Differential Op Amps - Non Ideal

The differential amplifier circuit is shown in Figure 6–7. The dummy variable, VE, is inserted to make the calculations easier, and a is the open loop gain.

Figure 6–7. Differential Amplifier Circuit

Equation 6–20 is the circuit transfer equation.

The positive input voltage, V+, is written in Equation 6–21 with the aid of superposition and the voltage divider rule.

The negative input voltage, V–, is written in Equation 6–22 with the aid of superposition and the voltage divider rule.

Combining Equations 6–20, 6–21, and 6–22 yields Equation 6–23.

After algebraic manipulation, Equation 6–23 reduces to Equation 6–24.

The comparison method reveals that the loop gain as shown in Equation 6–25 is identical to that shown in Equations 6–13 and 6–19.

Again, the loop gain, which determines stability, is only a function of the closed loop, and independent of the inputs.