are you trying to do the same things as shown here in fig13?
https://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdf
for the feedback amp, I guess it's a style choice between two op-amps and a bunch of transistors
Hi,
I have built and used the Jim Williams design that was published in the AN159 application note.
Here is a picture of my implementation:
One of the five channels got damaged, so it is running on ' 4 cylinders' instead of five.
I included a modification to add a current sink so that it can sink and source.
The original JW design could only source.
The output voltage of the JW design is limited to about 12V. The supply rail to the LT1220 op-amp, the VBEs of the output transistors and the ability of the LT1220 to work nears the positive rail.
The JW design uses 2N3866 which are fairly special. I would not trust some of the parts on eBay.
In the JW design the op-amp is in the signal path. It is essential fast LDO with the ability to modulate the output.
The design that I presented here, the op-amp, made out of BJTs, is not in the signal path. The op-amp is used only hold the
average voltage across the output device to 1V.
There is a commercial design made Picotest, that is just a MOSFET, a capacitor and resistor.
The design presented here is an improved version of that.
I will compare the two.
Regards,
Jay_Diddy_B