Page 102 - Troubleshooting Analog Circuits

8. Operational Amplifiers-

The Supreme Activators

External components often determine an op amp’s performance, and that’s why we
spent the previous seven chapters discussing them. But op amps aren’t always abso-
lutely trouble-free: Oscillations and noise are two possible areas of difficulty, among
others.
After many pages of fiddling around with many different components, we finally
amve at the operational amplifier itself. And the good news is that half of our op amp
troubleshooting problems are already solved. Why? Because it’s the components
around op amps that cause many of their problems. After all, the op amp is popular
because external components define its gain and transfer characteristics.
So, if an amplifier’s gain is wrong, you quickly learn that you should check the
resistor tolerances, not the op amp. If you have an AC amplifier or filter or integrator
whose response is wrong, you check the capacitors, not the op amp. If you see an
oscillation, you check to see if there’s an oscillation on the power-supply bus or an
excessive amount of phase shift in the feedback circuit. If the step response looks
lousy, you check your scope or your probes or your signal generator because they’re
as likely to have gone flaky as the op amp is. These failures are the reason we studied
so many passive components: The overall performance of your circuit is often deter-
mined by those passive components. And yet, there are exceptions. There are still a
few ways an op amp itself can goof up.

Don’t Sweat the Small Stuff

Before we discuss serious problems, however, you should be aware of the kinds of
op-amp errors that aren’t significant. First of all, it generally isn’t reasonable to ex-
pect an op amp’s gain to be linear, nor is its nonlinearity all that significant. For ex-
ample, what if an op amp’s gain is 600,000 for positive signals but 900,OOO for nega-
tive signals? That sounds pretty bad. Yet, this mismatch of gain slope causes a
nonlinearity of about 10 pV in a 20-V p-p unity-gain inverter. Heck, the voltage
coefficients and temperature-coefficient errors of thefeedbuck resistors will cause a
lot more error than that. Even the best film resistors have a voltage coefficient of 0.1
ppm/V, which will cause more nonlinearity than this gain error.
Recently I heard a foolish fellow argue that an op amp with a high DC gain such as
2,000,000 or 5,000,000 has no useful advantage over an amplifier with a DC gain of
300,000 because, unless your signal frequency is lower than 0.1 Hz, you cannot take
advantage of the high gain. Obviously, I don’t agree with that-if you have a step
signal, the output settles to the precise correct value in less than a millisecond-nor 1
second or more. The amplifier with the higher gain just settles to a more precise
value-it does not take any more time. I guess he just doesn’t understand how op
amps work. Especially since he doesn’t even want to talk about gain nonlinearity!
Many amplifiers such as the old OP-07 had low DC gain and poor gain linearity,

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