whats the difference between these two differential amp...?

[desperado1]

double ended differential

whats the difference between common mode single ended and common mode double ended differential amplifier coz i have read that in a single ended diff amp one of the input is grounded and common mode means the same signal is applied ?

 

[mo.khairy.mo]

Re: whats the difference between these two differential amp.

at single ended differential amp. you take the output from only one drain (or collectors) and the ground.

at double ended differential amp. say you will take the output from the 2nd drain you will make your Reference the 1st drain
in another word you will take the output with reference to the another drain not with reference to the ground.

in case of common mode the input will be the same input at the 2 gates (or bases) of the transistor

in case of common mode if u take the output double ended it'll be = zero
vout=v1-v2 ; v1=v2=vcm

if u take the output single ended
vout=v1 or v2

 

[desperado1]

Thanks a lot........but what about the CMRR ? which gives better CMRR ? single ended diff. amp or double ended..?

 

[mo.khairy.mo]

Re: whats the difference between these two differential amp.

in case of common mode
if u take the output single ended:-
the common mode gain =RD/2RSS or RC/2REE
CMRR=gm.REE


if u take the output double ended
the common mode gain = zero
CMMR=infinity

so as u see the double ended is much better than single ended

hope this help u