Inverting Amplifier:

When V₁ = 0, V₂ = V_in, then circuit reduces to the one illustrated in Figure (a) with

V₀=- (R₂/ R₂)V_in

This circuit is known as an inverting amplifier. Even though the inverting input terminal is not connected to ground, thus far the infinite-gain and negative feedback force the voltage appearing on this terminal to be ground potential. Hence it is called a 'Virtual ground'. Note that circuit ideally has zero output impedance but input impedance is equal to R₁ due to the virtual ground at the inverting input terminal of the op-amp.

(a)                                                   (b)

Figure: (a) Inverting Amplifier; and (b) Non-inverting Amplifier

Case 2

If V₂ = 0, V₁ = V_in, then the circuit takes the form illustrated in Figure (b) with

                                    V₀  =  (1 +( R₂ /R₁)  ) V _in

This circuit is called a non-inverting amplifier. Note down that this circuit does have ideally infinite input impedance.