The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Trans-impedance amplifier
https://designers-guide.org/forum/YaBB.pl?num=1361233397

Message started by raja.cedt on Feb 18^th, 2013, 4:23pm

Title: Trans-impedance amplifier
Post by raja.cedt on Feb 18^th, 2013, 4:23pm

hello,
can any one tell me how this amplifier works, don't know how it will work without -ve feedback?

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by Vladislav D on Feb 19^th, 2013, 12:26am

The first stage is a common gate , the second is common drain. The AC current converted to a voltage at the source of M2 and amplified by CG stage. CD stage performs many function (like lowering output impedance, current amplification, level shifting, whatever) except voltage amplification. If ro_M1 >> Rd, the input impedance without feedback 1/gm1. Applying feedback, the impedance decreases even more by amount of Loop Gain. This is desirable since, the input source is current.

Title: Re: Trans-impedance amplifier
Post by Lex on Feb 19^th, 2013, 2:19am

Vladislav nailed it pretty much, though it looks like two non inverting stages are connected. Is this thing actually stable? Maybe I'm missing something here...

Btw, common gate input stage is used to lower the impedance and can be quite useful when Cpd is not well defined.

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Feb 19^th, 2013, 3:05am

hello all,
i agree with that feedback stuff, basically it is not -ve feedback, did you observed that?

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by Praveen K on Feb 19^th, 2013, 3:54am

Hi Vladislav,

Can you explain me intuitively, how the impedance reduces by loop gain with feedback. probably if i sit and write small single models equations i may get it. But can you give me an intuitive explanation of it.

Since M1 and M2 are connected to Vb, M2 will be in triode region. What is the significance of M2 here. Can we replace it with a current bias??

thanks for your insight,
Praveen

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Feb 19^th, 2013, 4:46am

hello praveen,
Here Vb indicates bias, they are not equal. Imagine entire circuit is an open loop amp and rf is feedback resister, look at the impedance without Rf, it will be 1/gm. With feedback it will lower just because resister draws some current which is not the case in without res.

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by Larry_80 on Feb 20^th, 2013, 5:05pm

Is this supposed to be a RCG TIA?

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Feb 20^th, 2013, 10:15pm

RCG means?

Thanks,
raj.

Title: Re: Trans-impedance amplifier
Post by Larry_80 on Feb 21^st, 2013, 10:35am

Regulated Cascode. (Regulated common Gate)

Title: Re: Trans-impedance amplifier
Post by RobG on Feb 23^rd, 2013, 1:11pm

Lex wrote on Feb 19^th, 2013, 2:19am:

yeah, you need an inversion somewhere. Perhaps M3 was supposed to be a PMOS.

Title: Re: Trans-impedance amplifier
Post by RobG on Feb 23^rd, 2013, 1:25pm

Even if the polarity was correct Rd sets the bias current in M1 so the output voltage will be dependent on Vdd (a change in I(M1) will go through the feedback resistor since the input current doesn't change). The best use for this circuit is to send it to a competitor that you don't like.

Title: Re: Trans-impedance amplifier
Post by Vladislav D on Feb 23^rd, 2013, 3:47pm

RobG wrote on Feb 23^rd, 2013, 1:25pm:

I think it's quite difficult to say if this is a good or bad circuit without knowledge about the application where it is used. And the function it implements. Probably, it can be stable if loop gain < 1. Ya, of course this circuit can be only a textbook example =)

Title: Re: Trans-impedance amplifier
Post by RobG on Feb 23^rd, 2013, 3:51pm

Vladislav D wrote on Feb 23^rd, 2013, 3:47pm:

RobG wrote on Feb 23^rd, 2013, 1:25pm:

It would be good for sensing power supply variations ;).

I'd be interested in where it came from because it looks incorrect to me unless M3 is a PMOS.

Title: Re: Trans-impedance amplifier
Post by Vladislav D on Feb 24^th, 2013, 2:42am

RobG wrote on Feb 23^rd, 2013, 3:51pm:

I'd be interested in where it came from because it looks incorrect to me unless M3 is a PMOS.

I agree =)

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Feb 24^th, 2013, 3:22am

Dear all,
it is a trans-impedance amplifier. I found from prof Bertan Bakkaloglu student thesis (page 11, fig 6).

http://repository.asu.edu/attachments/56952/content/LaFevre_asu_0010N_10920.pdf

@Vladislav D:yes i agree that +ve fb circuits works with <1 gain, but i don't think it has PVT insensitive property like -ve feedback. Can you point out me any design with +ve feedback, if you know.

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by RobG on Feb 25^th, 2013, 10:30am

raja.cedt wrote on Feb 24^th, 2013, 3:22am:

That is a quality team on the thesis but that circuit polarity just seems wrong. I can't see it doing anything except seeking one rail or the other, but I've been wrong before. On the other hand, writing is very hard work and it is easy to overlook mistakes like this.

Have you tried it?

Title: Re: Trans-impedance amplifier
Post by Lex on Feb 26^th, 2013, 1:46am

RobG wrote on Feb 23^rd, 2013, 1:25pm:

... The best use for this circuit is to send it to a competitor that you don't like.

Brilliantly commented. =D

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Feb 26^th, 2013, 7:44am

hello,
@robg: i forgo to ask, can you please explain how it will be useful for sensing power supply variations?

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by RobG on Feb 26^th, 2013, 7:52am

raja.cedt wrote on Feb 26^th, 2013, 7:44am:

hello,
@robg: i forgo to ask, can you please explain how it will be useful for sensing power supply variations?

Thanks,
Raj.

