Page 91 - Troubleshooting Analog Circuits

78 7. Identifying and Avoiding Transistor Problems

current beta of a transistor is generally not degraded by this zenering, so if you are
hammering the VEB of a transistor in a switch-mode regulator, that will not neces-
sarily do it any harm, nor degrade its high-current beta.
Transistors are also susceptible to ESD-electrostatic discharge. If you walk
across a rug on a dry day, charge yourself up to a few thousand volts, and then touch
your finger to an npn’s base, it will probably survive because a forward-biassed junc-
tion can survive a pulse of a few amperes for a small part of a microsecond. But, if
you pull up the emitter of a grounded-base NPN stage, or the base of a PNP, you risk
reverse-biassing the base-emitter junction. This reverse bias can cause significant
damage to the base-emitter junction and might even destroy a small transistor.
When designing an IC, smart designers add clamp diodes, so that any pin can
survive a minimum of + and -2000 V of ESD. Many IC pins can typically survive
two or three times this amount. These ESD-survival design goals are based on the
“human-body” model, in which the impedance equals about 100 pF in series with
1500 a. With discrete transistors, whose junctions are considerably larger than the
small geometries found in ICs, ESD damage may not be as severe. But in some cases,
ESD damage can still happen. Delicate RF transistors such as 2N918s, 2N4275s, and
2N2369s sometimes blow up “when you just look at ‘em” because their junctions are
so small.
Other transistor-related problems arise when engineers make design assumptions.
Every beginner learns that the VBE of a transistor decreases by about 2 mV per de-
gree Celsius and increases by about 60 mV per decade of current. Don’t forget about
the side effects of these rules, or misapply them at extreme temperatures. Don’t make
sloppy assumptions about VBEs. For instance, it’s not fair to ask a pair of transistors
to have well-matched VBEs if they’re located more than 0.1 in. apart and there are
heat sources, power sources, cold drafts, or hot breezes in the neighborhood.
Matched pairs of transistors should be glued together for better results. Of course, for
best results, monolithic dual transistors like the LM394 give the best matching.
I’ve seen people get patents on circuits that don’t even work-based on miscon-
ceptions of the relationships between VBE and current. It’s fair to assume that two
matched transistors with the same VBE at the same small current will have about the
same temperature coefficient of VBE. But you wouldn’t want to make any rash as-
sumptions if the two transistors came from different manufacturers or from the same
manufacturer at different times. Similarly, transistors from different manufacturers
will have different characteristics when going into and coming out of saturation,
especially when you’re driving the transistors at high speeds. In my experience, a
components engineer is a very valuable person to have around and can save you a lot
of grief by preventing unqualified components from confusing the performance of
your circuits.
Another assumption engineers make has to do with a transistor’s failure mode. In
many cases. people say that a transistor, like a diode, fails as a short circuit or in a
low-impedance mode. But unlike a diode, the transistor is normally connected to its
leads with relatively small lead-bond wires; so if there’s a lot of energy in the power
supply, the short circuit will cause large currents to flow, vaporizing the lead bonds.
As the lead bonds fail, the transistor will ultimately fail as an open circuit.

More Beta-More Better?
It’s nice to design with high-beta transistors, and, “if some is good, more’s better.”
But, as with most things in life, too much can be disastrous. The h-parameter, hrb, is
equal to AVBE J AVc~ with the base grounded. Many engineers have learned that as