Question

Draw an inverting amplifier using an op-amp and derive the expression for its output voltage.

Answer 1

Apply KCL at A,

i_i = i_b + i_f

i_i = i_f (∵ i_B = 0)

\(\frac{V_i-V_A}{R_f}=\frac{V_A-V_o}{R_f}\)

\(\frac{V_i}{R_f}=\frac{-V_o}{R_f}\)  {as V_A = 0}

\(V_o=-(\frac{R_f}{R_i})V_i\)

Voltage gain,

A_v = \(\frac{V_o}{V_{in}}=-\frac{R_f}{R_i}\)

The output of op-amp inverting amplifier is 180° out of phase with respect to its input.