[sdiy] Discrete Op Amps

[Florian Teply]

Am Sat, 8 Apr 2017 05:45:35 -0500
schrieb Elain Klopke <functionofform at gmail.com>:

> Here's a question that's surprisingly not related to my Organ Donor
> module project.
> 
> Discrete op amps are kinda a big thing in recording electronics DIY,
> but do they have a place in SynthDIY or are they best left for people
> making preamps, compressors, EQs, and mixers?

Well, in the end it all comes down to circuit design. In principle,
and under certain circumstances, an opamp built from discrete parts
could potentiallly outperform even the best integrated opamps. There's
a big BUT however: In very few cases that's actually worth the effort.
With proper design, the tiny imperfections in operation like finite
Common-Mode rejection and such are on one hand negligible, and on the
other hand just the result of slight mismatches between individual
transistors.

So, one could potentially do better with discretes if transistors are
carefully selected. But that involves significant effort in transistor
selection, and essentially boils down to buying more parts than actually 
needed and testing them all for all parameters deemed important.
Testing (here this means full characterization of the device in
question) takes lots of time and equipment, and if one really wants to
outperform integrated chips, we're talking about several orders of
magnitude more parts to buy and test. At the same time, there are
comparably huge parasitics to take into account, thermal mismatch
between devices and all that crud.

On the other hand, most of that essentially comes for free with
integrated circuits: thermal coupling on chip is much better than on a
PCB, matching between side-by-side transistors is already pretty good,
and its pretty cost-efficient, even if a full custom design is to be
made. In the end, for those two or three applications in the world where
a discrete opamp could make a difference by delivering higher
performance, the people involved already know that. And they have the
ressources available to pull that off. And still they're likely to go
for an integrated circuit.

Best regards,
Florian