Page 90 - Troubleshooting Analog Circuits

7. Identifying and Avoiding

Transistor Problems

Although transistors-both bipolars and MOSFETs-are immune to many problems,
you can still have transistor troubles. Robust design methods and proper assumptions
regarding their performance characteristics will steer you past the shoals of transistor
vexation and the rocks of transistor disasters.
Transistors are wonderful-they ‘re so powerful and versatile. With a handful of
transistors, you can build almost any kind of high-performance circuit: a fast op amp,
a video buffer, or a unique logic circuit.
On the other hand, transistors are uniquely adept at causing trouble. For example, a
simple amplifier probably won’t survive if you short the input to the power supplies
or the output to ground. Fortunately, most op amps include forgiving features, so that
they can survive these conditions. When the pA741 and the LMlOl op amps were
designed, they included extra transistors to ensure that their inputs and outputs would
survive such abuse. But an individual transistor is vulnerable to damage by excessive
forward or reverse current at its input, and almost every transistor is capable of
melting. So it’s up to us, the engineers, to design transistor circuits so that the transis-
tors do not blow up, and we must troubleshoot these circuits when and if they do.
A simple and sometimes not-so-obvious problem is installing a transistor incor-
rectly. Because transistors have three terminals, the possibility of a wrong connection
is considerably greater than with a mere diode. Small-signal transistors are often
installed so close to a printed-circuit board that you can’t see if the leads are crossed
or shorted to a transistor’s can or to a PC trace. In fact, I recall some boards in which
the leads were often crossed and about every tenth transistor was the wrong gender-
pnp where an npn should have been, or vice versa. I’ve thought about it a lot, and I
can’t think of any circuits that work equally well whether you install a transistor of
the opposite sex. So, mind your Ps and Qs. your Ps and Ns, your 2N1302s and
2N1303s, and your 2N3904s and 2N3906s.
In addition to installing a transistor correctly, you must design with it correctly.
First of all, unless they are completely protected from the rest of the world, transis-
tors require input protection. Most transistors can withstand dozens of milliamperes
of forward base current but will die if you apply “only a few volts” of forward bias.
One of my pet peeves has to do with adding protective components. MIL-HDBK-217
has always said that a circuit’s reliability decreases when components are added. Yet
when you add resistors or transistors to protect an amplifier’s input or output, the
circuit’s reliability actually improves. It just goes to show that you can’t believe
everything you read in a military specification. For detailed criticism of the notion of
computing reliability per MIL-HDBK-217, see Ref. 1.
Similarly, if you pump current out ofthe base of a transistor, the base-emitter junc-
tion will break down or “zener.” This reverse current-even if it’s as low as nanoam-
peres or very brief in duration-tends to degrade the low-current beta of the tran-
sistor, at least on a temporary basis. So in cases where accuracy is important, find a
way to avoid reverse-biassing the inputs. Bob Widlar reminded me that the high-

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