An amplifier should have high input impedance so that it will not load the preceding stage or input voltage source. High input impedance for an amplifier can be achieved with Voltage series negative feedback.

Let I_i and V_i be input current and input voltage to the basic amplifier. V_s is input to feedback amplifier, V_f is feedback voltage and V_o is the output voltage.

Input impedance of basic amplifier is

\(z_i=\frac{V_i}{I_i}\)   ....(1)

Input impedance of feedback amplifier is

\(z_v= \frac{V_s}{I_i}\)    ....(2)

Input voltage to basic amplifier with negative feedback is

V_i = V_s - V_f

= V_s - βV_o  {∴ β = \(\frac{V_f}{V_o}\), V_f = BV_o}

= V_s - β.AV_i {∴ A = \(\frac{V_o}{V_{in}}\), V_o = AV_in}

V_i(1 + β) = V_s  ....(3)

Put eqn (3) in eqn (2)

\(z_{if}\) = \(\frac{V_s}{I_i}\)

= \(\frac{V_i(1+A\beta)}{I_i}\)

= \((\frac{V_i}{I_i})(1+A\beta)\)

Z_if = Z_i (1 + Aβ) from eq …… (1)
As (1 + Aβ) > 1 for negative feedback, Z_if > Z_i
Hence with negative feedback, input impedance of an amplifier increases.