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In analog circuits, the power dissipated by a one pole system is
P=8(Vdd/Vpp)kTf(S/N)
The minimum power consumption of analog circuits at a given temperature is basically set by the required SNR and the frequency of operation. Since this minimum power consumption is also proportional to the ratio between the supply voltage and the signal peak-to-peak amplitude, power efficient analog circuits should be designed to maximize the voltage swing. Hence it is desirable to have a rail to rail output stage in low power amplifier design
I would like to add on to Alles' answer. Well, a rail to rail amplifier is actually consisting of complementary differential pairs which operate in operating conditions which are complement to each other. This will enable us to meet very good ICMR conditions. The PMOS circuit will work when the operating conditions are more near to VDD and NMOS differential pair will work when the operating conditions are near to ground rail. You should notice that the ICMR for this pretty good. The minimum supply voltage this circuit will work is about 4VDS,sat + |VTP| + VTN. Importantly you need a circuit which balances the bias currents in order to maintain constant transconductance.
I agree with Vamsi Mocherla. Complementary input stages are very useful in rail-to-rail design. in order to have a rail-to-rail output stage, sometime class-AB configuration is very useful. I hade recommended a very useful (at least for me!) paper about rail-to-rail design in some other topic like this before.
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