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