The following are the advantages of negative voltage feedback in amplifiers :

**Advantages of Negative Voltage Feedback**

The following are the advantages of negative
voltage feedback in amplifiers :

**(i) Gain
stability. **An
important advantage of negative voltage feedback is that the resultant gain of
the** **amplifier can be made
independent of transistor parameters or the supply voltage variations.

For negative voltage feedback in an amplifier
to be effective, the designer deliberately makes the product Aν mν much greater
than unity. Therefore, in the above relation, 1 can be neglected as compared to Aν mν and the expression becomes :

It may be seen that the gain now depends only upon feedback fraction mν i.e., on the characteristics of feedback circuit. As feedback circuit is usually a voltage divider (a resistive network), therefore, it is unaffected by changes in temperature, variations in transistor parameters and frequency. Hence, the gain of the amplifier is extremely stable.

**(ii)
Reduces non-linear distortion. **A large signal stage has non-linear distortion because its voltage
gain** **changes at various points in
the cycle. The negative voltage feedback reduces the nonlinear distortion in
large signal amplifiers. It can be proved mathematically that :

It is clear that by applying negative voltage feedback to an amplifier, distortion is reduced by a factor 1 + Aν mν .

**(iii)
****Improves frequency response. **As feedback is usually obtained through a
resistive network,** **therefore,
voltage gain of the amplifier is *independent of signal frequency. The result
is that voltage gain of the amplifier will be substantially constant over a
wide range of signal frequency. The negative voltage feedback, therefore,
improves the frequency response of the amplifier.

**(iv) ****Increases
circuit stability. **The
output of an ordinary amplifier is easily changed due to variations in** **ambient temperature, frequency and
signal amplitude. This changes the gain of the amplifier, resulting in
distortion. However, by applying negative voltage feedback, voltage gain of the
amplifier is stabilised or** **accurately
fixed in value.

This can be easily explained. Suppose the
output of a negative voltage feedback amplifier has increased because of
temperature change or due to some other reason. This means more negative
feedback since feedback is being given from the output. This tends to oppose
the increase in amplification and maintains it stable. The same is true should
the output voltage decrease. Consequently, the circuit stability is
considerably increased.

**(v)
Increases input impedance and decreases output impedance. **The negative voltage feedback** **increases the input impedance and
decreases the output impedance of amplifier. Such a change is profitable in
practice as the amplifier can then serve the purpose of impedance matching.

**(a)
Input impedance. **The
increase in input impedance with negative voltage feedback can be explained** **by referring to Fig. 13.5. Suppose the
input impedance of the amplifier is Z_{in} without feedback and Z ′_{in}
with negative feedback. Let us further assume that input current is i_{1}.
Referring to Fig. 13.5, we have,

But eg/i_{1} = Z ′_{in} , the
input impedance of the amplifier with negative voltage feedback.

It is clear that by applying negative voltage
feedback, the input impedance of the amplifier is increased by a factor 1 + A_{ν}
m_{v}. As A_{ν} m_{v} is much greater than unity,
therefore, input impedance is increased considerably. This is an advantage,
since the amplifier will now present less of a load to its source circuit.

**(b)
Output impedance. **Following
similar line, we can show that output impedance with negative** **voltage feedback is given by :

It is clear that by applying negative feedback,
the output impedance of the amplifier is decreased by a factor 1 + A_{ν}
m_{ν}. This is an added benefit of using negative voltage feedback.
With lower value of output impedance, the amplifier is much better suited to
drive low impedance loads.

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Electronic Circuits - Feed Back Amplifiers : Advantages of Negative Voltage Feedback |

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