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Design >> Analog Design >> Lead compensation VS cascode compensation
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Message started by chase.ng on Apr 29^th, 2006, 12:34pm

Title: Lead compensation VS cascode compensation
Post by chase.ng on Apr 29^th, 2006, 12:34pm

Hi all,

I would like to get some opinion from you all, which scheme is a better compensation scheme for CMOS 2-stage opamp? Lead compensation or cascode compensation?

Thanks,
Chase

Title: Re: Lead compensation VS cascode compensation
Post by vivkr on Apr 30^th, 2006, 5:45am

Hi,

I assume that you are referring to the Ahuja compensation (Ahuja, B., JSSC, Dec. 1983) when you say "cascode compensation", and the lead compensation involves using a MOS transisor in triode to realize a series compensation with an R and Cc (by Tsividis I believe).

From my experience, I can list the following pros and cons:

1. Ahuja compensation:

+ Breaks the feedforward path completely, gives excellent phase margin
+ Owing to above fact, compensation relatively independent of process etc.
+ Easier to design
+ Good PSRR at high frequencies
- 1 extra stage (level shifter)
- Systematic offset introduced, since the tail of the level shifter is tied to the output of first stage
- Slew rate of opamp affected by current flowing in the intermediate stage as well

2. Lead compensation (assuming a good MOS implementation as described in Johns & Martin):

+ Good phase margin, but less than what one would get from the former approach.
+ Fairly good control over process variation etc. when the gate bias of the compensating device generated properly to ratio with input Gm
+ No extra stage (less current consumed)
+ No systematic offset
+ Slew rate should not be affected
- PSRR worse, and similar to that of a standard Miller compensated stage without R
- 1 extra pole (Other scheme also introduces an extra pole, but typically at a much higher frequency)
- Slightly harder to design (Ahuja compensation can be made to work first time even by a novice)

Both schemes will add extra noise, and a good comparison on this point eludes me.

I prefer the lead compensation scheme in most cases, although I feel that the first approach is much more elegant and simple.

Regards
Vivek

Title: Re: Lead compensation VS cascode compensation
Post by raul on May 1^st, 2006, 11:17am

I think the Ahuja compensation is generally good. But it is necessary to run large signal transient steps to verify that there is no large signal oscillations. This is due to the way the compensation capacitor is connected. Because the compensation capacitor can introduce enough charge into the node to turn off the common gate device and then the loop is open.