[sdiy] Discrete Op Amps

[Gordonjcp]

On Mon, Apr 10, 2017 at 01:08:36AM +0000, cheater00 cheater00 wrote:
> Tom would you mind posting what you think are a few (guessing ~10?) main
> types of differently behaving op amps to check out?
> 

You just need to use the TL07x family, unless you've got ridiculously specialised needs.  When you open up a synthesizer (a proper one, not one of those silly "boutique" modules) have a look at what opamps it uses.

I bet you'll find it's full of TL07x opamps, or something at a similar price point.

> Everyone: do you think the assertion is true that a discrete op amp can be
> made cheaply with SMD? Do you think it's a good idea to use that as a way
> of tuning an op amp to your circuit? Do you have characteristics you desire
> most? For me, one application could probably be for late gain stages: high
> headroom and running off relatively large voltage rails, high current
> capability, soft clipping, and low bandwidth not far beyond 20 kHz in the
> gain i use it in. Some circuits might better like 2nd and 4th harmonic
> clipping, others odd harmonic. Has anyone here tried their hand at building
> a simple discrete op amp?

I've built the circuit that I gave you a link to several posts ago.  It works, but not particularly well.  That is the simplest possible opamp design with the lowest possible parts count.  It's got a really low input impedance, a relatively high output impedance, it's unstable, it draws a lot of current and with carefully-selected components you might be able to squeeze 10dB gain at 20kHz out of it.

You couldn't make a surface-mount discrete opamp any cheaper than any normal IC opamp because the PCB alone would cost more than the IC.

If you actually wanted a usable opamp, you'd need a hell of a lot more complicated design - look at the LM741 schematic, there are 19 transistors, four diodes and a whole bunch of resistors to make an opamp that most consider slow, noisy and power-hungry.

I'm not sure why you'd be "tuning an op amp to your circuit".  You select an opamp based on the criteria you require and then you design your circuit around that.  In most cases the biggest driving factor is cost, and then you pick a JFET opamp that's available in the package and quantities you require that's most suitable for audio work, and order a whole bunch of TL074s because they're just about perfect for the job.

If you want soft clipping, design a soft clipper stage.  Don't rely on edge characteristics of your opamp to give the clipping performance you desire.

-- 
Gordonjcp MM0YEQ