CMFB amplifier with cascoded diode load

[Junus2012]

Hello All

in the image below two forms of CMFB, I usuallyuse the CMFB to correct the biasing voltage of the amplifier transistors MA3 and MA4 shown at the end of picture. But I have seen some authors are using the circuit of CMFB2, what is the advantagues of this circuit over CMFB1 ? surely matching current will be better but will this improve for example the offset voltage ? or there is other consideration ?

Because in my thinking (which mostly wrong ) the CMFB2 has less gain , right ?

Thank you in advance

 

[sutapanaki]

Both should have about the same gain. CMFB2 provides better matching with the opamp stage that you controll, which means less systematic offset between the two, which means that the CMFB should not work to compensate also for that systematic offset.

 

[Junus2012]

Both should have about the same gain. CMFB2 provides better matching with the opamp stage that you controll, which means less systematic offset between the two, which means that the CMFB should not work to compensate also for that systematic offset.

Thank you dear Suta and nice to see you again

I agree on your second point, but how both have the same gain when the cascaded load has higher resistance?

 

[sutapanaki]

For CMFB2, although it has cascodes, you still see 1/gm of M3/4 looking into the diode connection. Remember, a diode connection either around a single transistor or cascoded transistor is a feedback configuration that samples the drain voltage and hence reduces the output resistance.

 

[Junus2012]

Thank you Suta for making it clear,
I do appreciate your kind help