Importance/Need of negative feedback in amplifiers
An amplifier is a piece of technology or an electronic device used to amplify an input signal. Even the alterations or noise in the incoming signal are amplified throughout this amplification procedure.
Negative feedback is used in amplifiers because it significantly lowers the noise level in the output signal by feeding a portion of the output signal into the amplifier's input in a phase opposite to the input signal.
Advantages of Negative Feedback in Amplifier are as follow:
1. Increases the Circuit Stability - Variations in the temperature, frequency, or amplitude of the signal have an impact on an amplifier's output without negative feedback, which further alters the amplifier's gain and causes the output signal to be distorted. Therefore, negative feedback is used to stabilize the amplifier's gain.
2. Stabilizes Amplifier Gain - By decreasing the dependence of amplifier gain on different transistor characteristics or variations in supply voltage, negative feedback stabilizes the amplifier's gain.
Gvf = Gv / (1 + β . Gv)
Here, Gvf stands for the amplifier's resultant gain in this instance. Gv is amplifier gain when there is no feedback, and β represents the feedback ratio or feedback fraction.
According to the above equation, the amplifier gain with negative feedback shows its major dependency on the feedback fraction or feedback ratio.
3. Reduces Non-Linear Distortion - The level of non-linear distortion in the big signal amplifiers is also decreased by the application of negative feedback.
Dvf = D / (1 + β . Gv)
Here, Dvf stands for the final distortion level with adverse feedback. D stands for distortion without feedback. Gv is amplifier gain when there is no feedback. And β represents the feedback ratio or feedback fraction.
The above equation demonstrates clearly that a negative feedback amplifier reduces distortion by a factor of (1 + β . Gv).
4. Increases Input Impedance / Resistance - When negative feedback is used, the amplifier's input resistance or impedance increases.
Z'in = Zin. (1 + β . Gv)
Here, Z' is the output impedance as a result of the negative feedback. Z represent the input impedance without feedback. Gv is amplifier gain when there is no feedback, and β represents the feedback ratio or feedback fraction.
According to the above mentioned relationship, with negative feedback, amplifier's input impedance increases by a factor of (1 + β . Gv).
5. Decreases Output Impedance / Resistance - Negative feedback reduces the amplifier's output impedance or resistance.
Z'o = Zo / (1 + β . Gv)
Here Z'o is the output impedance after negative feedback in this case. Zo is output impedance, not including feedback. Gv is amplifier gain when there is no feedback, and β represents the feedback ratio or feedback fraction.
According to the relationship shown above, a negative feedback amplifier's output impedance is reduced by a factor of (1 + β . Gv).
6. Reduces Noise Level - Because the negative feedback we apply to the amplifiers is in the opposite phase of the applied input signal, it cancels out the disturbances that the amplifier circuit introduces into the output signal. As a result, there is less noise in the output signal.
7. Improves Bandwidth and frequency response - The amplifier with negative feedback has the gain that is independent of signal frequency because the negative feedback is a resistive network we apply to the amplifiers. Thus, the frequency responsiveness of the amplifier with negative feedback is enhanced as the gain becomes constant throughout a broad range of signal frequencies.
f'cf = fcf . (1 + β . Gv)
Here f'cf stands for the consequent cut-off frequency with adverse feedback. The fcf is the cut-off frequency without feedback, Gv is the gain of the amplifier without feedback, and β is the feedback percentage or feedback ratio.
The relationship mentioned above makes it abundantly evident that the amplifier with negative feedback results in a higher cut-off frequency, which is enhanced by a factor of (1 + β . Gv). As a result, in the case of an amplifier with negative feedback, we get a wider bandwidth that is more usable.
8. Higher Linear Operations - The output signal does not depend on the applied input signal when using a normal amplifier since we obtain a very high output signal value even for a small input signal value. However, the applied feedback network controls every parameter in the case of an amplifier with negative feedback. As a result, the relationship between the output and the input signal is more linear for an amplifier with negative feedback.
As we have already covered a number of benefits of employing negative feedback in amplifiers, it is crucial also to be aware of its principal drawback, which is a reduction in the overall gain of the amplifier by the factor of
(1 + β . Gv).