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Is transistor in saturation region?

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cjupiter

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saturation region

To check if a transistor in a circuit is saturated i know Vds > Vgs - Vth. Is Vds the voltage drop accross the transistor. For example, in my folded cascode amplifier, how would i check if transistors M1-M4 and M7-M11 are in saturation. Thanks for any help. :D
 

saturation region of transistor

Making sure that VDS>VGS-Vt is not a very good idea, because this relation is only (roughly) valid when the transistor is operating in strong inversion.
It may happen that VGS<Vt (there is still current - sub-threshold operation) and this does NOT mean that you can have a zero (or negative ...) VDS and still have the MOS in saturation.


I don't know exactly what simulator you are using. In HSPICE, for example, there is an indication of VDS and VDSsat. To have the transistor in saturation you must have VDS>VDSsat (with some margin, like 100 mV or so).

Another possibility is to compare the 'gm' and 'gds' of the transistor. When you are in saturation, the drain current is mostly defined by the VGS and the influence from the variations on VDS should be small. You should have gm (which relates the drain current variations with vgs variations) >> gds (which relates the drain current variations with vds variations).

Regards
 

saturation region in transistor

maxwellequ said:
In HSPICE, for example, there is an indication of VDS and VDSsat.
->in orcad9.1 capture-cis simulation(orcad pspice) how to indicate vds and vdssat.
->also how to use bsim4 in capture-cis
 

transistor saturation

Print the voltage of each net, then check Vgs Vds and Vt
You can decide which device is in saturation region
 

transistor in saturation region

When you do the simulation, make sure you have extract all the transistor on the output file!

Then, check the output file...check wish transistor is not is saturation region! :D
 

transistor saturation region

Just to add to the previous post - do an operating point simulation (for example in Pspice) and look in the .out file. At the end there is a list with the operating point for all transistors. Compare their Vds and Vsat. However, make sure that the output stage which you want to verify is in good region of operation. That means the output is not saturated at one of the rails. This will happen if you don't put feedback around the amplifier. You can also try to set the input voltages such that the output is somewhere in the middle but this is not a good approach. Better use feedback. This way you can also put the DC voltage at the output up or down from the center and see where axactly the output transistors go in linear.
Just out of curiosity, wht is the purpose of M11 and M12 - they seem to do nothing, just waste current.
 

saturation region

sutapanaki said:
Just out of curiosity, wht is the purpose of M11 and M12 - they seem to do nothing, just waste current.

Same with M16 and M17.
 

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