Page 118 - Troubleshooting Analog Circuits

Rely Only on Guaranteed Specs I05

out to have a beta of 20 at a collector current of 50 PA, versus a beta of 100 for the
“good’ ones. I convinced the user that keeping a few hundred of these inexpensive
parts in a safe (yes, literally) would be a lot cheaper than getting the manufacturer to
sort out high-P devices.
Op amps and other linear ICs can also have errors due to thermal “tails.” These
tails occur when the change of heat in one output transistor causes a thermal gradient
to sweep across the chip. This change occurs gradually, often over milliseconds, and
causes uneven heating of input transistors or other sensitive circuits. Many high-
power circuits and precision circuits, such as the LM3 17, LM350, LM338, LM396,
LM333, and LM337, have been tested for many years for thermally caused error.
These tests aren’t performed only on power ICs, but also on precision references such
as LM368 and LM369, and on instrument-grade op amps. In fact, a recent article by a
Tektronix engineer (Ref. 9) points out that thermal tails can be a major source of
error in fast signal amplifiers and that innovative circuit design can minimize those
overdrive-recovery errors.
If you have ever studied the gain errors of older OP-07-type amplifiers, you have
probably recognized that these errors and nonlinearities were caused by thermal
errors-which were related to a bad layout. These days, most OP-07s have a better
layout, and the thermal distortions have been banished.
Another characteristic that is not specified or discussed is the change of offset
voltage vs. stress. This is most noticeable on BET amplifiers, as the FETs are much
more sensitive to stresses in the silicon die than bipolar transistors are. When you
install and solder a plastic DIP op amp in a PC board, and then warp the board, you
can monitor the V,, and watch it shift. Some amplifiers are better than others of
similar types. It has a lot to do with the layout and also with the die attach. If you
need the lowest offset, watch out for this. If the board is vibrated, the AC warping
and stress can cause microphonic AC noises, too. CER-DIP amplifiers have a
stronger ceramic base, and have a little less of a problem.
Here’s where it gets really wild: Buy your BIFET amplifiers in an SO (Small-
Outline, surface-mount) package. The smaller plastic package is able to take up even
less of the stress, and the die gets warped even more, and the change of VOs gets even
worse than in ordinary DIPS. So, when you think you can pack even more of a good
thing onto a board by going to surface-mount (Small-Outline) packages, you may
also pack in more trouble. There is no specification on this on any data sheet. So a
SPICE analysis has no way to warn you about this potential problem. Even a bread-
board does not necessarily tell you about this. The actual prototype units, on the real
PC boards, must be checked out.
These days, just about every manufacturer’s monolithic op amps will survive a
short from the output to ground. (Hybrids are often unprotected.) But it’s not always
clear whether an op amp will survive a short circuit to the positive or negative supply
or, if so, for how long. You may have to ask the manufacturer, and you can expect
some kind of negative answer. You’ll be told to avoid overheating the device above
its absolute maximum junction temperature. Even if an amplifier or regulator does
recover fairly quickly from current limit, nobody will guarantee that it won’t oscillate
when in current limit. Nor will the manufacturer have much knowledge about how
the circuit recovers from the thermal gradients caused by current limit. If an op amp
survives a high-power overload, it’s not fair to ask the device to recover its full accu-
racy very quickly. The most you really can ask for is that it survives with no degrada-
tion of reliability-that’s the standard.
Some op amps (such as LM12 and LM10) and most voltage regulators (and other
power ICs) have an on-chip temperature limiter. Thermal shutdown circuits can