about common-mode feedback

[xz781122]

Is it necessary to design common-mode feedback for any amplifier? In some books common-mode feedback is used in two inputs two outputs amplifier.what about other types?

 

[eng_Semi]

I think that CMFB is a must in OPAMP's with active loads. Coz when u r using active loads, u can't define a certain output common mode level. While if u r using load resistors, it's easy to define ur common mode level.

Also u may use it with fully differential OPAMP's and single ended ones. For the fully differential case, u will sense the common mode voltages of both output terminals, then compare them to the needed common mode level, u will use to resistors or two caps (to avoid loading) to sense both voltages. While for the single ended case u will sense only the voltage of one side, which is easier.

Take care from the CMFB and the stability issues

 

[jiangwp]

Generally, for the single output op, the CMFB is not need.

the op is used with feedback, the CMFB is used to determine input common mode

voltage when full differential op is used by feedback loop.

In conculsion, the CMFB is used to determined common mode input voltage in

order to prevent the op from unwork.

 

[eng_Semi]

In conculsion, the CMFB is used to determined common mode input voltage in

order to prevent the op from unwork.

I don't agree with u.

After Razavi, p.316 (from the international edition 2001):
"In high-gain amplifiers, the output CM level is quite sensitive to device properties and mismatches and it cannot be stabilized by means of differential feedback. Thus, a CMFB network must be added to sense the CM level of the two outputs and accordingly adjust one of the bias currents in the amplifiers. We divide the task of CMFB into three operations: sensing the output CM level, comparison with a reference, and returning the error to the amplifier's bias network."


So the we need the CMFB network to adjust the putput CM level and not to prevent the OPAMP from unwork.

 

[beckchm]

yes

the single end output is very differernt from the differential output

 

[safwatonline]

I think that CMFB is a must in OPAMP's with active loads. Coz when u r using active loads, u can't define a certain output common mode level. While if u r using load resistors, it's easy to define ur common mode level.

Also u may use it with fully differential OPAMP's and single ended ones. For the fully differential case, u will sense the common mode voltages of both output terminals, then compare them to the needed common mode level, u will use to resistors or two caps (to avoid loading) to sense both voltages. While for the single ended case u will sense only the voltage of one side, which is easier.

Take care from the CMFB and the stability issues

hello Semi,
i just wanted to know y we need CMFB in case of single ended output, as "the diff-to-single ended stage that i know" will contain a diode connected which will set the CM , can u tell me the cases when we need CMFB in single -ended?
also i always thought that if we didnt put the CMFB one of the transistor will go to Triode , isnt that true or we can do active load w/o CMFB,
regards,
a.safwat

 

[tsb_nph]

Is it necessary to design common-mode feedback for any amplifier? In some books common-mode feedback is used in two inputs two outputs amplifier.what about other types?

These lecture notes will answer all your questions

**broken link removed**
**broken link removed**

 

[surianova]

Is it necessary to design common-mode feedback for any amplifier? In some books common-mode feedback is used in two inputs two outputs amplifier.what about other types?

My point of view is any DC amplifier do not need CMFM. The DC amplifier is the single ended output amplifier like error amplifier in voltage regulator (The DC voltage in regulator always changed to get desired output current). So there is
no need any CMFB.

For differential output amplifier, u need to have constant common mode output, if not, it will cause non-symetrical to the AC signal / the output swing or the signal will be clipped. That why, u need CMFB.