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Design >> Analog Design >> Comparator with positive feedback (internally)
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Message started by AMSA on Oct 8^th, 2014, 4:34pm

Title: Comparator with positive feedback (internally)
Post by AMSA on Oct 8^th, 2014, 4:34pm

Hi dear fellows.

I have not much experience designing comparators, so I need your help.

From the research I did, there are basically two types of comparators, those which use clock (latch type) and those who not, for example the basic 3-stage comparator (pre-amp, positive feedback, ouput drive) or the comparator with internal feedback (cross coupled bistable) and yet another variant known as Clamped CMOS voltage comparator (Allen's book).

Because I am not interested on using a clock, those last three are of my interest.

From what I read, I concluded that maybe the best one is the comparator with internal feedback (cross coupled bistable) whose circuit is this one:

Is fast, has less delay, relatively good gain;

Regarding this, I would like to hear from you, in your opinion which one is best.

The next question is: What consideration I must have in mind when designing the comparator? Should I use the lowest L in all transistors? The differential pair must have a large W? What current should I point? (I don't have restrictions) All the transistors must be in deep saturation or close to subthreshold? (100mV to 200mV(?)) The tech that I am using is UMC 130nm.

To finish, I tried to design that comparator with internal feedback (cross coupled bistable) based on some examples and the results were:

The circuit with dc operating points (both inputs with 1.65V):

The circuit operating as a comparator:

And the DC characteristic (don't know if it is a relevant detail):

The waveforms:

Last, the AC response, using the AC analysis on the Vin- pin, with a DC source around 1.65V:

The differential pair dimensions are 40um/0.3um.

The overdrive voltages I think are very high! ::)

I would like to hear from you and I appreciate any tip.

Thank you very much in advance.

Best regards.

Title: Re: Comparator with positive feedback (internally)
Post by loose-electron on Oct 8^th, 2014, 5:58pm

A cross coupled load pair changes the impedance of the amplifier load - Consequently, small signal and AC analysis are not a lot of use, except right at the crossover point.

Transient analysis is how you are going to get it done.

That said - the important questions are:

what are your tolerable offsets?
What type of speed do you need?
how small of a voltage difference do you need to respond to?
do you need hysterisis or not allowed?
power consumption?
common mode signal range?

what have I forgotten?

Title: Re: Comparator with positive feedback (internally)
Post by AMSA on Oct 9^th, 2014, 1:33am

HI Electron and thank you for your answer.

I was predicting that what you said, that the AC analysis is not of great help/use.

I must say in first place that there is nothing defined. It's all open. That said, regarding the questions that you did I don't know if I can answer them all.

The comparator will be operating @ 500MHz. It will compare a sawtooth waveform with an almost DC value, almost because that DC value will have a small ripple.

what are your tolerable offsets?
Don't have clue, but I imagine that must be as low as possible.

What type of speed do you need?
500MHz.

how small of a voltage difference do you need to respond to?
This must have with the resolution of the comparator, right? Well, the lower as possible would be good, taking into account that the DC value that I referred before is coming from the output of a buck converter. So, in the limit, the comparator must be able to response to the rapid load transient.

do you need hysteresis or not allowed?
Because the DC reference has a small ripple, I don't know at which point it is necessary hysteresis.

power consumption?
Well, power consumption. That another issue. I don't have restrictions, however, knowing that the converter will supply 100mA, I think that 1% of that would be the limit (1mA current budget; The power supply is 3.3V), but if I cannot met the specs, I can go higher on that value.

common mode signal range?
As I said, the comparator will be comparing a sawtooth, which will vary from ~0V up to 2V, with a DC voltage, around ~1.2V, so I would say that the input common mode signal range should be around the minimum possible (at least > 400mV or so) and 1.9V.

Thanks for the help Electron!

Kind regards.

Title: Re: Comparator with positive feedback (internally)
Post by loose-electron on Oct 9^th, 2014, 11:35am

Many of these things play off of each other and thus the list of questions.

How many gain stages? Ties to the minimum signal to respond to.

I would get a set of numbers on those items first. SOme of them will be estimates or educated guesses, but that is ok.

Also, this is a time domain problem so the 500MHz thing is not what you are interested in as much as how long it takes the device to respond.

Title: Re: Comparator with positive feedback (internally)
Post by AMSA on Oct 10^th, 2014, 1:31am

I see. However I didn't know what your comment can help me. Thanks anyway.

With the above mentioned comparator that I have done (in a first iteration), what is troubling me is the fact that he doesn't produce a rectangular waveform when he has to compare in a certain level (@ 1.9V):

I don't know if it might be related to the delay that he suffers.

Title: Re: Comparator with positive feedback (internally)
Post by aaron_do on Oct 10^th, 2014, 11:55am

What is your application? i.e. how are you going to use the comparator? As loose-electron has been saying, you need to know (at least roughly) how accurate your comparator needs to be and how quickly it needs to produce an answer.

For you last graph, it seems you may be operating the comparator too quickly for it to respond. If you slow down the input, you will see more precisely where it is triggering (and it will look more rectangular). Also, I think you will need a triangle waveform to see the hysteresis.

Aaron

Title: Re: Comparator with positive feedback (internally)
Post by AMSA on Oct 10^th, 2014, 2:26pm

The comparator will be used in the feedback loop of a buck converter.

The inputs are a sawtooth waveform and a "almost" dc voltage. Almost because it has a high frequency ripple.

That's the application.

I know that any kind of amplifier can be a comparator, but for this application I was thinking on that bi stable comparator because it has the cross couple which improves the gain (negative resistance).

Another option could be the Symmetrical OTA, because it can have a good slew rate.

Kind regards.

Title: Re: Comparator with positive feedback (internally)
Post by loose-electron on Oct 13^th, 2014, 8:14pm

aaron_do wrote on Oct 10^th, 2014, 11:55am:

For you last graph, it seems you may be operating the comparator too quickly for it to respond. If you slow down the input, you will see more precisely where it is triggering (and it will look more rectangular). Also, I think you will need a triangle waveform to see the hysteresis.
Aaron

Do a DC sweep to get the basic response characteristics.

If you are using this for the buck-boost comparator system in the feedback, need to ask the question also of how much input referred offset can you tolerate, which goes right back to the error of the converter circuit.