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PARALLEL-POSITIVE LIMITER WITH BIAS.—Figure 4-10, view (A), 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.
Figure 4-10A.—Parallel limiter with negative bias.
Figure 4-10B.—Parallel limiter with negative bias.
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 view (B). 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 4-11, view (A), 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 view (B).
Figure 4-11A.—Parallel-positive Limiter with positive bias.
Figure 4-11B.—Parallel-positive Limiter with positive bias.
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.
A PARALLEL-NEGATIVE LIMITER is shown in view (A) of figure 4-12. Notice the similarity of the parallel-negative limiter and the parallel-positive limiter shown in view (A) of figure 4-9. From T0 to T1 of the input signal, the diode is reverse biased and does not conduct, as shown in view (B) of figure 4-12. The output signal follows the input signal and the positive alternation is not limited.
Figure 4-12A.—Parallel-negative limiter.
Figure 4-12B.—Parallel-negative limiter.
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.
Figure 4-13A.—Parallel-negative limiter with negative bias.
Figure 4-13B.—Parallel-negative limiter with negative bias.
Figure 4-14, view (A), 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 view (B). The parts of the signal more negative than +4 volts are limited.
Figure 4-14A.—Parallel-negative limiter with positive bias.
Figure 4-14B.—Parallel-negative limiter with positive bias.
Q4. What component is in parallel with the output in a parallel limiter?
Q5. What is the condition of the diode in a series limiter when an output is developed? In a parallel limiter?
The last type of limiter to be discussed in this chapter is the DUAL-DIODE LIMITER, shown in figure 4-15, view (A). 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 (view (B)), 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.
Figure 4-15A.—Dual-diode limiter.
Figure 4-15B.—Dual-diode limiter.
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 4-16, views (A) through (E), illustrates some examples of waveforms created by clampers. However, before we discuss clampers, we will review some relevant points about series RC circuits.
Figure 4-16A.—Clamping waveforms. WITHOUT CLAMPING.
Figure 4-16B.—Clamping waveforms. WITH CLAMPING, LOWER EXTREMITY OF WAVE IS HELD AT 0V.
Figure 4-16C.—Clamping waveforms. WITH CLAMPING, LOWER EXTREMITY OF WAVE IS HELD AT +100 V.
Figure 4-16D.—Clamping waveforms. WITH CLAMPING, UPPER EXTREMITY OF WAVE IS HELD AT 0V.
Figure 4-16E.—Clamping waveforms. WITH CLAMPING, UPPER EXTREMITY OF WAVE IS HELD AT
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 4-17, view (A), 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 view (B). With this in mind, let's consider the operation of the circuit.
Figure 4-17A.—RC coupling.
Figure 4-17B.—RC coupling.
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.
Figure 4-18, view (A), 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 view (B) 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.
Figure 4-18A.—Positive damper and waveform.
Figure 4-18B.—Positive damper and waveform.
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.