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.
