Datasheet

OUT

UNSTABLE

ROC = 40 dB/decade

STABLE

ROC ± 20 dB/decade

FREQUENCY (Hz)

GAIN

LM6211

SNOSAH2C –FEBRUARY 2006–REVISED MARCH 2013

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STABILITY OF OP AMP CIRCUITS

Stability and Capacitive Loading

The LM6211 is designed to be unity gain stable for moderate capacitive loads, around 100 pF. That is, if

connected in a unity gain buffer configuration, the LM6211 will resist oscillation unless the capacitive load is

higher than about 100 pF. For higher capacitive loads, the phase margin of the op amp reduces significantly and

it tends to oscillate. This is because an op amp cannot be designed to be stable for high capacitive loads without

either sacrificing bandwidth or supplying higher current. Hence, for driving higher capacitive loads, the LM6211

needs to be externally compensated.

Figure 35. 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 at 20

dB/decade at the unity gain bandwidth of the op amp, 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 35). 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 a ROC of 20 dB/decade, which

ensures stability.

In the Loop Compensation

Figure 36 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

, is used to isolate the amplifier output from the load capacitance, C

, and a small capacitance, C

is inserted across the feedback resistor to bypass C

at higher frequencies.

Figure 36. In the Loop Compensation

Product Folder Links: LM6211