Analog Electronics – Page 38
Bandwidth Improvement Using Negative Feedback
One of the major advantages of negative feedback in amplifiers is the increase in bandwidth.
Although feedback reduces gain, it significantly increases the frequency range over which the amplifier operates.
Gain with Negative Feedback
The closed-loop gain of a feedback amplifier is given by:
Af = A / (1 + Aβ)
Where- A = Open loop gain
- β = Feedback factor
- Af = Closed loop gain
Bandwidth Relation
For an amplifier without feedback:
BW = fH − fL
When negative feedback is applied:
BWf = BW (1 + Aβ)
This means bandwidth increases by the same factor by which gain decreases.
Gain-Bandwidth Product
The gain-bandwidth product of an amplifier remains approximately constant.
A × BW = Constant
After feedback:
Af × BWf = A × BW
Example Problem
If an amplifier has:
- Open loop gain A = 100
- Bandwidth = 10 kHz
- Feedback factor β = 0.04
Find new bandwidth.
1 + Aβ = 1 + (100 × 0.04)
1 + Aβ = 5
BWf = 10 kHz × 5 = 50 kHz
Important GATE Points
- Negative feedback increases bandwidth
- Gain reduces but stability improves
- Gain × Bandwidth remains constant
- Used in most practical amplifier circuits
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