Sag and its Relation to Lower Cut-off Frequency:
The amplifier low frequency RC network consists of coupling and bypass capacitors make amplifier output to decrease with large time constant. As a result, the output voltage has sag or tilt associated with it as shown in the following figure.
The tilt or sag in time t1 is given by,
The lower 3 dB frequency can be determined from the output response by carefully measuring the tilt.
The lower 3 dB frequency is given as
So, the lower 3 dB frequency can be represented in terms of tilt is measured from the following figure
For a circuit shown in the following figure, calculate percentage tilt. Assume approximate h-parameter circuit for the transistor.
Equivalent h-parameter circuit for the given circuit is,
Here R1 = RC + RL = 4K + 2K = 6KΩ
We know that, P = (∏fL / f) * 100
Assuming f = 200 Hz
P = (∏ * 2.65 / 200) * 100
P = 4.1%