4-19 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. POSITIVE-DIODE CLAMPERS 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.

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