With Negative Bias
Figure 56 is a positive clamper with negative bias. Observe that with no input signal, the capacitor charges through R1 to the bias battery voltage; the output voltage equals −10 volts. The circuit has negative bias because the positive side of the battery is grounded. The output waveform is shown in Figure 57. Study the figure and waveforms carefully and note the following important points. Once again the peak-to-peak amplitude and shape of the output wave are, for all practical purposes, the same as the input wave. The lower extremity of the output wave is clamped to −10 volts, the value of the battery. Let’s look at the circuit operation. The capacitor is initially charged to −10 volts with no input signal, and diode D1 does not conduct.
The −25 volt input signal provides forward bias for D1. The capacitor charges to +15 volts and retains most of its charge because its discharge through R1 is negligible. The +25 volt input signal is series aiding the capacitor voltage and develops +40 volts between the output terminals. When the input voltage is −25 volts, D1 conducts and the output voltage is −10 volts (−25 volts plus +15 volts). In this way the output reference is clamped at −10 volts. Changing the size of the battery changes the clamping reference level to the new voltage.
4.3. Negative-Diode Clampers
Figure 58 illustrates the circuit of a negative-diode clamper. Compare this with the positive-diode clamper in Figure 52. Note that the diode is reversed with reference to ground. Like the positive clamper, resistor R1 provides a discharge path for C1; the resistance must be a large value for C1 to have a long discharge time. The low resistance of the diode provides a fast charge path for C1. Once C1 becomes charged, it acts as a source of voltage which will help determine the maximum and minimum voltage levels of the output wave shape. The input wave shape shown in Figure 59 is a square wave which varies between +25 and −25 volts. The output wave shapes are shown in the bottom half of Figure 59. You will find that the operation of the negative clamper is similar to that of the positive clamper, except for the reversal of polarities.
At T0 the +25 volt input signal applied to the circuit appears across R1 and D1. This makes the anode of D1 positive with respect to the cathode and it conducts heavily. Diode resistance is very small causing C1 to charge quickly. As the voltage across C1 increases, the output voltage decreases. The voltage across C1 reaches 25 volts quickly; during most of T0 to T1, the output voltage is 0.
At T1 the voltage across the capacitor and the input voltage are series aiding and result in −50 volts appearing at the output. At this time the diode is reverse biased and does not conduct. Because of the size of R and C, the capacitor discharges only 2 volts to approximately 23 volts from T1 to T2. Using Kirchhoff’s voltage law to determine voltage in the circuit, we find that the output voltage decreases from −50 to −48 volts.
At T2 the +25 volt input signal and the 23 volts across C1 are series opposing. The output voltage is +2 volts. The anode of D1 is positive with respect to the cathode and D1 will conduct. From T2 to T3, C1 charges quickly from 23 to 25 volts through D1. At the same time, the output voltage falls from +2 to 0 volts.
At T3 the input and capacitor voltages are series aiding and the total output voltage is −50 volts. From T3 to T4, D1 is reverse biased and C discharges through R. The circuit operation is now the same as it was from T1 to T2. The circuit operation for the following square-wave cycles duplicates the operation which occurred from T1 to T3.
As was the case with the positive clamper, the amplitude and wave, shape of the output is almost identical to that of the input. However, note that the upper extremity of the output wave shape is clamped to 0 volts; that is, the output wave shape, for all practical purposes, lies entirely below the 0-volt reference level.
4.3.1. Negative-Diode Clamper With Negative Bias
Figure 60 is the circuit of a negative clamper with negative bias. Again, with no input signal the capacitor charges to the battery voltage and the output is negative because the positive side of the battery is ground. The bottom of Figure 61 shows the output of the circuit. Study the figure carefully, and note the following important points. The peak-to-peak amplitude and shape of the output wave, for all practical purposes, are the same as that of the input wave. The output wave is clamped to −10 volts which is the value of the battery. Since this is a negative clamper, the upper extremity of the waveform touches the −10 volt reference line (and the rest of it lies below this voltage level).
Let’s review the important points of circuit operation. The capacitor is initially charged to −10 volts with no input signal. Applying Kirchhoff’s law we find that the +25 volt input signal and the 10-volt battery are series opposing. This series opposing forward biases D1 and the capacitor charges to −35 volts. The output voltage is equal to the sum of the capacitor voltage and the input voltage. Thus, the output voltage is −10 volts and the wave shape is clamped to −10 volts. With a −25 volt input, the charge maintained across C1 and the input are series aiding and provide a −60 volt output. C1 will discharge just before the next cycle begins and the input becomes positive. The +25 volt input signal and the approximately −23 volt charge remaining on C1 will forward bias D1 and the output will be clamped to the battery voltage. C1 will quickly charge to the input signal level. Thus, the output voltage varies between −10 and −60 volts and the wave shape is clamped to −10 volts.
