How many transistors can I stack on top of each other in single stage cascode?

[tobben24]

Hi,

How can i find out how many transistors I can stack on top of each other with a given Vdd before loosing to much output swing? I have a specification which states that my Vdd = 2V and an output swing of 90% of Vdd. I am using 0.35u.

 

[checkmate]

Re: Single stage cascode

That would leave you with 0.1V of headroom. Don't think you can afford cascodes with any technology.

 

[keremcant]

Re: Single stage cascode

hi,
for every transistor you put you loose an overdrive voltage, Vov. so assuming Vov = 0.2, you will be capable of using only one transistor, it seems.

 

[tobben24]

Re: Single stage cascode

Thanks for the answers But how can you calculate this Vov?

 

[keremcant]

Re: Single stage cascode

Vov is the difference between your Vgs and threshold voltage. i.e. Vov = Vgs - Tth.

the theory behind is this:

you know that the voltage drop on a trs is the voltage drop from drain to source, i. e Vds.

for an n-mos transistor you need Vds > Vgs - Vth for the trs to be in saturation region. this is given.


so for sat Vds > Vov. but when you bias your transistor for the maximum output swing you bias your transistor with a voltge s.t Vds is approximately equal to Vov or just a little bit larger. so for the hand calculation you can assume Vds = Vov. for an p-mos it is similar but Vds < Vov.

 

[tobben24]

Re: Single stage cascode

thanx keremcant