A transistor amplifier has voltage gain of 100 and Ri =

A transistor amplifier has voltage gain of 100 and R_i = 1 K. Negative voltage feedback with β = 1/5 is applied using unbypassed R_E. The R_i with feedback is:

Since the given amplifier uses voltage series feedback. Therefore after feedback input resistance increases.
R_if = R_i (1 + Aβ)

or R_if = 1 k		1 + 100 ×	1
			5

or R_if = 1 k (21)
or R_if = 21 k ohms.

1	k ohms
21