Datasheet

V
X
R
1
+
R
F
V
OUT
- V
X
=
R
C
V
X
- V
IN
(V
IN
± V
X
) G = V
OUT
-
+
R
F
V
OUT
R
1
R
C
V
X
V
IN
A
OL
G
min
= 20 dB
f
1
f
2
1 + = 6 dB
R
F
R
F
LMP7707, LMP7708, LMP7709
www.ti.com SNOSAW5B –JUNE 2007–REVISED MARCH 2013
Proper selection of the value of R
C
results in the shifting of the 1/F function to G
MIN
or greater, thus fulfilling the
condition for circuit stability. The compensation technique of reducing the loop gain may be used to stabilize the
circuit for the values given in the previous example, that is G
MIN
= 20 dB and R
F
= R
1
= 2 kΩ. A resistor value of
250 Ω applied between the amplifier inputs shifts the 1/F curve to the value G
MIN
(20 dB) as shown by the
dashed line in Figure 55. This results in overall stability for the circuit. This figure shows a combination of the
open and closed loop gain and the inverse feedback function.
This example, represented by Figure 52 and Figure 53, is generic in the sense that the G
MIN
as specified did not
distinguish between inverting and non-inverting configurations.
Figure 55. Compensation with Reduced Loop Gain
The technique of reducing loop gain to stabilize a decompensated op amp circuit will be illustrated using the non-
inverting input configuration shown in Figure 56.
Figure 56. Closed Loop Gain Analysis with R
C
The effect of the choice of resistor R
C
in Figure 56 on the closed loop gain can be analyzed in the following
manner:
Assume the voltage at the inverting input of the op amp is V
X
. Then,
where
• G is the open loop gain of the op amp (10)
(11)
Combining Equation 10, Equation 11, and Equation 9 produces the following equation for closed loop gain,
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