Datasheet

= 1 + = 1 + +
R
F
R //R
1 c
1
F
R
F
R
1
R
F
R
c
-
+
R
F
V
OUT
R
1
R
C
A
OL
G
min
= 20 dB
f
1
f
2
= 6 dB
= 1 +
R
F
R
1
1
F
1
F
|A
CL
|(min) = G
min
- 1
LMP7707, LMP7708, LMP7709
SNOSAW5B –JUNE 2007–REVISED MARCH 2013 www.ti.com
For an inverting configuration:
(8)
If R
1
and R
F
and are chosen so that the closed loop gain is lower than the minimum gain required for stability,
then 1/F intersects the open loop gain curve for a value that is lower than G
MIN
. For example, assume the G
MIN
is
equal to 10 V/V (20 dB). This is shown as the dashed line in Figure 53. The resistor choice of R
F
= R
1
= 2 kΩ
makes the inverse feedback equal 2 V/V (6 dB), shown in Figure 53 as the solid line. The intercept of G and 1/F
represents the frequency for which the loop gain is identical to 1 (0 dB). Consequently, the total phase shift at the
frequency of this intercept determines the phase margin and the overall system stability. In this system example
1/F crosses the open loop gain at a frequency which is larger than the frequency where G
MIN
occurs, therefore
this system has less than 45° phase margin and is most likely instable.
Figure 53. 1/F for R
F
= R
1
and Open Loop Gain Plot
RESISTIVE COMPENSATION
A straightforward way to achieve a stable amplifier configuration is to add a resistor R
C
between the inverting and
the non-inverting inputs as shown in Figure 54.
Figure 54. Op Amp with Compensation Resistor between Inputs
This additional resistor R
C
will not affect the closed loop gain of the amplifier but it will have positive impact on
the feedback network.
The inverse feedback function of this circuit is:
(9)
22 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LMP7707 LMP7708 LMP7709