ROUT

-

+

VIN

RF

CF

RIN

RL

CL

RS

0

UNSTABLE

ROC = 40 dB/decade

STABLE

ROC ± 20 dB/decade

FREQUENCY (Hz)

15

LMV641

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SNOSAW3D – SEPTEMBER 2007 – REVISED AUGUST 2016

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7.4 Device Functional Modes

7.4.1 Stability of Op Amp Circuits

If the phase margin of the LMV641 is plotted with respect to the capacitive load (CL) at its output, and if CL is

increased beyond 100 pF then the phase margin reduces significantly. This is because the op amp is designed

to provide the maximum bandwidth possible for a low supply current. Stabilizing the LMV641 for higher

capacitive loads would have required either a drastic increase in supply current, or a large internal compensation

capacitance, which would have reduced the bandwidth. Hence, if this device is to be used for driving higher

capacitive loads, it will have to be externally compensated.

Figure 36. 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, then 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 36). 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.

7.4.1.1 In The Loop Compensation

Figure 37 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, RS, is used to isolate the amplifier output from the load capacitance, CL, and a small capacitance, CF,

is inserted across the feedback resistor to bypass CL at higher frequencies.

Figure 37. In the Loop Compensation