Cut-off Frequencies and Bandwidth with Feedback

Cut-off Frequencies and Bandwidth with Feedback

AU : May-04, 05, 10, 13, Dec.-04

We know that, Af = A / 1 + β A

Using this equation we can write,

• Now we analyse the effect of negative feedback on lower cut-off and upper cut-off frequency of the amplifier.

Lower cut-off frequency

• The lower cut-off frequency with feedback is given as,

f_Lf = f_L / 1 + A_mid β  ... (9.7.4)

• From equation (9.7.4), we can say that lower cut-off frequency with feedback is less than lower cut-off frequency without feedback by factor (1 + Amid P). Therefore, by introducing negative feedback low frequency response of the amplifier is improved.

Upper cut-off frequency

• The upper cut-off frequency with feedback is given as

f_Hf = (1 + A_mid β ) f_H ... (9.7.5)

• From equation (9.7.5), we can say that upper cut-off frequency with feedback is greater than upper cut-off frequency without feedback by factor (1 + Amid P). Therefore, by introducing negative feedback high frequency response of the amplifier is improved.

Bandwidth

• The bandwidth of the amplifier is given as

BW = Upper cut-off frequency- Lower cut-off frequency

Bandwidth of the amplifier with feedback is given as

• It is very clear that (fHf - fif)> (fH - fL) and hence bandwidth of amplifier with feedback is greater than bandwidth of amplifier without feedback, as shown in Fig. 9.7.1.

Important Concept

Since bandwidth with negative feedback increases by factor (1 + Aβ) and gain decreases by same factor, the gain-bandwidth product of an amplifier does not altered, when negative feedback is introduced.

Review Questions

1. Compare the frequency response characteristics of an amplifier with and without negative feedback.

AU : ECE ; May-04, Marks 8

2. 'The gain bandwidth product of an amplifier is not altered, when negative feedback is introduced'. Justify the statement.

AU : ECE : Dec.-04, Marks 5

3. Discuss the effects of negative feedback on the frequency response of an amplifier.

AU : ECE : May-05, Marks 8

4. Explain how negative feedback acts on bandwidth of a circuit.

AU : ECE : May-10, 13, Marks 2