Once I was googling for discrete op amp ...
There is a lot of circuits on net. From standard API2520 to who knows what else. OK. I checked few of them with simulator and noticed that most of them will not work! Or could work conditionally. I will omit the details why, I'm sorry.
Recently I ended up the day with the idea to make my own discrete op amp. Nothing fancy, just stable amplifier with no pretencies. 70's style :)
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I did it. Results:
Looks good to me
Well, I started with SPICE of course ... but I used what I have as transistors !
I have SR1 Audio Analyzer in my work. Here are some measurements:
THD+N = -106dB
Amplifier G=11 22k/2.2k
THD+N = -96dB
Noise = -122dB
Not bad at all !
Board is milled with this ... another story.
:) fobi 05/03/2019
PS I intend to do dozen for my audio friends. Nothing commercial.