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LMV641

SNOSAW3C – SEPTEMBER 2007 – REVISED FEBRUARY 2013

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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 40). 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 41 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

Figure 41. In the Loop Compensation

by the presence of the zero, and that the ROC is maintained at 20 dB/ decade. For the circuit shown in Figure 41

2 k

(1)

Table 1.

Phase Margin (°)

0.5

680

10

17.4

1

680

20

12.4

1.5

680

30

10.1

The LMV641 is capable of driving heavy capacitive loads of up to 1 nF without oscillating, however it is

recommended to use compensation should the load exceed 1 nF. Using this methodology will reduce any

excessive ringing and help maintain the phase margin for stability. The values of the compensation network

tabulated above illustrate the phase margin degradation as a function of the capacitive load.

Although this methodology provides circuit stability for any load capacitance, it does so at the price of bandwidth.

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