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Differential Op Amps
6.5 Differential Op Amps
The differential amplifier circuit is shown in Figure 6–7. The dummy variable, V E , is inserted
to make the calculations easier, and a is the open loop gain.
V 1
V 2
V +
Z G
Z G
V –
V E
Z F
Z F
_
a
+
V OUT
Figure 6–7. Differential Amplifier Circuit
Equation 6–20 is the circuit transfer equation.
V OUT aV E V V
(6–20)
The positive input voltage, V + , is written in Equation 6–21 with the aid of superposition and
the voltage divider rule.
V V 2
Z F
Z F Z G
(6–21)
The negative input voltage, V – , is written in Equation 6–22 with the aid of superposition
and the voltage divider rule.
V V 1
Z F
Z F Z G
V OUT
Z G
Z F Z G
Combining Equations 6–20, 6–21, and 6–22 yields Equation 6–23.
V OUT a V 2 Z F
V 1 Z F
V OUT Z G
Z F Z G Z F Z G Z F Z G
(6–22)
(6–23)
After algebraic manipulation, Equation 6–23 reduces to Equation 6–24.
aZ F
V OUT
Z F
Z G
V 2 V 1
1 aZ G
Z F
Z G
(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.
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