ROUT

-

+

VIN

RF

CF

RIN

RL

CL

RS

0

UNSTABLE

ROC = 40 dB/decade

STABLE

ROC ± 20 dB/decade

FREQUENCY (Hz)

14

LMV651, LMV652, LMV654

SNOSAI7K – SEPTEMBER 2005 – REVISED MAY 2016

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7.4 Device Functional Modes

7.4.1 Stability and Capacitive Loading

If the phase margin of the LMV65x is plotted with respect to the capacitive load (CL) at its output, it is seen that

the phase margin reduces significantly if CL is increased beyond 100 pF. This is because the operational

amplifier is designed to provide the maximum bandwidth possible for a low supply current. Stabilizing it 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 of the operational amplifier. Hence, if these devices are to

be used for driving higher capacitive loads, they would have to be externally compensated.

Figure 34. Gain vs Frequency for an Operational Amplifiers

An operational amplifier, 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 unity-gain bandwidth of the operational amplifiers is stable. If, however, a large capacitance is

added to the output of the operational amplifier, it combines with the output impedance of the operational

amplifier to create another pole in its frequency response before its unity-gain frequency (see Figure 34). 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.

7.4.2 In The Loop Compensation

Figure 35 illustrates a compensation technique, known as in-the-loop compensation, that employs an RC

feedback circuit within the feedback loop to stabilize a noninverting 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 35. In-the-Loop Compensation