Two long-tailed pairs with FETs instead of BJTs because I learned how to make long-tailed pairs work with FETs first.
CMRR = common-mode rejection ratio, meaning the ratio with which it rejects noise which appears equally on both inputs. In this case, there is 1kHz common-mode noise at 2Vp-p, and 2kHz signal at 0.4Vp-p. I think the two long-tailed pairs do a good job!
Keeping the FETs in their linear region seems to be a bit tricky. I think it's enough to make sure the output voltage never drops below the input.
Change all 3 resistors of a FET long-tailed pair to change the amplification.
I didn't experiment with mis-matched transistors. I suppose it matters, so I guess I'd do this IRL with matched pairs.
