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Using fully differential amplifier in unity feedback by shorting input to output

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safwatonline

safwatonline

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hello,
in Razavi's book , i noted a fully diffrential amp. used in unity feedback configuration by just shorting the i/p to the output.
my question is when we are connecting the i/p to the o/p directly ,how the diffrential pair will affect them (i.e. we are shorting the diff pair)
i get it in making a bufer with diff. to single-ended where the o/p is connected to the inverting terminal and the i/p is connected to non-inverting terminal ,but in this case they are not shorted .
thnx for the help
 

Re: unity feedback

Generally, for buffer you just need amplifier with gain, and possibility of output signal range to track input signal range. Then just make unity negative feedback no matter if you have just simple differential pair or multistage amplifier. For good stability make phase margin higher than 60, for speed adjust supply current, and that´s it.
 

unity feedback

hello pixel,
thnx for the reply,
i know what u said , but my question is that i am shorting the i/p to the o/p, so i cannt see how the amp. will affect the o/p ,since it is already shorted to the i/p.
sorry for my bad english , hope i explained what i want.
regards,
a.safwat
 

Re: unity feedback

safwatonline said:
hello pixel,
i know what u said , but my question is that i am shorting the i/p to the o/p
Click to expand...
If you want negative feedback you should short i/n with o/p, and i/p to o/n, just as at the bottom picture. In other case (i/p, o/p) you have positive feedback, and no buffer.
You can watch both mosfets as two inverting amplifiers.
 

unity feedback

hello pixel,
thnx for the reply again,
but u didnt get me again, what i am asking is what is the diffrence between this circuit and the case when i just connect the i/p to the o/p directly,
i.e. in both cases there is no isolation between the i/p and o/p, so what did i gain and how was that gain.
regards,
a.safwat
 

Re: unity feedback

safwatonline said:
hello pixel,
thnx for the reply again,
but u didnt get me again, what i am asking is what is the diffrence between this circuit and the case when i just connect the i/p to the o/p directly,
i.e. in both cases there is no isolation between the i/p and o/p, so what did i gain and how was that gain.
regards,
a.safwat
Click to expand...

I just wanted to say that you should forget to conect positive input with positive output directly or indirectly, becuse of positive feedback.
Where is positive Vout node (left or right in the schematic),if M1 gate is positive input ?
 

    safwatonline

    safwatonline

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Re: unity feedback

hi again,
thnx again but u miss understand me :cry: ,
i/p == input "not input +"
o/p == output "not output +"
i think i got it , if i applied supperposition i will get a case like the diff. to single end buffer , so there is some sort of isolation, i think this may be correct.
anyway thanks alot for ur help. "i better go take an english course " :D,
regards,
a.safwat
 

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