Section V. HIGHPASS RL CIRCUIT
24.
GENERAL.
When EL is taken as the output voltage, the series RL circuit
is known as a highpass filter. A highpass filter attenuates all
frequencies below a certain determined value, while passing
frequencies above this value.
25.
SHORTTIME CONSTANT.
a. The pulse shown in Figure 37B is applied to the highpass RL
filter in Figure 37A. The time constant 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 to the pulse rise time, the output voltage across the
inductor (Figure 37C) will increase to only a fraction of the input
voltage.
b. When the voltage is first applied to the circuit, current
attempts to flow. During the first microsecond, the inductor
develops a back emf which is considered to be equal to the applied
voltage. As the applied voltage continues to increase, the rate of
current increase becomes proportional to the rate of increase of E.
As a result, the output voltage EL remains constant, while ER closely
follows the rise of the input pulse. At 10 usec, the output voltage
EL is still at this low value and ER is equal to E EL.
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