relationship of bandwidth of CMFB and signal loop

[don1105]

when designing a fully-differential opamp,
how to determine the bandwidths of CMFB and the main signal loop?
should be the bandwidth of CMFB less than that of signal loop?
what range should its bandwidth's ratio be fallen in?

thaniks a lot

 

[tatakt]

Hi,
you can use following instructions to measure the bandwith:
.measure ac dcgain find v(out) at=.1
.measure ac f3db when v(out) ='dcgain*.707'
In this case to determine the opamp bandwidth use the ideal CMFB,i.e. a voltage controlled current source and then measure the bandwith. To determine the CMFB bandwidth apply input signal to it as a seperate circuit and use the above instructions.
Good Luck,
Tata

 

[don1105]

Thank you, Tata,
But what I mean is that for a robust circuit, the bandwidth of CMFB should be larger or smaller than that of the signal loop.
For example, if we design a fully-differential opamp with the bandwidth of 20MHz, then how do we choose the bandwidth of its CMFB? how large is the value for the well-designed opamp, 10MHz or 30MHz?
If the bandwidth of CMFB is not chosen to the suitable value, the opamp maybe oscillate. is it?

DON

 

[rvenigal]

Hi,
rule of thumb says that the cmfb bandwidth should be greater then the bandwidth of the loop. Resone:- to compensate for common mode errors, like power supply noise, input common mode noise of previous circuit.

 

[tatakt]

Thank you, Tata,
But what I mean is that for a robust circuit, the bandwidth of CMFB should be larger or smaller than that of the signal loop.
For example, if we design a fully-differential opamp with the bandwidth of 20MHz, then how do we choose the bandwidth of its CMFB? how large is the value for the well-designed opamp, 10MHz or 30MHz?
If the bandwidth of CMFB is not chosen to the suitable value, the opamp maybe oscillate. is it?

DON

Hi,
Generally, The bandwidth of CMFB must be greater than bandwidth of the opamp.Otherwise, the cmfb can't hold the output level at the desired level and the output level may varies in a wide range due to variations in input dc level.
Good Luck,
Tata

 

[bobxing]

WHAT ABOUT LOOP GAIN? When stability is guranteed, loop gain the big,the better?

 

[ocarnu]

Yes, this will decrease your CM error. but you should be really carefull about the stability in common mode. And, gain comming at the expense of BW, is a trade off.

 

[sunbeam]

at least your CM GBW is compared to your DM GBW.