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

Differential Op Amps
6.5 Differential Op Amps
6-8
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.
V1
V2
ZG
ZG
Figure 6–7. Differential Amplifier Circuit
VE
V–
V+
ZF
_
a
+
Equation 6–20 is the circuit transfer equation.
V OUT aV E V V
ZF
VOUT
(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
Z F Z G
V 1 Z F
Z F Z G
V OUT Z G
Z F Z G
After algebraic manipulation, Equation 6–23 reduces to Equation 6–24.
VOUT
V2 V1 aZ F
Z F Z G
1 aZ G
Z F Z G
(6–22)
(6–23)
(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.