Sag and its Relation to Lower Cut-off Frequency

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

Problem 1:

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 = R_C + R_L = 4K + 2K = 6KΩ

We know that, P = (∏f_L / f) * 100 

Assuming f = 200 Hz

P = (∏ * 2.65 / 200) * 100 

P = 4.1%