Fully Differential Opamp and CMFB

[ccw27]

differential output op amp

For simulation analysis only is it necessary to build a CMFB for a differential amplifier? It seems hard to bias a telescopic or folded cascode on a 1.8V supply voltage without CMFB.

Thanks

 

[cristianb]

ideal cmfb

U need to used a CMFB to set the output ot the fully-differential op-amp, otherwise is is even harder to bias it. And u can do this evenwith lower voltage supply than 1.8V

 

[yeewong_su]

fully differential folded cascode

Full differential output opamp need the cmfb, you can use the sc ckt, and if your supply is 1.8v, you'd better using the foldcascode and gain boost, may be it's diffcult if you select the telscopic

 

[markty]

how to design cmfb differential amp

CMFB is necessary. Otherwise the output common voltage may float to the supply in the real ic.

 

[bear7679]

fully differential opamp

I think you must use CMFB circuit, you can use SC type or RC type. For detail you can see razavi's book.

 

[arsenal]

fully differential amplifier with opamp

Full differential output opamp need the cmfb, you can use the sc ckt, and if your supply is 1.8v, you'd better using the foldcascode and gain boost, may be it's diffcult if you select the telscopic

u a right,and sc is more preferrable as compared to resistors arctec

 

[ccw27]

differential output operational amplifier

How can I design an ideal CMFB ckt first?

 

[cristianb]

gain of fully differntial opamp

you can simulate an ideal CMFB by using ideal op-amp but the topology for the real circuit is dependent on the maximum voltage swing of the output stage of the main fully differential amp and its current capability. you will find a good analisys of fully differential amps and CMBF topologies (including compensationi) in the last chaper of Gray & Meyer last edition (I think its the 4th) of Analogue Integrated Circuits.

 

[ccw27]

fully differential folded-cascode

This is my ideal CMFB circuit, the amplifier is taken from ahdlLib. For initial design of low voltage fully differential amplifier this can be quite useful as it can help you focus on the design of differential amplifier rather than the whole thing. Of course, later you should substitute this with an actual CMFB circuit found in textbooks.

Does anyone know how I can implement the amplifier in the ideal CMFB circuit with finite gain and GBW? This can help the designer decide the minimum spec he needs to design the actual CMFB circuit.

Thanks

 

[cristianb]

problems with fully differential amplifier

First of all I there's a problem with the way u read the input signal (voc) for the error amplifier in the CMFB. voc=von+vop, but this is equal to 2*VCM so u need (vop+von)/2. for this use a simple resistive voltage divider. As for your question, the gain for the error amplifier is direcly linked to the precision u want to control the common mode at the ouput related to the voltage swing. so lets say if u need a 10mV precision for a output voltage |Vo|=1V this leads to a gain of 40dB. Now having and open loop amplifier in silicon the gain will vary so u need to go even a little higher. if u need more precision u cane use a simple VGA for gain control and the main limitation is given by the offset of the error ampliefier.

 

[ccw27]

common mode variation fully differential

Actually the VCVS have gain of 0.5 each so voc=(vop+von)/2, forgot to point that out.

Also if I have a two stage amplifier, folded cascode followed by common source. Is it ok to put the CMFB to track the output of the folded cascode instead of the output of the common source? From my simulations I needed a much higher gain amplifier if the CMFB input is taken at the output of the common source and its output drives the PMOS cascode mirror in the folded cascode.

Thanks

 

[cristianb]

differential op amp cmfb gain

This depends on the aplication of your amp. If is just stand alone there is no problem. if is part of a bigger block you'll have to take into consideration the behavior of the follwing stage. If this stage does not tolerate common mode variations (for example a high order adc) then you to take the common mode signal at the output of the common source stage, otherwise if u use the signal from the output of the folded cascode its ok. in the first case u should take in consideration that higher gain could splell in stability problems of the common mode loop and compensaiting this is tricky.

 

[wjxcom]

schema ideale cmfb

Dear ccw27：do you know how to simulate a full differential OPamp? for example we want to simulate ac character, PSRR character and so on, what circuit can we use?

 

[gte582w]

fully differential folded cascode amplifier

I personally prefer to use RC to sense the CM signal if the swing is large.
The down side is it takes quite a bit area. Source degenerated xtor in triode
is good but has limited sensing range.

 

[ahmad_abdulghany]

fully differential folded cascode opamp

Dear ccw27：do you know how to simulate a full differential OPamp? for example we want to simulate ac character, PSRR character and so on, what circuit can we use?

A simple OCEAN script can do it easily.

 

[raditsme]

hello everyone... i have to design a telescopic amplifier on 0.18um Spectre. my ugb requirement is > 300MHz and gain > 80db .Although power diss. limit is given i decided my current be 320uA to be ok. I calculated all W,Ls according to the basic equation for SINGLE STAGE TELESCOPIC AMPLIFIER topology as given in Razavi on Oage no.299-300,, of course giving enough margins of Vod for each transistor. But when i simulated my design on cadence tool it is becoming impossible for me to bias all transistor in saturation for given current.i am considering only single stage in initial exercise. I have spent a lot of time on the the exercise but i got no sucees .....please help and suggest some idea.