Question about DC Biasing

[Ansonng]

I have a question for DC biasing.
Suppose I have 4 MOS connecting like this.
___ Vpp
|
_|
Vb4- o||_ mp1
|
_|
Vb3-o||_ mp2
|_Vout
_|
Vb2 -||_ mn2
|
_|
Vb1 -||_ mn1
|
--- Vss

Suppose both Vthn and Vthp are 500mV and over-drive voltage is 200mV
So, Vb1 = Vss + 500mV + 200mV

However, I have questions to set Vb2
Since in DC mode, I suppose the Vds for all MOS are the same, i.e. Vds1 =450V. However, considering AC, Vds1 may be dropped to 200mV .

Vb2 = Vss + Vds1 + Vthn2 + Vover_drive
So, my question is whether
Vb2 = Vss + 200mv + 500mv + 200mv
or
Vb2 = Vss + 450mv + 500mv + 200mv
is correct.

Thank you very much for your answer.

Anson

 

[hr_rezaee]

Hi
First send a clear schematic. i can't understand your schematic.

 

[Ansonng]

Here is the schematic.
I want to bias the output part and set CM Vout = 0V

 

[Vamsi Mocherla]

Dear annsong,

If you are using Unipolar supplies as in the figure, there is no way the circuit will operate when the CM level of the output is set at 0 V. Even if you use a low voltage biasing, the minimum value of the output will have to be 2*vds,sat for the amplifier to act as an amplifier, where vds,sat is the saturation voltage of the NMOS devices.

Suppose, you use bipolar supplies, then you can have a zero bias at the output.

Coming to the point about what your Vb2 equation shall be, you can use

Vb2 = VGS2 + Vb1 - VTH1

where VGS2 = sq.root(2*Id/Kp*(W/L))+ VTH2

which will ensure that both the transistors are in saturation. Make sure that the lower transistor has enough room for the current.

I hope that it helps