Differential Amplifier With Three OP Amp:

Differential Amplifier With Three OP Amp, depicted in Fig. 34.47 consists of two stages—one formed by op-amps A₁ and A₂ and second by op-amp A₃. Hence for determination of overall voltage gain of the circuit shown in Fig. 34.47, it becomes imperative to determine the voltage gain of each stage.

The first stage may be viewed as two differential amplifiers, as depicted in Fig. 34.48.

The output voltages of the two differential amplifiers shown in Fig. 34.48 are determined by using superposition theorem.

For circuit depicted in Fig. 34.48 (a)

and for circuit depicted in Fig. 34.48 (b)

The output voltage of the first stage may be given as

Substituting the values of v₃ and v₄ respectively from Eqs. (34.54) and (34.55) in above equation, we have

The gain of the second stage may be determined using Eq. (34.42) and is given by

The overall voltage gain is given by

Substituting the values of v₃₄/v₁₂ and v_out/v₃₄ respectively from Eqs. (34.56) and (34.57) in above equation, we have

Thus the obtainable gain can be changed by adjusting the potentiometer (R_P) but it should never be set at zero or infinity. In case R_P is made zero, the obtainable voltage gain A_d will become indeterminant, whereas in case R_P is made infinity (open-circuit), the first stage will act as a voltage follower.

Input Resistance: The input resistance of the differential amplifier with three Op amp circuit depicted in Fig. 34.47 is the same as the input resistance of the first stage (the resist­ance determined at input v₁ and v₂) looking into the circuit with other ter­minal grounded.

In Fig. 34.48 (a), for example, when v₄ is reduced to zero (i.e. when v₂ is grounded), the circuit is a noninverting amplifier and the input impedance is given as

Similarly, the input resistance determined at input v₂ will be the same as determined above [Eq. (34.59)].

If the differential amplifier circuit depicted in Fig. 34.47 is slightly modified by replacing the first stage by voltage followers, as depicted in Fig. 34.49, we have a circuit that has the main advantage of possessing highest input resistance of three amplifiers and this resistance is given by

where R_in is the input resistance of the op-amp and A is the open-voltage gain of the op-amp.

However, the voltage gain of the circuit given in Fig. 34.49 is the same as the voltage gain of the differential amplifier with one op-amp i.e.