Discrete fully differential amps and no CMFB

rtarbell · 24th August 2006, 04:56 AM

Another question about fully differential amplifiers:

In my analog circuits class, when we covered fully differential opams, we learned about the common mode feedback (CMFB) circuit needed to keep the common mode voltage at the outputs from drifting.

I've seen discrete audio power amps that are designed to be fully differential (two inputs, two outputs), but they do not have the CMFB circuit. Some just have the symmetrical differential feedback (like the Aleph-X). Why is this, and under what conditions is a CMFB circuit needed?

Tim__x · 24th August 2006, 06:22 AM

All the fully differential amplifiers that do away with common mode feedback have fairly low open loop DC gain, and an ouput voltage with well defined value i.e. a current source feeding a resistor, not a current source feeding another current source.

You'll get better responses by putting all of you question in one thread.

24th August 2006, 04:56 AM	#1
rtarbell diyAudio Member Join Date: Sep 2005	Discrete fully differential amps and no CMFB Another question about fully differential amplifiers: In my analog circuits class, when we covered fully differential opams, we learned about the common mode feedback (CMFB) circuit needed to keep the common mode voltage at the outputs from drifting. I've seen discrete audio power amps that are designed to be fully differential (two inputs, two outputs), but they do not have the CMFB circuit. Some just have the symmetrical differential feedback (like the Aleph-X). Why is this, and under what conditions is a CMFB circuit needed?

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24th August 2006, 06:22 AM	#2
Tim__x diyAudio Member Join Date: Jun 2004 Location: Edmonton area, Alberta	All the fully differential amplifiers that do away with common mode feedback have fairly low open loop DC gain, and an ouput voltage with well defined value i.e. a current source feeding a resistor, not a current source feeding another current source. You'll get better responses by putting all of you question in one thread.

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