Wayne
The December 1, 2008 Electronic Design has an interesting article by Pease:
http://electronicdesign.com/Articles/Pr ... leID=20110
He says two interesting things:
Question for JR: Would the above trick (figure 2) be a means to offset the FB capacitor size in a VCA current summed mixer where the I-V converter was an op amp? See: viewtopic.php?f=6&t=183&start=12However, I remembered a good circuit I cooked up 40 years ago in Fort Wayne, Ind. A customer was using a photoresistor to vary an amplifier’s gain, as in Figure 2. But when the Rf went up to 5M, the amplifier became much too slow, due to the 5-pF capacitance inherent in the photoresistor. What to do?
At first I was going to use an extra op amp to make a negative capacitor to cancel out the Cf. But then I figured out it might work well if I just connected an adjustable C' back to the positive input. We tried it and it worked fine! It cancelled out the Cf under all conditions and extended the BW by 10×.
(For our readers: Ignore the fact that Rfb is a photoresistor in Pease figure 2.) If the combined output C of 16 VCAs was 15 pF x 16 (240 pF) and the required Rfb was 20K, then Cfb would have to be >240 pF. That would put the corner frequency around 33 kHz. Could this trick be used to speed up the IV stage?
And then there's this "gem":
I remember once JR where you characterised slew rate limiting as "clipping in the time domain." That simple explanation stuck with me. I'm curious to see what kind of mail Bob gets where he sort of says the same thing.I’ve heard arguments that every audio amplifier should have a 7:1 or 10:1 safety factor between its actual slew rate and the biggest, fastest signal it will have to handle. I used this circuit to show that a 3:1 margin would probably not cause 0.01% distortion in that signal. I was all set to give the demo, when I discovered that nobody wanted to listen to the demo. They had made up their minds and didn’t want to listen!
Anyhow, a factor of 1.5:1 or 2:1 is probably not safe, but 3 should be plenty. Don’t waste a lot of money on ultra-fast amplifiers to get a safety factor of 7 or 10. Also, don’t worry too much about slew-rate symmetry. If an amplifier is fast enough in one direction, and faster in the other direction, that’s not a big deal. I mean, who the heck spends a lot of time LISTENING TO SQUARE WAVES?! (Don’t answer that question...) (If you wanted to be sure to get matched positive and negative slew rates, you could throw out P2 and use an op amp to invert the voltage at the wiper of P1 to put the same magnitude of voltage at the foot of R2.)