"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

The Differential Amplifier
3.5 The Differential Amplifier
3-6
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.
V1
V2
Figure 3–5. The Differential Amplifier
R1
R3
R2
V+
V–
+
_
R4
VOUT
The op amp input voltage resulting from the input source, V 1, 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, V OUT1.
V V 1
R 2
R 1 R 2
V OUT1 V (G ) V 1
R 2
R 1 R 2 R 3 R 4
R 3
(3–10)
(3–11)
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.
V (3–12)
OUT2 V2 R4 R3 V OUT V 1
R 2
R 1 R 2 R 3 R 4
R 3
V 2
R4 R3 When R 2 = R 4 and R 1 = R 3, Equation 3–13 reduces to Equation 3–14.
V OUT V 1 V 2 R 4
R 3
(3–13)
(3–14)
It is now obvious that the differential signal, (V 1–V 2), is multiplied by the stage gain, so
the name differential amplifier suits the circuit. Because it only amplifies the differential