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# transmission gate question

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#### anantha_09

##### Member level 4
why dont we use only either p or n mos device in Transmission gate

also

why dont we size the p mos and n mos devices in TG unlike we do in cmos devices.

PMOS transistors are able to pass "0" levels and NMOS are able to pass "1" levels. that config allows for the noninverting propagation of the input signal.

Hi,
PMOS effective in passing '1' and NMOS in '0', both combination makes the transfer of data without any degradation, these are used in gating and hence in flip flops etc.

Yeah... PMOS can pass a strong one and a NMOS can pass a strong zero...And i am not sure whether we wont size the transistors in transmission gates....as far as i know it should be sized... size it will be uneven resistance for zeros and ones...

a common way to size transmission gate is to make both pmos and nmos the same size though this will make the rising edge delay and the falling edge delay different, but this is also true for dynamic logic and skewed cmos

why do we make both pmos and nmos of same size in transmission gates...?

Resistance will be different between high and low signal, if they are different size.

Hi,
Please go through the details about Transmission gate, How can we size them...
let me know if any thing is unclear!!

I think that the P Mos must be larger (2 times) that N Mos because of the mobility of carriers.

this type of implementation of the Transmission Gate (by pmos and nmos) makes it suitable to be used, along with inverters, to build any two-input logic gate...and this is according to Shannon's expansion theorem:

F(A,B) = A F(1,B) + Abar F(0,B)

So if you have 2 transmission gates...the input of one of them would be connected to F(1,B) and its control is A on NMOS and Abar on PMOS...and the input of the other would be F(0,B) and its control is Abar on NMOS and A on PMOS...the output of both is just one wire=> F(A,B)...which takes its value according to the level of A

Hi all,
Salma could you please explain more the tchnique you mensionned coz I couldnt get it. Please if you have material on this matter upload it.
Thanks a lot.

I hope these help and explain a little:

Shannon's Expansion states that:

F(A,B)= A F(1,B)+ Abar F(0,B)

Great !
Chahia tayba !
Regards,
MasterPicengineer

Thanks a lot...
I'll leave the tech side of me now and go continue with the cooking
Heheheee..!

useful.. material....

yes very much useful my thanxxxx t oall