Lecture - 10: Voltage Shunt Feedback

Input Resistance with Feedback:

To find the input resistance Miller equivalent of the feedback resistor Rf, is obtained, i.e. Rf is splitted into its two Miller components as shown in fig. 2. Therefore, input resistance with feedback Rif is then

Fig. 2

Output Resistance with Feedback:

The output resistance with feedback Rof is the resistance measured at the output terminal of the feedback amplifier. The output resistance can be obtained using Thevenin's equivalent circuit,shown in fig. 3.

iO = ia + ib

Since RO is very small as compared to Rf +(R1 || R2 )

Therefore,i.e. iO= ia

vO = RO iO + A vd.

vd= vi – v2 = 0 - B vO

Similarly, the bandwidth increases by (1+ AB) and total output offset voltage reduces by (1+AB).

Fig. 3

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