Negative feedback is a fundamental concept in amplifier circuits that involves the process of taking a portion of the output signal and feeding it back to the input with an inverted phase. This technique is widely used to improve the performance, stability, and linearity of amplifier circuits.
In an amplifier, the goal is to amplify a weak input signal to a higher magnitude output signal. However, amplifiers can suffer from various issues such as distortion, nonlinear behavior, and instability. Negative feedback is employed to mitigate these problems by reducing the overall gain and imposing greater control over the amplifier's characteristics.
Here's how negative feedback works in amplifier circuits:
Basic Setup: A typical amplifier consists of an input stage that processes the weak input signal and an output stage that generates the amplified output signal. Negative feedback is usually applied to the output stage.
Feedback Path: A portion of the output signal is taken and passed through a feedback network, which typically includes resistors and possibly capacitors. This network modifies the amplitude and phase of the feedback signal. The feedback signal is then fed back to the input stage of the amplifier, but with an inverted phase (180-degree phase shift).
Summation at the Input: At the input stage, the original input signal and the inverted feedback signal are combined. Due to the inverted phase of the feedback signal, it opposes the original input signal.
Effect on Gain: The negative feedback effectively reduces the overall gain of the amplifier. The degree of reduction depends on the amount of feedback applied. Higher feedback results in lower gain.
Advantages:
Improved Linearity: Negative feedback helps reduce distortion and nonlinearities in the amplifier's output signal. This is because any nonlinearities introduced by the amplification process are counteracted by the opposite nonlinearities in the feedback signal.
Stability: Negative feedback stabilizes the amplifier's operation by making it less sensitive to variations in component parameters and temperature changes.
Greater Bandwidth: The bandwidth of the amplifier is often extended with negative feedback, as it can counteract the frequency-dependent gain limitations inherent in many amplifying components.
Disadvantages:
Reduced Gain: While the reduction in gain is often desirable to achieve better linearity and stability, excessive negative feedback can lead to reduced overall amplification.
Noise Amplification: Negative feedback can amplify noise present in the feedback path, which might affect the signal-to-noise ratio.
In summary, negative feedback in amplifier circuits involves feeding back a portion of the amplified output signal to the input stage in an inverted phase. This technique helps improve linearity, stability, and other performance characteristics of the amplifier by reducing gain, mitigating distortion, and enhancing overall control.