The Differential Amplifier

The differential amplifier circuit amplifies the difference between signals applied to the inputs (Figure 3–5). Superposition is used to calculate the output voltage resulting from each input voltage, and then the two output voltages are added to arrive at the final output voltage.

Figure 3–5. The Differential Amplifier

The op amp input voltage resulting from the input source, V1, is calculated in Equations 3–10 and 3–11. The voltage divider rule is used to calculate the voltage, V+, and the noninverting gain equation (Equation 3–2) is used to calculate the noninverting output voltage, VOUT1.

The inverting gain equation (Equation 3–5) is used to calculate the stage gain for VOUT2 in Equation 3–12. These inverting and noninverting gains are added in Equation 3–13.

 

When R2 = R4 and R1 = R3, Equation 3–13 reduces to Equation 3–14.

It is now obvious that the differential signal, (V1–V2), is multiplied by the stage gain, so the name differential amplifier suits the circuit. Because it only amplifies the differential portion of the input signal, it rejects the common-mode portion of the input signal. A common-mode signal is illustrated in Figure 3–6. Because the differential amplifier strips off or rejects the common-mode signal, this circuit configuration is often employed to strip dc or injected common-mode noise off a signal.

Figure 3–6. Differential Amplifier With Common-Mode Input Signal

The disadvantage of this circuit is that the two input impedances cannot be matched when it functions as a differential amplifier, thus there are two and three op amp versions of this circuit specially designed for high performance applications requiring matched input impedances.