The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> slew recovery in opamps
https://designers-guide.org/forum/YaBB.pl?num=1195567012

Message started by vivkr on Nov 20^th, 2007, 5:56am

Title: slew recovery in opamps
Post by vivkr on Nov 20^th, 2007, 5:56am

Hi,

I have often seen discussion about use of slew recovery devices, especially for folded cascode opamps to prevent the transistors from turning completely off (Razavi, Johns & Martin). However, most of the schemes suggested in the texts seem to offer little help when actually used.

Maybe my opamp is not really suffering very badly from the problem, and so I see very little change on adding these devices (they are not really called into action). Or on the other hand, maybe I have implemented the slew recovey devices wrongly.

Can anyone provide some insight into this topic? The standard methods I have seen are (for NMOS input opamp with PMOS folding nodes):

1. Clamp the folding nodes if they should fall too far below VDD.
2. Clamp the folding nodes if the difference between them exceeds a certain limit.

Thanks & Regards
Vivek

Title: Re: slew recovery in opamps
Post by loose-electron on Nov 21^st, 2007, 10:37am

take a look at the recovery time characterisics both with and without the clamps. With smaller technology, sometimes it is faster to not have the added capacitance of the clamp circuits there.

Title: Re: slew recovery in opamps
Post by vivkr on Nov 22^nd, 2007, 7:34am

Hi Jerry,

Thanks for your comment. However, the problem is not really solved this way. If I don't design the clamp correctly,
then I may see no improvement in performance or even worsening. So, this question cannot be answered unless I have
a good working clamp, and if I had that, I wouldn't be asking the question.

Is there a good design guideline or some particular fact to keep in mind when inserting the clamps? I just did some rough
calculations treating the clamp device as if it would be a diode. The body effect was something I took a look at as well.
However, I don't know if there is anything else to watch out for. I am not using very fine line technology, and the amp is fully diff.
so that I don't really expect a drastic cutoff on the devices.

Any comments?

Regards
Vivek

Title: Re: slew recovery in opamps
Post by chase.ng on Nov 22^nd, 2007, 8:50pm

Hi vivkr,

I had a folded cascode design uses the clamp you mentioned. However, I found that the clamp just does not do any good when operating at low voltage and with smaller feature size devices. The clamp device must be sized so that it can support the large current thru it without a large VGS drop (ideally it should stay same regardless of the current). Since the small feature size devices has smaller voltage headroom, I found it simply putting a diode connected FET as clamp to be ineffective if it is small, and it degrades the bandwidth when it is big. I build a more elaborate clamp and it does help the slew rate. IMHO the simple clamp method is more useful if you have a large supply (relative to VT). Hope that help.

Thanks,
chase

Title: Re: slew recovery in opamps
Post by vivkr on Nov 22^nd, 2007, 10:55pm

Hi chase,

The ratio of supply to threshold is about 3x-4x in my case. Could you post a copy of your elaborate clamp or any references to it?
I found the same effects with my clamp circuit. However, I did not build my clamp to have a small VGS drop, reasoning to myself that
the clamp should be OFF unless there is a large transient. Otherwise, it would have become too big.

Regards
Vivek

Title: Re: slew recovery in opamps
Post by chase.ng on Nov 29^th, 2007, 3:09am

Hi Vivek,

I really never use a clamp circuit in an actual design. What I did is design a precision clamp using an opamp and a diode (the one that normally found in text book) and simulate it to see how does it performs. I prefer to use class AB amplifier to boost the slewrate. One of my favourite paper would be "The flipped voltage follower: a useful cell for low-voltage low-power circuit design".

Thanks,
Chase.

Title: Re: slew recovery in opamps
Post by sheldon on Nov 30^th, 2007, 8:56pm

Vivek,

Do you need to use a clamp at all? If you bias the pull-up current sources
at 2.X times the tail current, assuming n-ch input pair with p-ch loads, then
cascodes should not turn off during transients. As long as the pull-up current
is greater than the drain current of the input pair, the cascodes stay biased.
There is a trade-off of static power for dynamic performance and you will
need to be careful when optimizing, you will need to account for the effect
of device mismatch.

Best Regards,

Sheldon

Title: Re: slew recovery in opamps
Post by vivkr on Dec 2^nd, 2007, 11:06pm

Hi Sheldon,

That answers my question perfectly. Thanks for that. I do always use 2x bias current in the top mirrors, and
actually assumed that this would be something standard in all fully diff. foded cascode designs. By this I mean that
the total current through the input pair equals the current through any of the pullup pairs.

As I was unable to determine if I needed a clamp, so I raised this question.

Thanks for clearing up that one.

Regards
Vivek