urgent:how this amplifier works

[ramy_maia]

hello there i want to know how this amplifier works
as i understand it is a differential two stage amplifier and the first stage with pmos i/p and loaded by 2 nmos's as current source so we must insure same Vg for both so the BJT is used but i don't know how it make const. Vg for the nmos load
after that it goes for the 2nd stage with nmos input and take the o/p as single ebded

is my words are correct? if so how the BJT works?
thanks in advance

 

[wpchan05]

I guess the pnp are used as a source follower. They serve to keep track of the first stage output node voltage. So, whenever there is imbalance during the first stage differential output node voltage, the pnp will follow that imbalance node voltage and output to the second stage of the amp (common source amp)

Since the vbe of the pnp are kept constant by the constant current (the pmos transistor), the 2 nmos current source will experience the same terminal voltage if the inputs are equal. Thus, high CMRR is acheived.

 

[JPR]

The bipolar transistors ARE level shifters. The purpose for them is to increase common mode input voltage range, allowing input at ground (negative supply).

The devices MA10 and MA11 form a diode connected load.

Without the level shifters, MA10 and MA11 would have their gate tied to drain. The drain voltage for this would be Vtn + sqrt( (I*L)/(kp*W) ). The voltage available will be VinCM+Vtp. If Vtn is less than Vtp, then VinCM will not reach to ground. Even with Vtp a bit greater than Vtn, the input range will not reach to ground.

With the level shifters, the voltage at the drain will be lower by the Vbe of QA16 and QA17. This relaxes the requirements on Vtp-Vtn by about 0.7V, which should allow common mode range to ground.