Influence of lambda, current and Vds on ads of MOSFET in saturation region

[phanikiran]

[moved] influence of LAMBDA, current and Vds on rds of MOSFET in saturation regio

In MOS FET, specifically while designing amplifiers ( a simple Diode Load Differential amplifier) UGB will decides the gm of input differential pair width and slew rate decides the current, swing decides the Load PMOS widths. Here my problem is . if i go for higher slew rates current will increase and the LAMBDA parameter is drastically varying which varies rds of PMOS load finally it effecting my gain.

If i choose low currents ,input differential pair taking higher widths to give the reuired gm finally it also affecting gain.

I found rds is the main parameter which is not the value i expected.

I did experiment by varying Vds , and i got required rds for larger vds( depth of saturation is very high ex 1v)

how can i expect rds for a specific vds , how LAMBDA influences for currents (becauseit varying very drastically with high curretns?

 

[erikl]

Re: [moved] influence of LAMBDA, current and Vds on rds of MOSFET in saturation regio

... how can i expect rds for a specific vds , how LAMBDA influences for currents (becauseit varying very drastically with high curretns?

Find here an Id vs. vds plot of a MOSFET:


Here you can see rds vs. vds: in the linear region rds is essentially constant for very low vds, decreasing with increasing current.
This linear or triode region is a non-linear working region for the MOSFET, it lies outside of the FET's output common mode range (OCMR) and should be avoided in all gain stages of an OTA (resp. OpAmp).

The saturation region is the working region for the gain stages, and this is where the OCMR should lie within.
Here, lambda, the channel length modulation (CLM) parameter; Id = Id without Vds dependency · (1+lambda·Vds), increases with Id, particularly strong for short channel transistors.

What is not shown here: at voltages vds ≈ vdd(max), lambda is strongly increasing with vds, mainly for short channel lengths.

Steeper curve means lower resistance. Higher lambda also means lower (output) resistance.