Page 89 - Analog Circuit Design Art, Science, and Personalities

The Story of the P2

or impress Bob. He had to tailor the response of this operational amplifier with
about 75 kcps of unity-gain bandwidth, and he had to roll it off at aboul 11 dB per
octave or else have no output swing past IO cps. As it was, he got the full 20 V p-p
out to 500 cps, and even that was a struggle to accomplish. So Bob used the induc-
tors and anything else he could think of that would help, in addition to various ca-
pacitive damping circuits. And he got it all to work. He got it to work quite well.
So, what’s the big deal? Here’s a pretty crude operational amplifier with a voltage
gain of 10,000, and an output of +1 mA at f10 V, with a vicious slew rate of 0.03
V/Fsec. Who would buy an amplifier like that? It turned oul that thousands and
thousands and rhousunds of people bought this amplifier. because the input bias
current at either input was just a few picoamperes. What the heck is a picmmpere?
Most electrical engineers in 1960 didn’t even know what a picofarad was, not to
mention a picoampere, but they figured out it was a heck of a small fraction of a
microampere-at A, a picoampere is only 1 millionth of a microampere-and
for many high-impedance instrumentation applications, the P2 was by far the only
amplifier you could buy that would do the job. And it had this low bias current, only
a few picoamperes, because all those germanium transistors were running at 5 Mcps,
and their 5 or 10 pA of DC base current had no effect on the precision of the input
current. The input current was low, thanks to a well-matched bridge of four V47
varicaps. These were sold by Pacific Semiconductor, Inc. (PSI) for use as varactors
in parametric amplifiers, up in the hundreds of “megacycles,” in low-noise com-
munications receivers, mixers, and front-end amplifiers-parametric amplifiers.
The “V47” designation meant that they had a nominal capacitance of 47 pF at 4 V
reverse bias, which is where most RF engineers would bias them. But Bob Malter
biased them right around 0 V DC, with a minuscule f60 mV of AC drive.
At this level of drive, each diode would leak only 20 or 40 PA. But Bob had a
gang of technicians working day and night to match up the forward conduction
characteristics and the reverse capacitance voltage coefficients. and he was able to
make sets of four varactors that would cancel out their offset drift versus tempera-
ture, and also their reverse leakage. Of course, there was plenty of experimenting
and hacking around, plenty of experiments that didn’t work, but eventually a lot of
things that worked okay. After all, when you buy 10,000 V47s, some of them have
to match pretty well.
So, here’s a little do-hickey, a little circuit made up of just about as much parts as
a cheap $12 transistor radio, but there was quite a lot of demand for this kind of
precision. How much demand? Would you believe $227 of demand? Yes! The P2
originally started out selling for $185, but when the supply/demand situation heated
up, it was obvious that at S185, the P2 was underpriced, so the price was pushed up
to $227 to ensure that the people who got them were people who really wunted and
needed them. So, the people who really wanted a P2 had to pay a price that was
more than X the price of a Volkswagen Beetle-that was back when $227 was a real
chunk of money!
Meanwhile, what other kinds of “transistorized” op amps could you buy? Well.
by 1963, for $70 to $100, you could buy a 6- or 8-transistor amplifier, with lbias in
the ball-park of 60,000 to 150,000 PA, and a common-mode range of 1 1 V. The P2
had a quiet stable input current guaranteed less than 100 pA (5 or 10 PA, typical),
and a common-mode range of f200 V. (After all, with transformer coupling, the
actual DC level at the balanced bridge could be at any DC level, so there was no
reason the common mode rejection ratio could not be infinite.)
Wow. A $227 gouge. (You couldn’t call it a “rip-off,” because the phrase hadn’t
been invented, but perhaps that is the only reason.) Obviously, this must be a very

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