4.3.2. Negative Clamper With Positive Bias
Figure 62 illustrates the circuit of a negative clamper with positive bias. With no input signal the capacitor charges to the battery voltage and the output is positive because the negative side of the battery is grounded. The output is illustrated in the bottom half of Figure 63. Study the figure carefully and note the following important points. The peak-to-peak amplitude and shape of the output waveform, for all practical purposes, are the same as that of the input. The output wave is clamped to +10 volts, the value of the battery. Since this is a negative clamper (cathode to ground), the top of the output wave touches the +10 volt reference line.
Let’s go over a summary of the circuit operation. With no input signal the capacitor charges to 10 volts. The +25 volt input signal forward biases D1. With the 10-volt battery and the input in series, the capacitor charges to −15 volts. The capacitor remains charged, for all practical purposes, since its discharge through R1 (very large) is almost negligible. The output voltage is equal to the algebraic sum of the capacitor voltage and the input voltage. The +25 volt input signal added to the −15 volt capacitor charge provides a +10 volt output. With a −25 volt input at T1, D1 is reverse-biased and the charge across C 1 adds to the input voltage to provide a −40 volt output. From T1 to T2, the capacitor loses only a small portion of its charge. At T2 the input signal is +25 volts and the input returns to +10 volts. The wave shape is negatively clamped to +10 volts by the battery.
We can say, then, that positive clamping sets the wave shape above (negative peak on) the reference level, and negative clamping places the wave shape below (positive peak on) the reference level.
(long or short)? of a damper (long or short)? the output waveform (positive or negative)? negative potential? shape to what polarity (positive or negative)? extremity of the wave shape to be clamped above 0 volts? voltage swing from +50 to −50 volts?
5. Voltage Multipliers
You may already know how a transformer functions to increase or decrease voltages. You may also have learned that a transformer secondary may provide one or several ac voltage outputs which may be greater or less than the input voltage. When voltages are stepped up, current is decreased; when voltages are stepped down, current is increased.
Another method for increasing voltages is known as voltage multiplication. VOLTAGE MULTIPLIERS are used primarily to develop high voltages where low current is required. The most common application of the high voltage outputs of voltage multipliers is the anode of cathode-ray tubes (CRT), which are used for radar scope presentations, oscilloscope presentations, or TV picture tubes. The dc output of the voltage multiplier ranges from 1000 volts to 30,000 volts. The actual voltage depends upon the size of the CRT and its equipment application.
Voltage multipliers may also be used as primary power supplies where a 177 volt-ac input is rectified to pulsating dc. This dc output voltage may be increased (through use of a voltage multiplier) to as much as 1000 volts dc. This voltage is generally used as the plate or screen grid voltage for electron tubes.
If you have studied transformers, you may have learned that when voltage is stepped up, the output current decreases. This is also true of voltage multipliers. Although the measured output voltage of a voltage multiplier may be several times greater than the input voltage, once a load is connected the value of the output voltage decreases. Also any small fluctuation of load impedance causes a large fluctuation in the output voltage of the multiplier. For this reason, voltage multipliers are used only in special applications where the load is constant and has a high impedance or where input voltage stability is not critical.
Voltage multipliers may be classified as voltage doublers, triplers, or quadruplers. The classification depends on the ratio of the output voltage to the input voltage. For example, a voltage multiplier that increases the peak input voltage twice is called a voltage doubler. Voltage multipliers increase voltages through the use of series-aiding voltage sources. This can be compared to the connection of dry cells (batteries) in series.
The figures used in the explanation of voltage multipliers show a transformer input, even though for some applications a transformer is not necessary. The input could be directly from the power source or line voltage. This, of course, does not isolate the equipment from the line and creates a potentially hazardous condition. Most military equipments use transformers to minimize this hazard.
Figure 64 shows the schematic for a half-wave voltage doubler. Notice the similarities between this schematic and those of half-wave voltage rectifiers. In fact, the doubler shown is made up of two half-wave voltage rectifiers. C1 and CR1 make up one half-wave rectifier, and C2 and CR2 make up the other. The schematic of the first half-wave rectifier is indicated by the dark lines in Figure 65. The dotted lines and associated components represent the other half-wave rectifier and load resistor.
