Re: Transmission Gate question

Fets are controlled by the gate source voltage Vgs and its relationship
to the threshold voltage. When the fets are fully turned on, they act
as a resistance equivalent to the Rds-on parameter. Think of the
devices as being switches, either on or off and then modeling them as a
small or large resistance.

Be aware, the input - output side of the fets (drain and source) are
not really interchangable. I was taught that they were in school and
got bit by this on a circuit board design. The result was $500 and two
weeks down the toilet. I didn't understand it at first, until I
realized I was trying to turn the device on by controlling Vds not Vgs.

Most text books focus too much on the Id = k(Vgs - vt)^2 equation.
Unless you are getting into semiconductor physics, this equation is
probably about worthless. It may help for you to look at some product
data sheets for fets and look at the curves relating on resistance
versus the Vds and Vgs.

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