Following on from the work described in a previous thread, I have managed to accomplish my goal of approaching the performance of decent IC op-amps with a simple discrete circuit that can be impemented small enough to replace an IC.
The following results were achieved with through-hole components on a breadboard, and so are probably a little optmistic given that the available SMT parts tend to be inferior, but it's promising nonetheless.
I determined that the major source of distortion was not the output stage as I had first thought, but the Vas, presumably due to the nonlinear input impedance. Cascoding this stage fixed it up nicely, reducing measured distortion from worse than an OPA604 to immeasurable, maybe even better than any op-amp I currently possess.
The attached schematic shows the circuit used. Compared to my first attempt, this one has degeneration resistors on the current mirror (they don't help distortion much, but they do help DC offset a lot - hopefully enough that I can do away with the adjustment pot.) and a cascoded Vas. I did try a MOSFET input stage too, since it's easier to buy dual ones compared to JFETs, but they are far too noisy so I'll have to put up with hand-matching single JFETs instead.
The following results were achieved with through-hole components on a breadboard, and so are probably a little optmistic given that the available SMT parts tend to be inferior, but it's promising nonetheless.
I determined that the major source of distortion was not the output stage as I had first thought, but the Vas, presumably due to the nonlinear input impedance. Cascoding this stage fixed it up nicely, reducing measured distortion from worse than an OPA604 to immeasurable, maybe even better than any op-amp I currently possess.
The attached schematic shows the circuit used. Compared to my first attempt, this one has degeneration resistors on the current mirror (they don't help distortion much, but they do help DC offset a lot - hopefully enough that I can do away with the adjustment pot.) and a cascoded Vas. I did try a MOSFET input stage too, since it's easier to buy dual ones compared to JFETs, but they are far too noisy so I'll have to put up with hand-matching single JFETs instead.