Page 121 - Troubleshooting Analog Circuits

9. Quashing Spurious

Oscillations

Oscillations are the ubiquitous bugaboos of analog-circuit design. Not only can you
encounter oscillating op amps, as described in Chapter 8, but also oscillating transis-
tors, switching regulators, optoisolators, comparators, and buffers. And, if you think
about it, latched-up circuits are just the opposite of oscillating ones, so I included
them here, too.
Recall the corollary of Murphy’s Law that states: “Oscillators won’t. Amplifiers
will.. . .”-oscillate, that is. The knack of spotting and quashing spurious oscillations
is, for some fortunate people, a well-developed art. But others have not learned this
art well.
I obviously cannot tell you how to solve every kind of oscillation problem. But,
I will give you some general principles and then notes on what can go wrong with
various components, including comparators and buffers. This information, along
with a few suggested procedures and recommended instruments, will get you off to
a good start.
Here are some of the types of oscillations that can pop up unexpectedly:
Oscillations at very high frequencies-hundreds of megahertz-because of a single
oscillating transistor.
Oscillations at dozens of megahertz arising from stray feedback around a comparator.
Oscillations at hundreds of kilohertz because of an improperly damped op-amp loop,
an unhappy linear voltage-regulator IC, or inadequately bypassed power supplies.
Moderate-frequency oscillations of a switching-regulator loop because of improper
loop damping.
Oscillations at “60 Hz” or at “120 Hz,” or similar line-related frequencies.
Low-frequency oscillations coming from physical delays in electromechanical or
thermal servo loops.
As these general descriptions indicate, the frequency of an oscillation is a good
clue to its source. An electric-motor loop can’t oscillate at 10 MHz, and a single
transistor can’t (normally) rattle at 100 Hz. So when an engineer complains of an
oscillation, the first question I have is, “Oh, at what frequency?” Even though the
frequency is often a good clue, engineers often fail to even notice what the frequency
was. This omission tends to make troubleshooting by phone a challenge.
At very high frequencies, 20-1000 MHz, the layout of a circuit greatly affects the
possibility of oscillation. One troubleshooting technique is to slide your finger
around the circuit and see if at any point an oscillation improves or worsens.
Remember, knowing how to make an oscillation stronger is not worthless knowl-
edge-that information can provide clues on how to make the oscillation disappear.
I remember being very impressed when a colleague showed me that some of the
earliest IC amplifiers had a tendency to self-oscillate at 98 MHz with certain levels of

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