10-10-2012, 04:18 PM
Op amp stability and input capacitance
Op amp stability.pdf (Size: 173.86 KB / Downloads: 34)
Introduction
Op amp instability is compensated out with the addition of
an external RC network to the circuit. There are thousands
of different op amps, but all of them fall into two categories:
uncompensated and internally compensated. Uncompensated
op amps always require external compensation
components to achieve stability; while internally compensated
op amps are stable, under limited conditions, with
no additional external components.
Internally compensated op amps can be made unstable
in several ways: by driving capacitive loads, by adding
capacitance to the inverting input lead, and by adding in
phase feedback with external components. Adding in phase
feedback is a popular method of making an oscillator that
is beyond the scope of this article. Input capacitance is
hard to avoid because the op amp leads have stray capacitance
and the printed circuit board contributes some stray
capacitance, so many internally compensated op amp
circuits require external compensation to restore stability.
Output capacitance comes in the form of some kind of
load—a cable, converter-input capacitance, or filter
capacitance—and reduces stability in buffer configurations.
Stability theory review
The theory for the op amp circuit shown in Figure 1 is
taken from Reference 1, Chapter 6. The loop gain, Aβ, is
critical because it solely determines stability; input circuits
and sources have no effect on stability because inputs are
grounded for the stability analysis. Equation 1 is the loopgain
equation for the resistive case where Z = R.Beware of Equation 1; its simplicity fools people
because they make the assumption that A = a, which is
not true for all cases. Stability can be determined easily
from a plot of the loop gain versus frequency. The critical
point is when the loop gain equals 0 dB (gain equals 1)
because a circuit must have a gain ≥1 to become unstable.
The phase margin, which is the difference between the
measured phase angle and 180º, is calculated at the 0-dB
point. A typical open-loop-gain curve for the TLV278x
family of op amps is used as a teaching example and is
shown in Figure 2.
Conclusion
The addition of input, output, or load capacitance to an op
amp circuit decreases stability, which leads to overshoot in
the time domain. The stability decrease is worse for a
buffer than for other amplifier configurations. Reducing
the resistance value that makes up the RC circuit reduces
the effect of the capacitance; this effect leads to the rule
of thumb that high frequencies and low resistance go
together. Input capacitance is easily compensated by
adding a feedback capacitor into the circuit. The value of
the feedback capacitor should be just large enough to
achieve the desired overshoot response, because larger
values cause a loss of high-frequency performance.