Simple question: Why do most (almost all) negative feedback amplifiers use Miller compensation rather than lead/lag? The latter has a much flatter Bode plot of the OLG which, I believe, should increase the slew rates.
What’s the disadvantage of a lead/lag (other than the need for larger caps)?
What’s the disadvantage of a lead/lag (other than the need for larger caps)?
Most amplifiers use a Miller capacitor around the VAS because it moves the dominant pole (formed by the VAS's collector-base capacitance) to a much lower frequency, without creating a new pole. The last thing one wants is two low-frequency poles.
Successful use of lag compensation requires moving the VAS's collector-base pole to a much higher frequency. This is best done through cascoding. I like this approach because it does not degrade the PSRR at high frequencies.
A lead capacitor can be used with either technique.
Ed
Successful use of lag compensation requires moving the VAS's collector-base pole to a much higher frequency. This is best done through cascoding. I like this approach because it does not degrade the PSRR at high frequencies.
A lead capacitor can be used with either technique.
Ed
To use lead/lag compensation, you have to know exactly where the existing poles are.
You see tube amps almost use lead/lag compensation exclusively, because the major pole is from the output transformer. The manufacturers definitely know the location of the pole of the transformer.
You see tube amps almost use lead/lag compensation exclusively, because the major pole is from the output transformer. The manufacturers definitely know the location of the pole of the transformer.
Lead comp in a VFA gains you some phase margin around the unity loop gain frequency. But, if you over do it you can make a decently Miller comp’d amp’s transient response worse, so you should check this out in a sim and then on the bench.
(Never ever use lead comp on a CFA! You will have a fine power oscillator)
(Never ever use lead comp on a CFA! You will have a fine power oscillator)
Another problem with a lead capacitor is that it can couple RFI on the amplifier's output back to its input.
Ed
Ed
