Effect of Negative Feedback on Input Impedance:
High input impedance is always desirable in an amplifier as it will not load the preceding stage or the input voltage source. Such a desirable characteristic can be had with the help of negative feedback. The Effect of Negative Feedback on Input Impedance of an amplifier is explained below.
Voltage-Series Feedback:
A more detailed series feedback connection is shown in Fig. 19.13.
The input impedance can be determined as follows:
The input impedance with series feedback is seen to have the value of input impedance without feedback multiplied by the factor (1 + βA) and applies to both voltage-series and current series configuration.
Voltage-Shunt Feedback:
A more detailed shunt feedback connection is depicted in Fig. 19.14.
In this case,
and input impedance,
So, the input impedance with voltage shunt feedback is reduced by the factor (1 + βA).
Current-Shunt Feedback:
From Fig. 19.10(d)
and input impedance,
So, the input impedance with current shunt feedback is reduced by the factor (1 + βA).