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Common-mode feedback in the full differential opamp?

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joskin

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common mode feedback tutorial

In the full differential opamp,the common mode signal detector is formed with two resistors in parallel with two capacitors in most cases.
What's the function of the capacitors?
What will happen without the capacitors?
 

common mode feedback compensation

These capacitors are used for compensation differential loop.

It's possible to skip these capacitors if ur Op is compensated.
 
full diffrential opamp current source

i recommend the book of 'design of analog cmos integerated circuits--razavi' & 'analysis and design of analog integrated circuits--gray'. there are detailed explaination about the CMFB.
 

common mode feedback differential amplifier

I think the most completed description of CMFB u can find in Johns and Martin book, chap.6


**broken link removed**
 

cmfb circuit tutorial

Please read the Gray's book. Chapter12 handle the CMFB. I am sure you will understand how to design CMFB circuit after you understand this chapter.
 

op amp single-end differential

I am designing a op amp,the first stage is differential output and the second stage is single-end output,need I add a CMFB circuits to the op amp for the first stage?
 

common mode signal differential pair

is it for speed up?
 

fully differential op amp common mode

yyliang said:
I am designing a op amp,the first stage is differential output and the second stage is single-end output,need I add a CMFB circuits to the op amp for the first stage?
Click to expand...

If the amplifer is a DC amplifier, you no need a common feedback because you want the DC point to change when you are using it in the feeback system.If the amplifier is used as AC amplfier, you need the CMFB circuit to maintain the common mode so that there is no pulse width distortion.

Added after 2 minutes:

joskin said:
In the full differential opamp,the common mode signal detector is formed with two resistors in parallel with two capacitors in most cases.
What's the function of the capacitors?
What will happen without the capacitors?
Click to expand...

The two resistor use to extract the DC point and there will be some ripple there and the capacitor can reduce the ripple.
 
common mode detector capacitor

The resistors aimed at measuring the output common mode: (VoutP+VoutN)/2.

The measured output common mode is used has an input of the CMFB differential amplifier, thus it goes to the gate of a differential pair and thus a Cgs gate. This create a RC pole at the input of the CMFB which slows down the CMFB loop in HF. Thanks to the capacitor, you create a zero that partially compensate for this pole, and thus speeds up the CMFB loop at High Frequency.
 

differential op amp with common mode setting

The capacitor have two effects.
1. sense the common level of two output port.
2. reduce the load of output to easy the compensation and increse the drive capability.
 

extracting common mode voltage with op-amp

Mline7 said:
The resistors aimed at measuring the output common mode: (VoutP+VoutN)/2.

The measured output common mode is used has an input of the CMFB differential amplifier, thus it goes to the gate of a differential pair and thus a Cgs gate. This create a RC pole at the input of the CMFB which slows down the CMFB loop in HF. Thanks to the capacitor, you create a zero that partially compensate for this pole, and thus speeds up the CMFB loop at High Frequency.
Click to expand...

By common mode half circuit, the Resistor is in series with current source, how
it can create third pole? This's my confusion.
 

cmfb right pole

You can find much information about CMFB and OpAmp compensation in Jacob Baker "CMOS: Design, Simulation and Layout" book.
 

common mode signal+op amp

joskin said:
In the full differential opamp,the common mode signal detector is formed with two resistors in parallel with two capacitors in most cases.
What's the function of the capacitors?
What will happen without the capacitors?
Click to expand...

You can learn about this in the nex file:
 
why cmfb is needed

this type of feedback is one of many. please read in the lit
 

he high-frequency common-mode feedback

Miline7 is totaly right !

OMEsystem poles come from the Mos 's gate (Grid) , so you will compesate this pole by the shunt capacitor .
 

cmfb circuit cap

MLR67 said:
Miline7 is totaly right !

OMEsystem poles come from the Mos 's gate (Grid) , so you will compesate this pole by the shunt capacitor .
Click to expand...

I can figure out the additional pole comes from input cap of mos device in cmfb
error amplifier. But, pole is not only composed of cap but also resistor. The Ro
of current source is the one of the compositions. And, if that's the case, the cmfb
resistor which is in series with current source has negligible effect if the Ro is
infinite.
 

surianova said:
yyliang said:
I am designing a op amp,the first stage is differential output and the second stage is single-end output,need I add a CMFB circuits to the op amp for the first stage?
Click to expand...

If the amplifer is a DC amplifier, you no need a common feedback because you want the DC point to change when you are using it in the feeback system.If the amplifier is used as AC amplfier, you need the CMFB circuit to maintain the common mode so that there is no pulse width distortion.

Added after 2 minutes:

joskin said:
In the full differential opamp,the common mode signal detector is formed with two resistors in parallel with two capacitors in most cases.
What's the function of the capacitors?
What will happen without the capacitors?
Click to expand...

The two resistor use to extract the DC point and there will be some ripple there and the capacitor can reduce the ripple.
Click to expand...

WHY? can u state more clearly?
why no common mode feed back is needed in the first stage, when used as a dc amplifier?
the first stage, i think, will not function well, because no common operation points are well defined.
thanks.
jeff.yan
 

In my opinion,the R and C generated a zero,not a pole.
 

Resistor will change the loading of op. Cap will not.
But cap will affect the speed of OP
 

You may also want to look at the common mode return ratio (i.e. neg. of the loop gain) to determine the stability of the opamp. If the R and C for the CMFB circuit are not selected correctly, this could cause CM oscillations and the amplifier to go unstable even though in the differential case it looks like the amp is stable. Just a thought.
 

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