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[Moved]: Differential to single ended and no common mode feedback.

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tenso

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I know this is a question which has been asked here before but the old threads, while helpful, haven't answered my questions, so I would be grateful if someone can indulge me.

1) Why is the output common mode voltage only a concern in fully differential circuits? Why is there not a requirement to keep the output bias voltage at a required level in a circuit with only a single output (like the common diff. amp with a current mirror load?



2) What about a circuit with diode connected loads and a differential output? does it require CMFB?

3) is common mode only a feature of differential signals (either input or output) , i.e these signals have some common values, which doesn't apply if there is only one signal?

-Thanks
 

Re: Differential to single ended and no common mode feedback.

Hi,

Your questions are too common, and the answers often depend on the application.
Please give more informations, examokes, reference documents, part names.

*****
One example:
Imagine you have a fully differential output amplifier.
If you drive a fully isolated transformer with this sgnals then common mode voltage won't be a problem
But if you try to drive the input of a fully differential ADC with this signals, then the ADC might have limitations on common mode voltage.

Klaus
 

Re: Differential to single ended and no common mode feedback.

I guess my main question would be, is the concept of "common-mode" only for differential signals and not single ended signals?
 

Re: Differential to single ended and no common mode feedback.

Hi,

I recommend you to focus your question on an example...maybe the answer becomes obvious..
At least we can discuss on it.

Klaus
 

Re: Differential to single ended and no common mode feedback.

Vcm = (Va + Vb)/2
Vdiff = Va - Vb

So how observe Vcm if you only have one signal level, e.g. Va?

In so far the answers to question 1 and 3 are obvious.

Only question 2 isn't self-evident from the start. The answer depends on 1. circuit details, 2. circuit purpose, 3. operation conditions.

CMFB may be required either because you want a specific Vcm level at a differential amplifier output or because the circuit would saturate (not work as amplifier at all) without it. Decide what's your requirement and analyze the circuit of interest.
 
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    tenso

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Re: Differential to single ended and no common mode feedback.

Vcm = (Va + Vb)/2
Vdiff = Va - Vb

So how observe Vcm if you only have one signal level, e.g. Va?

In so far the answers to question 1 and 3 are obvious.

Only question 2 isn't self-evident from the start. The answer depends on 1. circuit details, 2. circuit purpose, 3. operation conditions.

CMFB may be required either because you want a specific Vcm level at a differential amplifier output or because the circuit would saturate (not work as amplifier at all) without it. Decide what's your requirement and analyze the circuit of interest.


Thanks for the answers . It helps a lot. Especially the bit about how a single ended signal has no common mode.
 

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