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What will decide the VDS drop of 4 transistors bias in cascode structure?

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rampat

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Hi All

I have one very basic question.if you have 4 transistors (2 pmos 2 nmos) in cascode structure,what will decide the vds drop across each of the transistor assuming all are in saturation ?

thanks
rampat
 

Re: bias for cascode

I can't understand you but I think you should consider these equ.:
nmos:Vds > Vgs -Vth pmos: Vds < Vgs -Vth
 

Re: bias for cascode

The vds will be decided by the output resistance, multiplied by the biasing current.
If you attempt to force the same biasing current both from the pmos side and nmos side, they are unlikely to match, whcih would force one side into the linear region. That is why a CMFB is required for fully differential amplifiers to force the output stage to remain in saturation.
 

Re: bias for cascode

Well this is a bit problem. If you use the basic rule amir88 suggested you will overdesign . This rule is valid for large geometries (more than 1um gate length) since it does not include all the effects.
If you go lower I would recommend to use vdsat parameter from the model. Search BSIM3 model for vdsat explanation.
 

Re: bias for cascode

The VDS of the "master" devices will be the cascode bias
voltages, less VT of the "guard" devices (VGS @ ID setpoint).
The guards' VDS presumably carries the output signal of
interest and would vary widely in application. You could
assume a common-mode point for purposes of analysis.

An amplifier which bears out the assumption that all are
in saturation, will probably have a very low dynamic range
over which that remains true. Outside that range you
would expect to see gain degrade, offset accrue, etc.
 

Re: bias for cascode

Teddy said:
Well this is a bit problem. If you use the basic rule amir88 suggested you will overdesign . This rule is valid for large geometries (more than 1um gate length) since it does not include all the effects.
If you go lower I would recommend to use vdsat parameter from the model. Search BSIM3 model for vdsat explanation.
Click to expand...
Teddy is right.:D
 

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