Page 111 - Troubleshooting Analog Circuits

98 8. Operational Amplifiers-The Supreme Activators

Also beware of op amp models and what they might mistakenly tell you. For in-
stance, the “standard” equation for a single-pole op amp’s gain is A = A,,( 1/1 + jtiST).
This equation implies that when the DC gain, &, changes, the high-frequency gain,
A, changes likewise. Wrong! With the growing popularity of computer modelling, I
have to explain this to a would-be analyst every month. No, there is almost no corre-
lation between the high-frequency response and the spread of DC gain, on any op
amp you can buy these days. There are several ways to get an op amp’s DC gain to
change: Change the temperature, add on or lift off a load resistor, or swap in an am-
plifier with higher or lower DC gain. Although the DC gain can vary several octaves
in any one of these cases, the gain-bandwidth product stays about the same. If there
ever were any op amps whose responses did vary with the DC gain, they were aban-
doned many years ago as unacceptable.
So, your op-amp model is fine if it gives you a fixed, constant gain-bandwidth
product. But if the model’s gain at 1 MHz doubles every time you double the DC
voltage by reducing the load, you’re headed for trouble and confusion. I once read an
op-amp book that stated that when the DC gain changed, the first pole remained at
the same frequency. In other words, the author claimed that the gain-bandwidth
product increased with the DC gain. Wrong. I wrote to the author to object and to
correct, but I never heard back from him.
I often see op-amp spec sheets in which the open-loop output impedance is listed
as 50 Q. But by inspecting the gain specs at two different load-resistor values, you
can see that the DC gain falls by a factor of two when a load of 1 k0 is applied. Well,
if you have an op amp with an output impedance of 1 kQ, its gain will fall by a factor
of two when you apply a 1-kQ load. But if its output impedance were 50 Q, as the
spec sheet claimed, the gain would only fall 5%. So, whether it’s a computer model
or a real amplifier, be suspicious of output impedances that are claimed to be unreal-
istically low.

Watch Out for Real Trouble
What real trouble can an op amp get you into without much help from the compo-
nents? Well, you could have a part with a bad Vas. Or if the temperature is changing,
the thermocouples of the op amp’s Kovar leads may cause small voltage differences
between the op amp leads and the copper of the PC board. Such differences can
amount to 1/10 or 1/20 of a Celsius degree times 35 pV per degree, which equals 2 to
3 pV. A good way to avoid this problem is to put a little box over the amplifier to
keep breezes and sunshine off the part. That’s very helpful unless it’s a high-power
op amp. Then just repeat after me, “Heat is the enemy of precision,” because it is.
After all, when an IC has to handle and dissipate a lot of power, it’s not going to be
nearly as accurate as when it is not overheating, and when all the components around
it get heated, too.
You should remember, too, that not all op amps of any one type have the exact
same output-voltage swing or current drive or frequency response. I get phone calls
every four or five months from customers who complain, “We have a new batch of
your op amps, and they don’t have as good an output swing (or output current or
frequency response) as the previous batches.” When I check it out, 98% of the time I
find that a part with extremely good performance is just a random variation. The
customer had got into the habit of expecting all the parts to be better than average.
When they got some parts that were still much better than the guaranteed spec but
worse than average or “typical,” they found themselves in trouble. It’s always painful
to have to tell your friends that you love them when they like you and trust you, but