Does RF circuit need CMFB circuit?

[wccheng]

Dear all,

I have seen that most of the mixed signal need to use the CMFB (Common Mode FeedBack) circuit to stablize the cicuit. However, I could not see the RF circuit will adopt it? WHY will this happen? Could anyone explain it to me?

Thanks

wccheng

 

[maxwellequ]

Common mode feedback circuits are used in fully differential amplifiers (which have 2 differential inputs and 2 differential outputs) to define the common-mode of the two differential output voltages.

This is needed because the external feedback path only defines the differential component of the input and output voltages.
For example in an inverting amplifier with the gain of -2, the differential output voltage must be -2x the differential input voltage. If you put -0.5 V at the input, you must have an output differential voltage of 1V- it can be Vop=3V and Von=2V or Vop=4.5V and Von=3.5V or any other pair of values which differ by 1V.

The CMFB sets the common-mode component of the output voltages (VCMo=(Vop+Von)/2). For example, if VCMo=2.5 V, in the previous example you would have Vop=3 V and Von=2 V.

This is needed in every fully differential amplifier. It does not depend on the frequency range were the amplifier is being used.

 

[wccheng]

Dear maxwellequ,

However, in the most of the paper shows that not much RF circuit, such as differential LNA, does not included the CMFB circuit. WHY?

Thanks

wccheng

 

[iamlog]

The purpose of CMFB is to fix the common mode voltage. Most of the RF circuit use resitors instead of active load, so the common mode voltage is already fixed by, for example I(bias)*R

 

[thcm]

It depends on your load.Also you can find some mixer circuit topologies ,such as Gilbert type,use CMFB circuits often.