Advantages of class AB output stage in fully-differential amplifier

[Junus2012]

Dear Friends,

The obvious advantage of the class AB output stage is its ability to sink/source output current that is far above the static condition. That I can see it no doupt when using single op-amp with a class AB output stage.

However, if we have a fully-differential amplifier, and in case if the CMFB is controlling the output class AB transistors like the circuit shown below, then the output current will be limited to the CMFB bias current, hence I can't see the class AB operation.

More details can be found here

A Power Optimized Continuous-Time <formula formulatype="inline"><tex>$\Delta \Sigma $</tex></formula> ADC for Audio Applications

We present design considerations for low-power continuous-time modulators. Circuit design details and measurement results for a 15 bit audio modulator are given. The converter, designed in a 0.18 mum CMOS technology, achieves a dynamic range of 93.5 dB in a 24 kHz bandwidth and dissipates 90 muW...

ieeexplore.ieee.org

I very welcome your discussion and thank you in advance

Regards

 

[Dominik Przyborowski]

CMFB is reacting on common mode while differential loop on differential signal. Thus, class AB operation in CMFB doesn't need to improve capability of differential path.
This is basically key problem in fully differential amplifiers. How make performance of both path similar.

 

[Junus2012]

CMFB is reacting on common mode while differential loop on differential signal. Thus, class AB operation in CMFB doesn't need to improve capability of differential path.
This is basically key problem in fully differential amplifiers. How make performance of both path similar.

Thank you Dominik for your reply, I agree with you that CMFB circuit reacting on the common mode loop,
but with this control loop, the circuit is attempting to make upper currents equal to the down currents and brings the common mode voltage to the desired level.

This can be understood from the first image in my post where the CMFB tries to make the current of M7 equal to the current of M71 and the same for M8 current equal to M81 current. From DC operating point and static current is very clear.

But as the circuit is class AB output, the M7 ,M71, M8, M81 are supposed to source and sink higher currents. However, with the CMFB, the maximum output current will be restricted to the CMFB biasing current of M9 in the next image and I cant see the class AB operation with this configuration.

In simple world, I am thinking that there is no class AB or push-pull advantague when the circuit is fully differential mode, and basically, all fully differential amplifiers can thus be classified as class A amplifiers.

That is what I am thinking

Thank you