Datasheet
R
OUT
-
+
V
IN
R
F
C
F
R
IN
R
L
C
L
R
S
0
UNSTABLE
ROC = 40 dB/decade
STABLE
ROC ± 20 dB/decade
FREQUENCY (Hz)
GAIN
LMV551, LMV552
SNOSAQ5G –FEBRUARY 2007–REVISED FEBRUARY 2013
www.ti.com
Figure 30. Gain vs. Frequency for an Op Amp
An op amp, ideally, has a dominant pole close to DC, which causes its gain to decay at the rate of 20 dB/decade
with respect to frequency. If this rate of decay, also known as the rate of closure (ROC), remains the same until
the op amp’s unity gain bandwidth, the op amp is stable. If, however, a large capacitance is added to the output
of the op amp, it combines with the output impedance of the op amp to create another pole in its frequency
response before its unity gain frequency (Figure 30). This increases the ROC to 40 dB/ decade and causes
instability.
In such a case a number of techniques can be used to restore stability to the circuit. The idea behind all these
schemes is to modify the frequency response such that it can be restored to an ROC of 20 dB/decade, which
ensures stability.
In the Loop Compensation
Figure 31 illustrates a compensation technique, known as ‘in the loop’ compensation, that employs an RC
feedback circuit within the feedback loop to stabilize a non-inverting amplifier configuration. A small series
resistance, R
S
, is used to isolate the amplifier output from the load capacitance, C
L
, and a small capacitance, C
F
,
is inserted across the feedback resistor to bypass C
L
at higher frequencies.
Figure 31. In the Loop Compensation
The values for R
S
and C
F
are decided by ensuring that the zero attributed to C
F
lies at the same frequency as the
pole attributed to C
L
. This ensures that the effect of the second pole on the transfer function is compensated for
by the presence of the zero, and that the ROC is maintained at 20 dB/decade. For the circuit shown in Figure 31
the values of R
S
and C
F
are given by Equation 1. Values of R
S
and C
F
required for maintaining stability for
different values of C
L
, as well as the phase margins obtained, are shown in Table 1. R
F
, R
IN
, and R
L
are to be 10
kΩ, while R
OUT
is 340Ω.
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