Page 129 - Troubleshooting Analog Circuits

I I6 9. Quashing Spurious Oscillations

any guaranteed values. Although these engineers have been getting away with crowd-
ing the limits for years, the latest batch of comparators gives them trouble. Some of
our best friends depend on us to have our parts meet those typical specs, and it’s always
painful for us to tell them that they really ought to depend only on guaranteed specs.
If you need three op amps and one comparator, can you use a single LM324? Well,
op amps are not necessarily bad as comparators, but they sure are slow, and the
LM324 is among the slowest. Not only is its slew-rate slow, but if you put in an over-
drive of just 5 mV more than VOS, the output will respond at only 0.01 V/ p-not
even as fast as its specified slew rate. An LF35 1 or one-fourth of an LF347 will re-
spond somewhat faster. So if you want to use an op amp as a comparator, you’d
better need merely a slow comparator. (Note, however, that one LM358 plus one
LM392 will give you effectively three-fourths of an LM324 plus one-fourth of an
LM339, and the space taken by the two 8-pin mini-DIPS would be only 4% more
than that taken by a single 14-pin DIP.)
But, even so, some people do use op amps as slow, precision comparators. Even
though op amps are generally not characterized as comparators, you can engineer
such a circuit successfully. For example, the LM709 minus its compensation capaci-
tors is a surprisingly competent, fairly quick comparator. But, please don’t overdrive
and damage the inputs.
Conversely, I am occasionally asked, “Can I put some damping capacitors on an
LM339 and use it as a unity-gain follower?” The general answer is NO! because the
LM339’s phase lags are too squirrelly to be controlled by any possible compensation
scheme. But I have used the slower LP339 and LP365 successfully this way, as a
slow inverter or slow follower.

Even Buffered Circuits Can Oscillate

Any circuit that adds current gain can oscillate+ven a buffer. Let’s agree that a
buffer is some kind of linear amplifier that has a lot of current gain. Some have a
voltage gain around 0.90 or 0.95. Others have gains as high as 10 or 20 because their
outputs must swing 50 or 100 V p-p-or more. Even emitter followers, which you’d
expect to be very docile because their voltage gain is less than 1, have a tendency to
“scream” or oscillate at high frequencies. So whether you buy a buffer or “roll your
own,” watch out for this problem.
Also, a buffer can have a high-frequency roll-off whose slope increases suddenly at
40 or 60 MHz and thus can contribute phase shift to your loop, back down at 6 or 10
MHz. You can beat this problem, but you have to plan. A buffer can also add a little
distortion, which the op amp cannot easily cancel out at moderate or high frequencies.
Since buffers don’t usually have a spec on this distortion, beware. Also, if you’re run-
ning the output’s quiescent bias current as Class AB, you must be sure that the DC
operating current is stable and not excessive. You must set it high enough so that you
don’t get distortion but not so high that power consumption becomes excessive.
One of my standard procedures for stabilizing a unity-gain follower stage is to put
feedback capacitance around just part of the loop (Figure 9.5). This circuit tolerates
capacitive loads, because the buffer decouples the load while the feedback capacitor
around the op amp provides local stability. Most unity-gain buffers, whether mono-
lithic, hybrid, or discrete, are unstable with inductive sources, so keep the input leads
short. A series resistor may help stability, as it does for the LM310, but it will slow
down the device’s response.
Many high-speed buffers have the chore of driving loads in the range of 50-150 Q.
Driving these loads can require a lot of current, which leads to overheating. Plan your
heat sinks carefully to keep the device from exceeding its rated maximum tempera-