Op Amps for Everyone - The Repeater Builder's Technical ...

The Differential Amplifier
3.5 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.
R 1
R 3
V +
+
V 1
R 2
V –
_
V OUT
V 2
R 4
Figure 3–5. The Differential Amplifier
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 .
R
V V 2
(3–10)
1
R 1
R 2
R
V (3–11)
OUT1
V (G ) V 2
1
R 1
R 2
R 3 R 4
R 3
The inverting gain equation (Equation 3–5) is used to calculate the stage gain for V OUT2
in Equation 3–12. These inverting and noninverting gains are added in Equation 3–13.
V (3–12)
OUT2
V 2
R 4
R 3
V OUT
V 1
R 2
R 1
R 2
R 3 R 4
R 3
V 2
R 4
R 3
(3–13)
When R 2 = R 4 and R 1 = R 3 , Equation 3–13 reduces to Equation 3–14.
V OUT
V1 V 2
R 4
R 3
(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
3-6