Question

The negative feedback in an amplifier

• A. increases noise and distortion
• B. reduces noise and distortion
• C. reduces noise and increases distortion
• D. increases noise and reduces distortion

Hint:

Negative feedback is used to trade gain for stability and fidelity. It stabilizes the gain, reduces distortion, reduces noise (improving the signal-to-noise ratio), increases the bandwidth, and changes the input/output impedances (depending on the topology).

Answer 1

The Correct Option is B

Step 1: Define negative feedback in an amplifier.
Negative feedback is a technique where a fraction of the output signal is sampled and fed back to the input, out of phase (180$^\circ$ phase shift) with the input signal. This creates an overall reduction in the net input signal to the amplifier.

Step 2: Analyze the effect of negative feedback on distortion.
Distortion refers to the change in the shape of the output waveform compared to the input waveform, often caused by non-linearity in the amplifier's active components. Negative feedback tends to linearize the amplifier's operation by reducing the gain for non-linear components, thus reducing all types of distortion (harmonic, intermodulation, and phase distortion).

Step 3: Analyze the effect of negative feedback on noise.
Noise is any unwanted signal generated within the amplifier stages themselves (e.g., thermal noise, shot noise). Negative feedback reduces the gain of the amplifier for all signals, including the noise signals generated after the initial input stage. The overall reduction in gain for both the desired signal and the unwanted noise results in a net reduction in noise and an improvement in the signal-to-noise ratio.

Step 4: Conclude the correct effect.
The primary advantage of using negative feedback is the significant improvement in amplifier stability and linearity, which is achieved by simultaneously reducing both noise and various forms of distortion.