Oh, I was joking, which maybe I shouldn't do since I could be taken seriously. As far as I can tell the positive feedback will force the output to the positive rail or the negative rail so the output will Vss or Vdd-Vgs. In other words, it is a measure of the power supply.

Maybe Rd is small enough that it will find an operating point, but in that case the M1 bias current will be (Vdd-(Vo+Vgs3)) so it will be dependent on Vdd.

I could be wrong, but I can't see how it would work.

Did you get the email I sent you?

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Feb 26^th, 2013, 12:11pm

Dear robg,
fine, i understood what you are saying. Before posting this thread i am pretty sure this was mistake while drawing fig. i just want to make sure once so posted . recently saw few loops working with +ve feedback with less than 1 loop gain (please find the attached fig) and some are both with +ve and -ve feedback.

Do you think +ve fb with <1 gain have all good things as -ve fb had?
can you tell me how this attached unity gain buffer works?
BTW..i didn't get your email...
Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by RobG on Feb 26^th, 2013, 3:07pm

The NMOS in that circuit acts more like a cascode for the PMOS that is bootstrapped to the input. I don't think of that as a feedback circuit but I suppose you could - the loop gain would be essentially nil from the PMOS drain to the input.

Title: Re: Trans-impedance amplifier
Post by RobG on Feb 28^th, 2013, 7:50pm

OK, heard through the grapevine that this transimpedance amplifier is supposed to be run open loop. Now it makes more sense. Start thinking about it by removing Rf. Then you will notice that the small signal voltage at the gate of M3 is I_IN*Rd. M3 then just serves as a voltage follower. The gain is simply Vo/Iin = Rd.

When you include Rf it adds positive feedback which will enhance the gain. My initial scratchings (which are often wrong) indicate the gain will be Vo/Iin = Rd/(1-Rd/Rf).

However, I'm not sure that is how Rf was intended to be used because the description in the thesis does not match the figure. It says that the gain is Vo/Iin =Rf and describes the operation as if it were a negative feedback transimpedance amplifier. (Especially one where the current from M1 is mirrored and then buffered by M3, which would work fine.) Therefore I think the figure is in error although it will function if Rf>Rd (also note you will have 0dB PSRR if you measure relative to ground, maybe worse when Rf is included).

Assuming the figure is in error, having written a few papers I know how easy it is to miss something like this in the final edit especially when it isn't the main focus of the paper. Every once in a while I have to remind myself not to be too critical of other people's work.

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Mar 1^st, 2013, 10:33am

Hello all,
i am extremely sorry for mis understanding this feedback. It's my mistake, it is -ve feedback. But still i don't have solid reason to say -ve feedback but some how i will try to figure out the reason.

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by RobG on Mar 1^st, 2013, 10:34am

raja.cedt wrote on Mar 1^st, 2013, 10:33am:

? I think it is positive feedback.

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Mar 1^st, 2013, 10:44am

No rogb,
I am sure it is -ve feedback. However i don't have solid ans...One simple test in small signal gain expression if you get all coefficients +ve then it's -ve feedback. Please find the attached doc.

As you might have already noticed circuits looks like +ve fb, but they have some where inherent strong -ve feedback. One simple example gm=1/R circuit.

I will update if i have some good answer.

Good night,
raj.

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Mar 3^rd, 2013, 2:48am

Dear Robg,
one more example, according to razaavi the following fb is -ve but i though it is +ve. do you know how it is -ve fb..

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by Lex on Mar 4^th, 2013, 1:33am

raja.cedt wrote on Mar 3^rd, 2013, 2:48am:

Dear Robg,
one more example, according to razaavi the following fb is -ve but i though it is +ve. do you know how it is -ve fb..

Thanks,
Raj.

Go through M1 (no inversion), then M2 (inversion). So, one inversion -> negative feedback.

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Mar 4^th, 2013, 2:07am

dear Lex,
M2 non-inverting.So +ve fb.

Thanks,
Raj.

Title: Re: Trans-impedance amplifier
Post by Lex on Mar 4^th, 2013, 2:16am

raja.cedt wrote on Mar 4^th, 2013, 2:07am:

dear Lex,
M2 non-inverting.So +ve fb.

Thanks,
Raj.

Oops. Sorry. You're right. I mistakenly saw M2 for a PMOS. I guess it is time for a coffee.

/edit
I think you're right about the positive feedback part, but if the loopgain is smaller than 1 it can still operate. Please provide the figure number next time (8.57). It helps to look up the context.

Title: Re: Trans-impedance amplifier
Post by RobG on Mar 4^th, 2013, 7:43am

That looks like positive fb to me. I found it in his book but I couldn't find where he said it was negative fb.

Title: Re: Trans-impedance amplifier
Post by raja.cedt on Mar 4^th, 2013, 10:32am

in the introduction....

Title: Re: Trans-impedance amplifier
Post by RobG on Mar 4^th, 2013, 12:46pm

Raja - Please be more specific so we can help you out. Which introduction? I find the circuit in several places, Figs 8.57, 7.54, and 6.38 but nothing that says it is negative fb. Also, when I do the input impedance on that circuit it is negative.

I see several examples of problems (such as finding input impedance) with positive feedback circuits in his book. Generally you have to add resistance to keep these circuits from latching up or otherwise hitting a rail.

I'm glad you asked however as it reminded me that I need to read this book.

Title: Re: Trans-impedance amplifier
Post by Vladislav D on Mar 6^th, 2013, 7:07am

raja.cedt wrote on Feb 24^th, 2013, 3:22am:

Can you point out me any design with +ve feedback, if you know.

Any supply insensitive current reference for example