The advantage of using differential input is Differential input voltage cancels the common mode input voltage and it only amplifies the differential input signal.Also the common mode input voltage is the voltage that is common to both the inputs of an amplifier.

The common mode input voltage may be an offset voltage to both inputs

The common mode input voltage may be noise generated by other elements in the circuit

These are the noises which are eliminated by the differential amplifiers and the first one is1) Power supply noise2) Electromagnetic Interference ( EMI) noise. Differential amplifier may implement single ended input amplifier by grounding the other input.

Basic Transistor Level CMOS Differential Amplifier:Recall the single ended Common source Amplifier In this particular circuit, we are not going to worry about DC biasing, The output voltage of Common source amplifier has two components called as AC and DC componentsThe DC components is given as Vo1=Vdd-IdRd Id is the DC currentor Vo1=Vdd-1/2KnW/L (Vgs-Vth)2(1+lambda Vds)Rd

This was all about single stage amplifiersNow we will see How the differential Amplifier is implemented using two identical common source amplifiers

Intuitive Analysisof the behavior of Differential Amplifier

All the transistors have to be in the saturation region in order to work as an amplifier.

To find the voltages Vx and Vy we need to apply superposition theorem and to apply superposition theorem all the current sources are open circuited and voltage sources are short circuited.So the final circuit becomes as shown above on the right

Now the above circuit has become a common source amplifier with degenerated resistor 1/gm2 We can solve for Vx due to V1 as by applying thevenins theorem