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Bias circuit for folded cascode with gain boosting OTA neede

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wael_wael

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ota ckt

Hi everubody,

here i have folded cascode withe gain boosting amplifiers, could someone help me and send me a good bias circuit valid for it.

best regards
 

gain boosting

I can tell you already that unless this is only meant for the academic world, (to be published only), I wouldn't use that CMFB circuit (I have put it in circles). It is not the best way by far.
 

gain-boosted folded cascode

thank you, but i am take about the bias network not about the CMFB circuit. do you have any idea for the bias network i hope that some can help
 

nmos current bias circuit

PaloAlto said:
I can tell you already that unless this is only meant for the academic world, (to be published only), I wouldn't use that CMFB circuit (I have put it in circles). It is not the best way by far.

Could you explain further ? At that point differential signal swing is limited, so using the diff pair to extract the CMFB seems appropriate to me.
The only issue I see in what you highlighted is the fact that he is implicitly comparing it to the supply rail, and I would actually use some other voltage generated by replica biasing.
 

bias circuit

Hi wael_wael,

What do you exactly need? Is it the generation of the gate voltages of the current mirrors P and N and the cascodes P and N?

Yeah, that's a good question. Because that is normally not publish, so it's not so easy to find.

Well, there's not a big mystery behind it. You somewhere would need a reference generation block (like a bandgap).

Now, the simplest is to use a current passing it through mos transistors connected in diode mode and changing their sizing. As a rule of thumb, for the mirrors the references will have the same size as the mirrors and for the cascodes, the diode connected transistors will roughly have the same W and 4*L.

There are slightly more sophisticated structures, but I don't know how to get those drawings right now.

I hope your question was in this direction, otherwise, what a lot of writing for nothing :-b

Added after 12 minutes:

n1cm0c,

There are better ways to sense the CM rather that re-using an already active circuit like the input diff pair, but anyway, there are two big problems with it:

* It s very sensible to mismatch. In fact, if you analyze the circuit, you'll see that the current that goes through the cm control transistor is the difference between the total current of the PMOS current mirrors and the NMOS current mirrors. The mismatch between the two can spoil the gain of the CMFB loop which probably is already quite small

* It has dangerous start up conditions. If you force the inputs of the Y amplifier to gnd, the transistor can be cut-off and not able to push up the cm voltage

I have personally experience problems with this type of circuit. It is not such a bad solution because it uses only one transistor but it is very dangerous. People tend to overlook this part of the circuit and it can make your complete system fail
 

    wael_wael

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gain boosted folded cascode ota

thank you so much, i think i get solutin and i would like to share it with you, download the project , there is bias network for the circuit above
 

biasing network diode-connected ota

I have to make gain boosted folded cascode OTA for pipeline ADC,,

may I refer to your project...

thank you very much
 

biasing circuit design ota

thanks you very much I do appreciate your cooperation.
thsnksssssssssss
 

hi!

id like to ask if it is possible to measure the slew rate of a fully differential gain boosted folded cascode OTA? how?

thanks in advance.

regards.
 

Hi!

I read a paper said no CMFB circuit is needed for the gain boosting ota (the X and Y blocks) since it's in negative feedback form. If I can make sure deadlock not happen during startup, then no CMFB circuit is needed?

Thanks in advance.
 

you can see a bias circuit design rule for cascades devices in Abo's phd thesis at UC, Berkeley
 

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