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Design >> Analog Design >> Rail to Rail Op Amp Input stage problem
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Message started by hai on Nov 21^st, 2011, 10:16am

Title: Rail to Rail Op Amp Input stage problem
Post by hai on Nov 21^st, 2011, 10:16am

Dear everyone

I recently have read a biosensor paper. The author adopts a rail to rail folded cascoding structure to design a fully differential amplifier. The schematic is attached below.

My questions are
1. What is the function of the circuit as I circled in red? As far as I know, we need to provide a constant gm at the rail to rail input stage but I really have no idea why this circuit can make it.
2. Why the sizes are different in the bias circuit as I circled? I assumed that the cascoding branch should provide 2uA current so the size should be the same.
3. Why the input stage is like a differential amplifier?

Thanks in advance for all the discussions and help!

Title: Re: Rail to Rail Op Amp Input stage problem
Post by loose-electron on Nov 21^st, 2011, 2:05pm

Terrible circuit architecture!

Due to the Vcm source follower (you circled it) the input differential pair is not getting their high impedance common mode rejection from a current source below the differential pair. It has a source follower driving it, and the input is no longer truly differential, rather it is now (sort of ) complimentary.

Generally you will control common mode by adjusting the active load, or the common current source. This method I would expect will make common mode rejection worse.

Strange methods to say the least!

Title: Re: Rail to Rail Op Amp Input stage problem
Post by hai on Nov 21^st, 2011, 4:02pm

loose-electron wrote on Nov 21^st, 2011, 2:05pm:

Thanks for the reply!
I totally agree on you. The CMRR will be worse because of the source follower. The tail node is not high impedance any more.
Do you find any constant gm scheme used in this circuit?
I have to say that this circuit really makes me mad!

Title: Re: Rail to Rail Op Amp Input stage problem
Post by RobG on Nov 21^st, 2011, 5:18pm

hai - The circuit had me baffled at first since I didn't realize it was a dual diff. However, I believe it's a legit circuit: the source followers are used to keep the total gm constant. This is helpful because the total gm contribution of the two diff pairs would change over the input common mode range, being 2x the value when both pairs are on when the input is in the middle of the common mode range.

It reminds me of a circuit published in the red rag, probably in the mid/late 90's.

Here is how I remember it working. 1) When Vin > VCM the PMOS source follower shunts all of the tail current away from the PMOS diff pair and only the NMOS diff pair is on. 2) When Vin < VCM, the current is shunted away from the NMOS pair, leaving the Pdiff on. 3) When Vin = VCM both diff pairs are one, but at a fraction of the current. The fraction is determined by the relative sizes of the source follower and diff pair fets.

So in all 3 cases of input common mode value the gm remains about the same (if the source followers are sized properly). Thus when it is used on both input pairs the large signal common mode rejection is actually increased. I expect the small signal CMRR is still good at the midpoint because as the current is shunted away from one pair, it is added to the other.

rg

Title: Re: Rail to Rail Op Amp Input stage problem
Post by hai on Nov 21^st, 2011, 9:41pm

RobG wrote on Nov 21^st, 2011, 5:18pm:

Thanks Rob, your explanation turns me up.
I have another question about the class AB output stage. Could you please tell me what is the function of using a floating current source in there? Is it used to raise the biasing up just like to transfer the very top and bottom biasing voltage to there?

Hai

Title: Re: Rail to Rail Op Amp Input stage problem
Post by rfidea on Nov 22^nd, 2011, 12:15am

Jerry, maybe I'm wrong but is not the idea with the two Vcm source followers to turn off one of the differential pair. For a high input common mode the top diff stage will work ok since the Vcm source follower will get a low Vgs and is turned off. For a low input common mode the differntial pair is turned off. In the last case to bottom stage is active.

