Completed Curve Tracer Circuit

Long overdue, the more or less final functional circuit of the curve tracer. The phase of the AC inputs is important so that the base bias is stable while the device  under test receives power. I used separate transformers for each. R6/R7 is a potentiometer, as is R8/R9. Vvar is supplied by a variable linear regulator with low dropout.

I used TL712CP comparators for U2 and U1, and IIRC a LM833 dual opamp for U3 and U4.

Below is the circuit as thrown together.

It's not pretty, but it works reliably for any small BJT or FET. Outputs to the oscilloscope are posts on the right side - only the ground connection is clearly visible. The rocker switch at the top switches between PNP and NPN modes by converting U3 and U4 from inverting to noninverting unity amplifiers.