Q.Which type of negative feedback increases the input impedance and decreases the output impedance of an amplifier.

Voltage series negative feedback is the most important of the four negative-feedback connections and the one analysed in full detail. Here the feedback network samples the output voltage and returns a feedback voltage $V_f = \beta V_o$ in series with the input signal, so that the net amplifier input is $V_i = V_s - V_f$.

This particular combination — voltage sampling at the output and series mixing at the input — gives the pattern of effects that make it ideal for a voltage amplifier:

• the voltage gain decreases to $A_f = \dfrac{A}{1+A\beta}$ but becomes highly stable;
• the input impedance increases, $Z_{if} = Z_i(1+A\beta)$, so the amplifier does not load the preceding stage or source;
• the output impedance decreases, $Z_{of} = \dfrac{Z_o}{1+A\beta}$, so it can deliver signal to a load with little loss;
• the bandwidth widens by the factor $(1+A\beta)$. …