Author Topic: What's inside an op-amp  (Read 773 times)

reddesert

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What's inside an op-amp
« on: June 24, 2018, 02:26:17 PM »
Here is a really interesting microscope photograph and analysis of the insides of a TL084 op-amp: http://www.righto.com/2018/06/silicon-die-analysis-op-amp-with.html, link posted by marcelomd over at DIYSB.  The author breaks down the structure of the various transistors and why they are shaped that way.  The amount of engineering that goes into even a "simple" op-amp has always impressed me.

Of note re the recent discussion of the LM308 and its need for an external frequency compensation capacitor, you can see that the TL084's internal capacitors are the largest area on the chip, even though they are a very small value cap (because there's no high-dielectric material to work with).

cooder

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Re: What's inside an op-amp
« Reply #1 on: June 24, 2018, 02:53:02 PM »
Very cool article indeed! Thanks for sharing!
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pickdropper

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Re: What's inside an op-amp
« Reply #2 on: June 24, 2018, 03:29:23 PM »
Yeah, it's pretty cool stuff.

I've had to do a double check on raw die before it got sent to fab.  We did it with two different teams of two engineers.  It got tedious; we were even measuring to make sure the dimensions of the caps to make sure they were in the ballpark.  It was tedious work and it took me a bit to get my head around it.

These days, they have sophisticated programs to do it.  I'm not sure why we did it by hand, other than to double check.  It look a couple of days.
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somnif

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Re: What's inside an op-amp
« Reply #3 on: June 24, 2018, 03:46:55 PM »
For comparison sake, Ken has also done a reverse engineer on an LM108 as well. (BUT, it ended up being a rather unusual specimen):
http://www.righto.com/2016/12/inside-lm108-op-amp-superbeta.html

And, an even older op-amp, the LM741
http://www.righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.html

Interestingly the 741 DOES have an internal cap (its biggest claim to fame), but, it uses lower gain transistors so it requires higher currents to function. The 108 family used fancier transistors, but they were more fragile, hence the need for external compensation.

somnif

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Re: What's inside an op-amp
« Reply #4 on: June 24, 2018, 03:57:41 PM »
Also, for fun, there is a kit out there letting you build a discrete version of the LM741 with through-hole parts.




(I bought it for my friend one Christmas ages ago. We proceeded to wire it into my IC Big Muff, and it did, in fact, function.)

Matmosphere

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Re: What's inside an op-amp
« Reply #5 on: June 24, 2018, 07:17:30 PM »
Also, for fun, there is a kit out there letting you build a discrete version of the LM741 with through-hole parts.




(I bought it for my friend one Christmas ages ago. We proceeded to wire it into my IC Big Muff, and it did, in fact, function.)

Wish those kits were a little less expensive. Id get a few 555s and make an Atari Punk console.