Common mode control circuitry for multi-stage operational amplifiers

1. A fully differential amplifier comprising:

a first amplification stage;

a second amplification stage coupled to the first amplification stage, wherein a feed forward path is provided by injecting a current signal to the output of the second amplification stage;

a third amplification stage coupled to the second amplification stage, having a non dominant pole that is compensated by a single Miller capacitor; and

a common mode amplifier that provides an output to control a current source that injects a control current into the second amplification stage.

2. The fully differential amplifier of claim 1 , wherein the first amplification stage is comprised of two stages.

3. The fully differential amplifier of claim 1 , wherein the first amplification stage provides a high gain that creates a dominant pole system.

4. The fully differential amplifier of claim 1 , wherein the first amplification stage includes a folded cascade.

5. The fully differential amplifier of claim 1 , wherein the third amplification stage provides a differential output of the fully differential amplifier.

6. The fully differential amplifier of claim 1 , wherein a voltage of a differential output is sensed and compared with a reference voltage, the difference of the voltage of the differential output and the reference voltage is amplified to provide a loop gain for a common mode signal.

7. A method for common mode regulation in a fully differential amplifier comprising:

sensing an output voltage of the fully differential amplifier;

comparing the output voltage to a reference voltage;

determining the difference of the output voltage and the reference voltage;

amplifying the difference to provide loop gain for a common mode signal;

injecting a feed forward current based on the loop gain, to one stage of the fully differential amplifier; and

providing a feed back control voltage to another stage, in parallel to the injecting, the feed back control voltage to change a biasing of an active load of the another stage.

8. The method of claim 7 , wherein the output voltage is representative of a common mode output voltage of the fully differential amplifier.

9. The method of claim 7 , wherein the sensing is performed by a summing resistive network.

10. The method of claim 7 , wherein the determining and amplifying is performed by a common mode amplifier.

11. The method of claim 7 , wherein the feed forward current changes biasing of the second stage.