T-Network Gain Stages
3-9
EVM Applications
Figure 3-9. Simplified EVM Schematic for Inverting Gain
VS-
R3
R4
-
+
U1
THS4601
2
3
6
7
4
Z4
Z6
J3
Vout
VS+
J1
Vin-
The following assumes that Z4 and Z6 are resistors.
Gain for the T-network stage is defined by the expression:
Vout
V
in
+
–
ǒ
R4
)
Z4
R3
)
Z4
Z6
R4
R3
Ǔ
Looking at the expression above, the gain of the circuit is the standard inverting
gain (the first term), modified by the second term. The second term is a voltage
divider on the output voltage, multiplied by a gain term formed by R4 and R3.
When the T-network is used for noninverting gain, as shown in Figure 3-10,
the gain for the T-network stage is defined by the expression:
Vout
V
in
+
1
)
R4
)
Z4
R3
)
Z4
Z6
ǒ
1
)
R4
R3
Ǔ
The noninverting gain expression is similar to that of the inverting gain; except
that the first inverting gain term is replaced by a noninverting gain term. The
second term is similarly modified—it is a voltage divider on the output voltage,
multiplied by a noninverting gain term formed by R4 and R3.