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Design >> Analog Design >> differential source follower
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Message started by nxing on Jan 2^nd, 2008, 5:35pm

Title: differential source follower
Post by nxing on Jan 2^nd, 2008, 5:35pm

Hi,

Can anybody refer me a differential source followers circuit with common-mode feedback? I can not think one of them.

Thanks

Nxing

Title: Re: differential source follower
Post by nxing on Jan 3^rd, 2008, 5:25pm

So nobody pick up this topic?

Title: Re: differential source follower
Post by vivkr on Jan 4^th, 2008, 5:50am

Hi,

I don't know of a differential source follower but perhaps you can get the job done by using a DDA (differential difference amplifier) in
unity gain configuration. Search for differential difference amplifier in Google or IEEE Xplore. The authors of the original paper are
Eduard Saeckinger and Walter Gugginbuehl (IEEE JSSC).

Regards
Vivek

Title: Re: differential source follower
Post by ACWWong on Jan 4^th, 2008, 8:15am

Hi nxing, when you say "differential source follower" do you mean two source followers which are differentially driven ? If so, the standard technique is to average the output signals by taking the centre-point of two resistors connected between the outputs, and using this in a feedback loop to control the common-mode levels into the source follower gates. This is covered in several textbooks, such as Razavi's "Design of Analog Integrated Cricuits" section 9.6.

Title: Re: differential source follower
Post by nxing on Jan 4^th, 2008, 3:56pm

Thanks for the replies!
The usage of this block is actually btw and mixer and filter. actually, I don't even sure there is a really "differential" source follower exists since I can not find a "virtual ground point" like the common source differential pair. The output of mixer will directly connect to the gate of the source follower (DC coupled) so, even I can get the common-mode output voltage , I don't know where to feed it back? Any suggestions?

Thanks

Title: Re: differential source follower
Post by thechopper on Jan 7^th, 2008, 5:50pm

Hi nxing,

I might be misunderstanding your question but from your last description it looks you are trying to force the output common mode of your differential source follower by means of sensing it and somewhere feed the signal back.
If that is the case I hardly see how that will work since there is no way to let the circuit work as a source follower and force the output common mode at the same time, if the CM signal is what you are interested in. If you only care about the differential signal you might end up using a differential pair.

tosei