Notice that C1 and CR1 work exactly like a half-wave rectifier. During the positive alternation of the input cycle (view A), the polarity across the secondary winding of the transformer is as shown. Note that the top of the secondary is negative. At this time CR1 is forward biased (cathode negative in respect to the anode). This forward bias causes CR1 to function like a closed switch and allows current to follow the path indicated by the arrows. At this time, C1 charges to the peak value of the input voltage, or 200 volts, with the polarity shown.
During the period when the input cycle is negative, as shown in Figure 66, the polarity across the secondary of the transformer is reversed. Note specifically that the top of the secondary winding is now positive. This condition now forward biases CR2 and reverse biases CR1. A series circuit now exists consisting of C1, CR2, C2, and the secondary of the transformer. The current flow is indicated by the arrows. The secondary voltage of the transformer now aids the voltage on C1. This results in a pulsating dc voltage of 400 volts, as shown by the waveform. The effect of series aiding is comparable to the connection of two 200-volt batteries in series. As shown in Figure 67, C2 charges to the sum of these voltages, or 400 volts.
The schematic shown in Figure 68 is an illustration of a half-wave voltage tripler. When you compare Figure 67 and Figure 68, you should see that the circuitry is identical except for the additional parts, components, and circuitry shown by the dotted lines. (CR3, C3, and R2 make up the additional circuitry.) By themselves, CR3, C3, and R2 make up a half-wave rectifier. Of course, if you remove the added circuitry, you will once again have a half-wave voltage doubler.
Figure 69 shows the schematic for the voltage tripler. Notice that CR3 is forward biased and functions like a closed switch. This allows C3 to charge to a peak voltage of 200 volts at the same time C1 is also charging to 200 volts.
The other half of the input cycle is shown in Figure 70. C2 is charged to twice the input voltage, or 400 volts, as a result of the voltage-doubling action of the transformer and C1. At this time, C2 and C3 are used as series-aiding devices, and the output voltage increases to the sum of their respective voltages, or 600 volts. R1 and R2 are proportional according to the voltages across C2 and C3. In this case, there is a 2 to 1 ratio.
The circuit shown in Figure 71 is that of a full-wave voltage doubler. The main advantage of a full-wave doubler over a half-wave doubler is better voltage regulation, as a result of reduction in the output ripple amplitude and an increase in the ripple frequency. The circuit is, in fact, two half-wave rectifiers. These rectifiers function as series-aiding devices except in a slightly different way. During the alternation when the secondary of the transformer is positive at the top, C1 charges to 200 volts through CR1. Then, when the transformer secondary is negative at the top, C2 charges to 200 volts through CR2. R1 and R2 are equal value, balancing resistors that stabilize the charges of the two capacitors. Resistive load RL is connected across C1 and C2 so that R L receives the total charge of both capacitors. The output voltage is +400 volts when measured at the top of R L, or point "A" with respect to point "B." If the output is measured at the bottom of RL, it is −400 volts. Either way, the output is twice the peak value of the ac secondary voltage. As you can imagine, the possibilities for voltage multiplication are extensive.
| Q38: |
A half-wave voltage doubler is made up of how many half-wave rectifiers? |
| Q39: |
If a half-wave rectifier is added to a half-wave voltage doubler, the resulting circuit is a voltage ____. |
| Q40: |
In a full-wave voltage doubler, are the capacitors connected in series or in parallel with the output load? |
6. Diode Switch
In addition to their use as simple rectifiers, diodes are also used in circuits that mix signals together (mixers), detect the presence of a signal (detector), and act as a switch "to open or close a circuit." Diodes used in these applications are commonly referred to as "signal diodes." The simplest application of a signal diode is the basic diode switch shown in Figure 72.
When the input to this circuit is at zero potential, the diode is forward biased because of the zero potential on the cathode and the positive voltage on the anode. In this condition, the diode conducts and acts as a straight piece of wire because of its very low forward resistance. In effect, the input is directly coupled to the output resulting in zero volts across the output terminals. Therefore, the diode, acts as a closed switch when its anode is positive with respect to its cathode.
If we apply a positive input voltage (equal to or greater than the positive voltage supplied to the anode) to the diode’s cathode, the diode will be reverse biased. In this situation, the diode is cut off and acts as an open switch between the input and output terminals. Consequently, with no current flow in the circuit, the positive voltage on the diode’s anode will be felt at the output terminal. Therefore, the diode acts as an open switch when it is reverse biased.
| Q27: |
What is a load? |
| Q28: |
What is the output of a half-wave rectifier? |
| Q29: |
What type of rectifier is constructed by sandwiching a section of semiconductor material between two metal plates? |
| Q30: |
What type of bias makes a diode act as a closed switch? |