[quote author=loose-electron link=1307475244/0#14

AC coupled signal, Gate drive of source follower is on OTA output, with a current source off of it to keep it biased properly (goes triode when its down, but still works). Source feeds a cap (drive the bottom plate side) and the other side of the cap goes to your feedback point. Done. [/quote]

Jerry,

Would you mind posting a schematic? Still cannot see how you get rail-rail output with the arrangement you are describing.

By the way, the Ahuja compensation scheme was published in JSSC about 30 years ago (Dec. 1983), around the time when MOS circuits started to phase out BJTs in common telecomm ICs, i.e. when MOS started to go mainstream, and the problem of the RHP zero started to get more attention as BJT amps didn't suffer much from this.

There were countless variations published around this time and earlier which all attempted to get rid of the nasty RHP zero. The scheme using a MOS biased in triode region as a resistor to push out the zero is a little older and was published by Tsividis if I recall correctly. The approach using a source follower which you describe was also published around this time although I cannot recall by whom (The output swing limitation was a problem there as most people want to use two-stage opamps precisely to get high swing).

The difference between using an active stage (source foll./Ahuja) and a passive R is of course one of topology. You are burning a bit more current but you have an active stage. In principle, you can be about gm*ro times better in mitigating the problem loosely speaking.

Vivek

fonseca.ha wrote on Jun 16^th, 2011, 9:26am:

Is there a reason you can't put the metal caps over the active area?

I was just responding to your comment that metal caps even look less attractive. They can be pretty capacitive dense (especially if stacked) and can be put over devices. This makes them pretty attractive to me, but figured that maybe something was going on in the process you were talking about. I've never been a fan of gate caps for compensation... the non-linearity is just one more thing to complicate the design.

loose-electron wrote on Jun 16^th, 2011, 4:35pm:


Hi Loose-electron,
Thank you for sharing the schematic. In this circuit the consumption of the output stage (Q8, Q9) seems to be supply dependent, I say this because the gate of Q9 is driven by the source of Q6 which shares its gate with Q8. Therefore VgsQ9+VgsQ6+VgsQ8=VDD. So if supply changes the Vgs change as well affecting the bias of the output stage.  It may impact psrr.
Humberto

fonseca.ha wrote on Jun 17^th, 2011, 12:44am:


No kidding - as I said before, I use this material in a class to illustrate a concept. The concept being illustrated here is the use of a source follower to eliminate a feedforward path and thus eliminate the resistor in the compensation circuit.

Jerry,

The scheme is described in the JSSC paper I already mentioned in my previous posts (JSSC, Dec. 1983, An Improved Frequency Compensation Technique for CMOS Operational Amplifiers). There is no diode connected MOSFET there. Many variants (with some improvements) also exist, but the concept can be seen here.

Vivek

Ah, its a folding method that injects a current differential to drive gain down. Similar to a folded cascode. Interesting.

Thing is, if you are doing op-amps, I would hope that the system is fully differential with common mode feedback. So the ground referenced op-amps shown here are essentially good for illustration but need to be converted appropriatly.

Jerry,

Yes! it is kind of a folding. Of course, the figure here is only an illustration. I think the original paper only had a single-ended amp though (it was 1983).
But a fully-differential version is preferable also because you will avoid systematic offset (The level shifter branch is sharing current with the input branch).

Rob,

Indeed, the most popular variant of this scheme is to return the feedback to the cascode. However, as you point out, that causes peaking. My experience was that this peaking makes life quite painful, so much so that you do not really reap the full benefits of using this scheme. It turns out to be better to just put the extra stage although you end up burning more power and adding an extra source of noise to your amp.

Vivek