Since E Ec = ER , then ER is increasing during this time but in a
negative direction. At 60 usec, E is equal to zero, and the output
voltage ER is equal to Ec, but is opposite in polarity. The
capacitor continues to discharge until, at the end of 7 time
constants (700 usec later), Ec and the output voltage ER are both
equal to zero.
Section IV. LOWPASS RC CIRCUIT
21.
GENERAL.
The output of a lowpass RC filter is taken across the
capacitor. The effect of the time constant on the output waveform
differs entirely from that of a highpass RC filter. When the time
constant of a lowpass RC filter is short compared with the reference
period, the output waveform closely resembles the input waveform.
When the time constant of a lowpass RC filter is long compared with
the reference period, the output waveform does not resemble the input
waveform.
22.
SHORTTIME CONSTANT.
a. The pulse shown in Figure 35B is applied to the lowpass RC
filter in Figure 35A. The time constant of the circuit is equal to 1
usec, which is short compared to the pulse rise (10 usec), duration
(30 usec), and decay (20 usec) times. Since the time constant of
this circuit is short compared with the pulse ripe time, the charging
rate of the capacitor quickly reaches the rate of increase of the
applied voltage. As a result, the output voltage Ec (Figure 35C)
closely follows the rise of the pulse. During this time, the current
and ER level off at a low value.
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