I think they do. However, the OP07 is probably good enough when the servo is taken from the output as shown in the OP.JR. wrote:Interesting to revisit this old thread... I've got at least one new question... Do they still make chopper stabilized opamps? IIRC they were slow as dirt, but very low offset voltage. Probably too noisy for a mic preamp.
If the servo input is taken from emitters, as in "Phantom Menace I," I don't think the LT1012 is even good enough.
IIRC, to minimize clicking at high gains, the Vos across the gain resistor had to be held <<30 uV. Below 10 uV I considered it to be good enough. I used the LT1012 in the DC preamp this way and it did work. But, using the new topo it could work better. If you add up all the LT1012 errors in the topology where the servo wraps from the emitters back to the input they quickly grow beyond 30 uV. And it's not just DC Vos errors from Vbe mis-match. The servo has to correct DC rectification from differential RF imbalance. That may just be a protoboard issue but I found it to be very real. I could "play" the servo like a Theramin.
I easily got <<30 uV at high gains with the "improved" topology. Also advantageous is the decrease in servo Rin. Fortunately, the servo makes it's greatest errors as the gain is stepped downward so the decrease in gain as the dynamic servo error increases tend to cancel. You'll note that the 1570/5171 uses this same topology. It's not new, one of our members here have used it in a commercial design. I just didn't know it when I came up with it. Thanks JP for letting us use it.