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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Comparator with positive feedback (internally) https://designers-guide.org/forum/YaBB.pl?num=1412811251 Message started by AMSA on Oct 8th, 2014, 4:34pm |
Title: Re: Comparator with positive feedback (internally) Post by loose-electron on Oct 8th, 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 9th, 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 9th, 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 aaron_do on Oct 10th, 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 10th, 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 13th, 2014, 8:14pm aaron_do wrote on Oct 10th, 2014, 11:55am:
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. |
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