
Reverse-engineering precision op amps from a 1969 analog computer - matt_d
https://www.righto.com/2019/09/reverse-engineering-precision-op-amps.html
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MisterTea
Recently I became interested in DiY audio gear and I spent a lot of spare time
on Rod Elliott's ESP audio pages. One project is [https://sound-
au.com/project07.htm](https://sound-au.com/project07.htm), a discrete op amp
project. It's been awhile since uni days so I bought a bunch of pnp and npn
bjt transistors from digikey and spent a few days at my bench tinkering with
the circuit. different gains, miller caps to set stability and bandwidth, how
input and output impedance affect gains, etc. Very satisfying seeing your
little mess of transistors come to life and amplify a signal as designed.

One thing it taught me was how the different sub circuits come together to
make a power or operational amplifier. Long tail pair aka differential
amplifier, current mirrors, current sources, Vbe multiplier, class A stages,
Class AB stages, etc. Another great article which builds on it is
[https://sound-au.com/amp_design.htm](https://sound-au.com/amp_design.htm).
This is where you get to see how these building blocks come together. This
basic topology is present in most amps be they little op amp chips or big
beefy audio power amps: input -> diff amp -> class A gain -> class AB buffer
-> output.

Interesting to learn that there isn't much difference between a little TL071
and my Rod Elliott P3A.

~~~
bcaa7f3a8bbc
We'll never have the luxury of laser-trimmed resistor or matched transistor,
an integrated opamp usually outperforms discrete designs, at least for low-
power applications. But yeah, it's much fun to build an integrated component
from scratch and see how it works.

BTW, NwAvGuy, the audio guru who have vanished, has some interesting
criticisms on the dismissal of opamp in the audiophile community.
[https://nwavguy.blogspot.com/2011/08/op-amps-myths-
facts.htm...](https://nwavguy.blogspot.com/2011/08/op-amps-myths-facts.html)

~~~
Kirby64
For laser trimmed resistors, there's always hand sorting resistors. Even
today, that's how I've seen it done in commercial op-amp characterization.
Tests like CMRR (Common mode rejection ratio) require precisely matched input
resistances or the result is thrown off by mismatches.

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rapjr9
I played with a large EIA analog computer at my first job, modeling a few
simple equations and building an oscillator. They had bought it many years
before to model guidance equations for missiles. Maybe one would be useful
today for trying out guidance algorithms for quadcopters? In some cases an
analog circuit can do the same things as a digital circuit and use less power.
Insect sized flying machines might benefit from analog guidance circuits also.

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gautamcgoel
I wonder how easy and cheap it would be to build a comparable analog computer
using modern ICs. I also wonder if analog computers could make a comeback.
Training a neural net seems very much like an analog computation to me...

~~~
kens
If you want an accurate analog computer, you'd still be paying a pile of money
for precision components. (0.01% resistors cost about $10 on Mouser.) And even
with modern ICs, you're not getting a Moore's Law scaleup on your op amps.
Plus, the interconnect doesn't scale nicely.

That said, there are claims that analog computers will make a comeback:
[https://spectrum.ieee.org/computing/hardware/not-your-
father...](https://spectrum.ieee.org/computing/hardware/not-your-fathers-
analog-computer)

~~~
jacquesm
A good neural net simulator would work just fine with imperfect components.

~~~
nicoburns
This is basically what a brain is, right?

~~~
Retric
Far less than you might think. For example the wide range of neurochemicals
would be largely unnecessary if that was the case.

It’s like how a city contains many buildings, but it also contains roads etc.

~~~
anticensor
Different neurotransmitters provide different types (speed, response function)
of activation.

~~~
Retric
The same neuron may release different neurotransmitters based on stimulus, but
it’s not universal.

[https://www.ncbi.nlm.nih.gov/books/NBK10818/](https://www.ncbi.nlm.nih.gov/books/NBK10818/)

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amatic
Nice read! I keep wanting to learn more about the hardware to build something
myself. Only simulations so far. Analog computers are absolutely fascinating.
I feel like there is a lot of lost lore in that field.

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sitkack
This reminded me of the TI acquisition of Burr-Brown, analogous to Oracle
buying Sun. Rip.

Some early tilt rotor aircraft used analog computers for stability control.

~~~
MisterTea
Interesting comparison. What exactly did TI do to BB which warrants such a
ghastly comparison?

I know BB well thanks to tinkering with their isolation amplifiers for an old
data acquisition project. also familiar with them due to working on equipment
from the 70's/80's with the really old BB 3451 monoblock isolation amplifiers.

~~~
sitkack
TI is antagonistic to all but the largest customers. Trying to get access to
manuals, instruction sets, etc was a pain in the 90s. Devkits commonly cost
>$500, even for the smallest of parts.

There is a big memory hole, at least in the people I interact with about the
personalities around the various semiconductor companies. Nerdy (Maxim,XMOS)
dorky (Green Arrays) serious (ST) irreverent (Fairchild), sloppy and fun
(Microchip). Companies like TI are in it for the money, others have a passion
for engineering and solving civilization's problems.