But maybe the circuit is crap anyway in the middle where both stages are avtive

Title: Re: Rail to Rail Op Amp Input stage problem
Post by raja.cedt on Nov 22^nd, 2011, 3:29am

@Robg ....due to sourse follower CMRR will be bad, how can you say that CMRR is still better when you are at midrail, becaz untill unless both source followers turn off CMRR is bad i guess

@hai where did you find this ckt? here commond feedback is not that effective as you control active load or tail current either.

Thanks,
Raj.

Title: Re: Rail to Rail Op Amp Input stage problem
Post by RobG on Nov 22^nd, 2011, 6:45am

raja.cedt wrote on Nov 22^nd, 2011, 3:29am:

@Robg ....due to sourse follower CMRR will be bad, how can you say that CMRR is still better when you are at midrail, becaz untill unless both source followers turn off CMRR is bad i guess

Raj - I didn't mean to say it was better at midrail, just that it was still good (I don't know for sure - but "eyeballing" the circuit I think it should be acceptable). The reason is that as one source follower (SF) shunts current, the other SF is debiased by approximately the same amount, thus the combined top+bottom tail current is about the same. Depending on how you sized the SFs you could optimize CMR or constant gm.

Hai should be aware that mismatched offsets in the diff pairs gives dual diffs bad CMRR. That is to say that when input is low, the offset is from the pDiff, and when the input is high the offset is from the nDiff. Thus the difference in offset results in an output voltage error that is dependent on input common mode. I expect that this error is at least as large as the one caused by the SFs.

Title: Re: Rail to Rail Op Amp Input stage problem
Post by RobG on Nov 22^nd, 2011, 6:54am

hai wrote on Nov 21^st, 2011, 9:41pm:

Hai - the schematic has no transistor numbers so it would be difficult to describe the operation. The original paper, Hogervost, JSSC, Dec 1994 is here: http://repository.tudelft.nl/assets/uuid:d59c343f-3550-4cdf-8e20-c9135ba5d533/ieee_hogervorst_1994.pdf
I'd also look at how they did the dual diff in that paper if it is important to your application.

Title: Re: Rail to Rail Op Amp Input stage problem
Post by RobG on Nov 22^nd, 2011, 7:07am

raja.cedt wrote on Nov 22^nd, 2011, 3:29am:

@hai where did you find this ckt? here commond feedback is not that effective as you control active load or tail current either.

At first glance the CMFB looks like it will work to me. Isn't it controlling the active load? At any rate, I bet there is a common-mode glitch when the input passes through midrail since the total tail current will change. However, it will change a lot less than if no SFs were used. In that case one of the tail current sources would completely shut off as the input approached the rails.

One other thing that I see that will help CMRR - note how the diff pair loads are created by mirroring a fraction of the tail current. If I understand the circuit, this would mean that tail current changes aren't "folded" into the output stage. Thus, to first order, a change in tail current will not change the current in the output stage.

rg

Title: Re: Rail to Rail Op Amp Input stage problem
Post by hai on Nov 22^nd, 2011, 7:53am

RobG wrote on Nov 22^nd, 2011, 7:07am:

raja.cedt wrote on Nov 22^nd, 2011, 3:29am:

@hai where did you find this ckt? here commond feedback is not that effective as you control active load or tail current either.

Thanks, Rob
I totally agree on you. The input diff pairs are isolated from the cascoding output part so that it will not affect the biasing. It is still connected like a folded cascoding one but the current in the folded part is actually provided by diff pair itself.

What is the function of the floating current source then at the output part?

This is the original paper with the schematic in it. The name of the paper is A Fully Differential Potentiostat
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4749400

Title: Re: Rail to Rail Op Amp Input stage problem
Post by RobG on Nov 22^nd, 2011, 8:10am

hai wrote on Nov 22^nd, 2011, 7:53am:

What is the function of the floating current source then at the output part?

Hai - the floating current source provides the gate voltage biases for the output stage. I think it is a misnomer to call it a current source in the configuration shown as it doesn't determine the current that flows through its terminals. The Hogervost paper that I linked uses that topology as an actual current source to bias the opamp in addition to providing the bias for the output stage. It is well worth the time to understand how this "floating current source" bias works.