A feedback amplifier is an amplifier in which a portion of the output signal is fed back to the input. This feedback can either enhance or oppose the input signal, thereby affecting the overall performance of the amplifier. Feedback amplifiers are widely used in electronics, communication systems, and control systems to improve gain stability, bandwidth, and linearity.
Table of Contents
Basic Concept of Feedback
In a feedback system:
- \(V_i\) = input signal
- \(V_o\) = output signal
- A fraction of output \((\beta V_o)\) is fed back
The input to the amplifier becomes:
\[
V_{in} = V_s \pm \beta V_o
\]
- \(\beta\) = feedback factor
- (+) → positive feedback
- (−) → negative feedback
Types of Feedback
Negative Feedback
- Feedback signal opposes the input
- Reduces overall gain but improves performance
\[
A_f = \frac{A}{1 + A\beta}
\]
- \(A\) = open-loop gain
- \(A_f\) = closed-loop gain
Positive Feedback
- Feedback signal adds to the input
- Increases gain and can cause oscillations
\[
A_f = \frac{A}{1 – A\beta}
\]
Used in oscillators.
Advantages of Negative Feedback
- Gain Stability: Gain becomes less sensitive to parameter variations
- Bandwidth Increase: Bandwidth increases as gain decreases
- Noise Reduction: Reduces noise and distortion
- Improved Linearity: Output becomes more faithful to input
- Reduced Distortion: Harmonic distortion is minimized
Disadvantages of Negative Feedback
- Reduces overall gain
- May lead to instability if not properly designed
Types of Feedback Amplifiers
Feedback amplifiers are classified based on:
- Type of signal sampled (voltage/current)
- Method of mixing (series/shunt)
Voltage Series Feedback (Series–Shunt)
- Output voltage is sampled
- Feedback added in series with input
- Increases input impedance
- Decreases output impedance
Voltage Shunt Feedback (Shunt–Shunt)
- Output voltage sampled
- Feedback applied in parallel
- Decreases input impedance
Current Series Feedback (Series–Series)
- Output current sampled
- Feedback in series
- Increases both input and output impedance
Current Shunt Feedback (Shunt–Series)
- Output current sampled
- Feedback in parallel
- Decreases input impedance
Effect of Feedback on Gain and Bandwidth
\[
A_f \times BW = \text{constant}
\]
Thus:
- When gain decreases, bandwidth increases
Stability of Feedback Amplifier
For a stable system:
\[
|A\beta| < 1
\]
If
\[
|A\beta| = 1
\]
the system may become unstable or oscillatory.
Applications
- Audio amplifiers
- Operational amplifiers (Op-Amps)
- Control systems
- Communication systems
- Signal processing circuits