controlling mosfet biasing directly in the circuit question

[john23]

Hello,I have created a circuit shown below in ltspice using tsmc 180nm mosfets.
unlike the standart way where we bias the mosfets using the large signal furmulas ,I would like to bias the M3,5,1,2 into saturation while M4,6 needs to be linear using charactersitics plots .
How can I make these setting in the circuit below?
Photos and Ltspice files are attached.
Thanks.

https://sites.bu.edu/engcourses/files/2016/08/mosfet-differential-amplifier.pdf

[Jay_Diddy_B]

Hi,

The circuit is a differential amplifier. It needs external feedback like this:



R1 sets the operating current. I chose 10uA.

R2 and R3 provide a feedback divider.

This is similar to the op-amp circuit I have added.

Regards,

Jay_Diddy_B

[john23]

Hello J_diddy_B,M3 needs to in saturation,M4 needs to be in linear biased.
then based on the biasing we calculating the gain.
The is a gm/Id medthod with lots of plots.
could you please say what plot I need to use to set the biasing?
What plots I need to create to set the gain?
Thanks.

[Jay_Diddy_B]

Hi,

You need to study the document:
https://sites.bu.edu/engcourses/files/2016/08/mosfet-differential-amplifier.pdf

that you provided in the first message.

It shows:



The author has chosen the set the current in MA at 20uA. I chose 10uA.
In the pdf document the author has indicated W/L ratios for the FETs.
In the model that you provided, and I edited, all the W/L ratios are the same.

Io sets the operating currents. These are mirrored using MA, MB and MC. In the authors example MB is operating at 1/5 of the current of MA. It is 1/5 because of W/L ratio.

M3 and M4 are the current mirror load for the differential pair M1 and M2

E2 is a voltage-controlled voltage source. This is used to provides feedback. I implemented this as a simple resistive divider, E2 = 0.5 x Vout.



Jay_Diddy_B

[john23]

Hello , I am trying to follow the example shown in the manual below.
In step 3 they find the proper W/L using mathematical expressions.
However I know that these formulas are aproximations ,the realistic way is to usr characteristic plots.
What caractestics plots  you reccomend me to use in order to get the proper W/L?
https://aicdesign.org/wp-content/uploads/2018/08/lecture19-150211.pdf
Thanks.