Whats the common mode swing ?

[khabib]

CM swing !!

Hi,
Can anyone please explain what is common mode swing and some qualitive description
about that. Why this is important for the VCO inputstage structure or op-amp input stage or in differential amplifier input stage. Please be little bit descriptive !!

Thanks,

Khabib

 

[maxwellequ]

Re: CM swing !!

I don’t know exactly what is the application that you are interested in. I will assume that you want to know what the common-mode input swing will has to do with an opamp input stage.

If you are asking about a common-mode issue, is certainly because the signals that you want to process are differential. This means that you have two signals (negative and positive) vp=VCM+DeltaV/2 and vn= VCM-DeltaV/2. In this case the differential voltage, which is you are processing is DeltaV. VCM is the common-mode voltage.

Your fully differential opamp amplifies the differential input voltage: for example if the gain is set to 3, the DIFFERENTIAL output voltage will be 3x larger than the differential input voltage. Ideally this amplification operation should be independent of the input common mode voltage…. but it is not.


Imagine that your amplifier as a NMOS input differential pair. If the common-mode voltage of your input signal is near the negative supply, the differential pair will not work properly (the transistors will cutoff). In this case you must increase the input common-mode voltage, to have the differential pair biased correctly. If you ensure this, the output voltage will be (almost) independent of the input common-mode voltage variations.

Hope this helps !

 

[goodboy_pl]

Re: CM swing !!

CM of Differential pair is at fixed voltage ONLY the input signal of it is small, for large inputs C will changes, this occure in VCOs since they are designed for large signal to decrease the phase noise. some architectures in VCO have a little changes in CM of the tail current source thus they have lower induced fliker noise by the tail!

BEST!

 

[hanjiemy]

CM swing !!

I guess that the input CM u said is the DC Range @ your opams input stage, in which the opams can work!
i agree with goodboy_pl, large swing will be useful to low phase noise.