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How does the overdrive voltage effects the maximum voltage swing??
a) If my VDD is 1.2V and overdrive voltage of M4A is 300mV, what can be the maximum output swing at output?
b) Incase the overdrive voltage of M4A is 100mV what can be the maximum output voltage then?
With supply of 1.2V you have theoretical 1.2V output voltage swing (rail to rail). In simple terms you get 1.2V at output by driving the upper transistor so it is low ohms, and the lower transistor so it is high ohms.
Conversely you get 0V output by driving the lower transistor so it is low ohms to ground, and the upper transistor is high ohms. (However we don't expect output to get close to the bottom supply rail if we have the tail resistance at bottom.)
Gain determines the amount of amplifier response to your input signal. There are ways you can adjust gain so that your amplifier responds slightly or strongly.
Say your gain is 4. Then 300 mV of input swing results in theoretical 1.2V output swing.
One way to determine input/output dynamic range is transisor's dc operating point. Usually mosfets are expected to working in saturation region to provide a high intrinsic gain. With this consideration, Vod(M4A)=0.3V means Vout should be smaller than Vdd-Vod(M4A)=0.9V.
While the output of this structure is able to swing to power rail, this means M4A is working in linear region and gain of 2nd stage is low.
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