Design of a Transmission Gate

[Jheof32]

Hi, currently I'm working on my Thesis about Fractional Order Differentiator. For my switch, I used transmission gate. Unfortunately, it does not allow my input signal to totally pass through the switch. Thus, I'm having a problem with my output signal. Do you have something to suggest for me a design of a tgate or a dimension though of a tgate using 0.35 um Technology more specifically for analog signal? Thank you very much.

 

[erikl]

For my switch, I used transmission gate. Unfortunately, it does not allow my input signal to totally pass through the switch. Thus, I'm having a problem with my output signal.

It will never pass totally. If your output impedance (the source in front of the switch) is Z1, your switch's impedance Z2, and your input impedance (the "receiver" behind the switch) Z3, you'll get a fraction Z3/(Z1+Z2+Z3). So just see that your input impedance Z3 is large in comparison to those of the 2 others.

 

[Jheof32]

It will never pass totally. If your output impedance (the source in front of the switch) is Z1, your switch's impedance Z2, and your input impedance (the "receiver" behind the switch) Z3, you'll get a fraction Z3/(Z1+Z2+Z3). So just see that your input impedance Z3 is large in comparison to those of the 2 others.

Would you suggest a dimension of a tgate for switching? Or any piece of advice for such? Thanks

 

[erikl]

Would you suggest a dimension of a tgate for switching? Or any piece of advice for such?

May I repeat: The dimension of Z2 (s.a.) depends on Z1 & Z3. If you know the latter ones, you can decide on Z2 and the dimensions of its transistor(s). Done this, you can analyse and verify the fraction which is transferred to the receiving gate.

If Z3 » Z1 , probably min. dimensions will do.

 

[FvM]

Jheof32 didn't tell how his transmission gate looks like. We don't even know if it's regular transmission gate using P and N FETS.

 

[erikl]

True, but for consideration of dimensions this doesn't (yet) matter, I think.

But you're right: some more info would be useful.

 

[FvM]

You could e.g. use a NMOS transistor as transmission gate and shouldn't be surprized if it doesn't "totally pass through" the input signal.

 

[Dominik Przyborowski]

Another question is about DC levels on input and output od this tgate and also about maximum signal speed.