What is diference between Vdsat, Vov ,Veffective (Vgh-Vth), and Vds......?

[ASHUTOSH RANE]

HELLO FRIENDS,

I am getting confused with term Vov(ovrdrive voltage) and Veffective (=vgs-vth).......!!
while designing a opamp in cadence i set
Vov of 0.3 for PMOS and
Vov of 0.25 for NMOS.....

the operating point info in Cadence shows Following parameters
Vdasat,
Vds,
Vgs-Vth (can be calculated)
then which i should consider as Vov drive ?:|:?:


The tool shown very different values than the designed one (it is obvious as it Uses BSIM3v3 model )!

 

[jimito13]

Overdrige Voltage Vov is identical with effective Voltage Veff and both are equal to Vgs-Vth.

Vdsat is the minimum drain-source voltage needed to ensure that the transistor works in saturation (Vds>Vdsat for sat. otherwise the fet works in triode).

As for Vds i don't think you need explanation...

 

[ASHUTOSH RANE]

@JIMITO13
Hi friend ,
thank u a lot for reply,
Vds> Veff (book considers this criterion for saturation)
Vds> Vdsat,(tool consider this criterion for saturation ....):roll:
which criterion is most accurate among this?

 

[jimito13]

For long channel devices or for devices of older processes with high allowed minimum length Vov(or Veff) equals Vdsat.
Additionally for hand calculations we assume Vdsat=Vgs-Vth for simplicity.
As for simulation and real design the criterion is Vds>Vdsat for saturation.

Take a look at chapter 3.7.3 of David Binkley's excellent book "Tradeoffs and Optimization in Analog CMOS Design" for more in-depth search.