Negative Feedback in Amplifiers: Improving Stability and Performance

Learn how negative feedback enhances amplifier performance. This guide explains the principles of negative feedback, its effects on amplifier gain, stability, distortion, and noise, and its overall impact on producing a cleaner, more predictable output signal.

Negative Feedback in Amplifiers

What is an Amplifier?

An amplifier is an electronic device that increases the amplitude (strength) of an input signal. Unfortunately, amplifiers can also amplify noise and distortion present in the input signal.

Negative Feedback in Amplifiers

Negative feedback is a technique used to improve the performance of amplifiers. It involves feeding a portion of the output signal back to the input, but with a phase shift of 180° (opposite polarity). This feedback signal counteracts variations and noise in the input signal, resulting in a more stable and predictable output.

Advantages of Negative Feedback

Negative feedback offers many benefits:

1. Increased Stability

Negative feedback reduces the impact of variations in temperature, supply voltage, and transistor characteristics on the amplifier's gain, resulting in a more stable output signal.

2. Stabilized Gain

The gain of a negative feedback amplifier is less dependent on the amplifier's characteristics and is more predictable. The closed-loop gain (Gvf) is given by:

Gvf = Gv / (1 + βGv)

Where:

  • Gvf is the gain with feedback.
  • Gv is the gain without feedback.
  • β is the feedback fraction (the ratio of feedback signal to output signal).

3. Reduced Non-Linear Distortion

Negative feedback reduces distortion, especially in high-amplitude signals. The reduction factor is (1 + βGv):

Dvf = Dv / (1 + βGv)

Where:

  • Dvf is the distortion with feedback.
  • Dv is the distortion without feedback.

4. Increased Input Impedance

Negative feedback increases the amplifier's input impedance (resistance to current flow):

Z'in = Zin (1 + βGv)

5. Decreased Output Impedance

Negative feedback decreases the amplifier's output impedance:

Z'o = Zo / (1 + βGv)

6. Reduced Noise

Negative feedback helps cancel out noise introduced by the amplifier circuit itself.

7. Improved Bandwidth and Frequency Response

Negative feedback makes the amplifier's gain more constant across a wider range of frequencies, increasing bandwidth. The closed-loop cut-off frequency (f'cf) is given by:

f'cf = fcf (1 + βGv)

8. Increased Linearity

Negative feedback results in a more linear relationship between input and output signals. This improves the accuracy of signal amplification.

Drawback of Negative Feedback

While offering many advantages, negative feedback reduces the overall gain of the amplifier by the factor (1 + βGv).

Conclusion

Negative feedback is a crucial technique in amplifier design, significantly improving stability, reducing distortion and noise, and enhancing frequency response. While it reduces gain, the benefits often outweigh this drawback.