Differential Amplifier with two Op Amps:

The gain as well as input resistance of the differential amplifier can be increased by using two op-amps. The characteristics of such an amplifier are identical to those of a noninverting amplifier. A practical circuit using differential amplifier with two op amp is given in Fig. 34.46.

The circuit is composed of a two stage amplifier, the first stage being of a noninverting amplifier and the second being of a differential amplifier, both having gains of different values.

Equations of First Stage and Second Stage:

The output of first stage,

Output from second stage,

Since R₃ = R₁ and R_f = R₂, we have

Voltage gain,

Input Resistance: Input resistance of first stage,

where R_in is the open-loop resistance of op-amp, as first stage is noninverting amplifier

With v_in2 grounded i.e. v_in2 = 0, the second stage also becomes a noninverting amplifier whose input resistance may be given as

where R_in is open-loop resistance of op-amp

Drawbacks:

The output signal may be of smaller magnitude than expected. This is a drawback of such an amplifier.