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The Reduction of Non-Linear Distortion in Feedback Amplifiers

Published Date
Author Cadence System Analysis

Key Takeaways

  • Negative feedback amplifiers have an important role in reducing amplitude distortion, frequency distortion, and phase distortion. 

  • With feedback, the overall gain of the amplifier becomes Af=A1+A𝛃

  • The distortion in the feedback amplifier gets reduced by a factor of 11+A𝛃.

Non-linear distortions

Non-linear distortions in amplifier waveforms

Non-linear distortions are common in electronic systems, especially in amplifiers. Non-linear distortions occur when an amplifier fails to reproduce the output waveform as the exact amplified replica of the input waveform. In such cases, the non-linearity of the amplifier results in distortions in the amplitude, frequency, and phase of the output waveform. However, there is a fair amount of reduction observed in non-linear distortions in feedback amplifiers. 

Negative feedback amplifiers have an important role in reducing amplitude distortion, frequency distortion, and phase distortion. The reduction of non-linear distortions can be considered the greatest advantage of feedback amplifiers. Let’s take a closer look at how this phenomenon works.

Non-Linear Distortion in Feedback Amplifiers

The non-linearity in transfer characteristics of an active device is the main reason for the occurrence of non-linear distortion in amplifiers. When the operation of an active device in an amplifier—for example, a transistor—goes beyond the linear region, then the output waveform gets distortions in amplitude, frequency, and phase. 

A “feedback amplifier” generally refers to a negative feedback amplifier, which is a widely used, closed-loop configuration in amplifiers. When a fraction of the output signal is fed back to the input side with its phase or amplitude value opposite to the input signal, then the closed-loop system is called a negative feedback system. 

In a negative feedback amplifier, if there are any variations in the output, the feedback signal influences the input so that the changes in the output waveform are counteracted. The introduction of negative feedback reduces the effective input to the amplifier, which brings the device back to the linear region of operation. With negative feedback, the level of non-linear distortions in the amplifier is reduced.

With feedback, the overall gain of an amplifier is reduced to: 

Af=A1+A𝛃(1)

Even though negative feedback reduces the overall gain of an amplifier, there are many advantages to negative feedback, including:

  1. Improved stability

  2. Reduced distortion 

  3. Increased signal-to-noise ratio

  4. Decreased output impedance 

  5. Increased input impedance 

  6. Increased bandwidth of the amplifier operation

The Impact of Negative Feedback on Distortion

The reduction in distortion is a desirable characteristic of negative feedback. The non-linearity of an active device in a basic amplifier distorts the sinusoidal input by flattening the peaks. The feedback voltage vf gets subtracted from the input voltage vi to make the net input equal to v’i to the amplifier. Since the peaks of the vf are flattened when vf is subtracted from vi, the peak of resulting output voltage v’o will become more peaked. The net input voltage v’i is pre-distorted so that it partially compensates for the flattening caused by the amplifier. When the peaked input v’i gets amplified, the output will typically be sinusoidal, because the amplifier always tries to flatten peaks. 

Consider an amplifier with gain A, which produces distortion D without feedback. This distortion appears in the output. With feedback, the gain becomes Af and the distortion becomes Df. The 𝛃Df of the distortion Df is fed to the input. When it gets amplified by the basic amplifier, the distortion becomes A𝛃Df. This gets added to the original distortion D to make net distortion Df. The distortion in the feedback amplifier gets reduced by a factor of 11+A𝛃.

Df=D-A𝛃Df(2) Df=D1+A𝛃(3)

Non-linear distortions in feedback amplifiers impact output waveform amplitude, frequency, and phase. No amplifiers are free from distortions, however, feedback amplifiers can mitigate distortion at the expense of reduced overall gain. By properly designing the feedback loop of an amplifier, a considerable reduction in distortions can be achieved. 

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