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Transmission gate VS nmos switch

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okiiatama

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nmos switch

Hi,

Transmission gate has its benefit that the total equivalent resistance almost independent of the output voltage
While the single nmos switch has varying on-resistance.

however i found that transmission gate is only popular in digital circuit.
In some high-frequency design such as the RF switch, normally nmos switch is used and some modifications is made, for example adding a shunt transistor or adding a dummy transistor.

What made the nmos switch is chosen rather than transmission gate?

What are the advantages of using nmos switch as compared to transmission gate?
Is that depends on the applications?the size? or the frequency?


Thanks!!
 

transmission gate design

I think the application and the real circumstance will decide you use a transmission gate or only P or N mos device. for example, when there is only ground availabe, it will be hard to connect the source of a PMOS. also, if the resistance from NMOS is small enough for your application, there is really no need to use the Tx gate.
 

transmission gate sizing

when trans low, NMOS works in linear region, just like a resistor;
when trans high, NMOS works in saturation region, connect as a diode, and also have large body effect;

PMOS have the reverse result.

use single MOS for switch can save die size, especially when many many switchs used. but the transmission voltage range should keep MOS in linear region.
 

pmos nmos switch

okiiatama said:
Hi,


In some high-frequency design such as the RF switch, normally nmos switch is used and some modifications is made, for example adding a shunt transistor or adding a dummy transistor.



Thanks!!
Click to expand...
any more detail on this "shunt transistor or dummy transistor"?
 

advantages of transmission gates

why the RF switch use nmos switch not Tx one?
 

nmos pmos switch

the idea is that u use Trans. gate to get arger ON region but with modern technology and knowing that Vth is not small , the ON region is not getting any benefits i.e. the PMOS and NMOS ON states are not connected together , this adds to the fact that charge injection optimum sizing isn't equal to the constant resistance "conductance" of the Transmission gate.
i remember reading a good lecture about this issue in Berkeley's ADCs course try to refer to it for more details
 

transmission gates dummies

When it comes to current control like in current mirror for ON/OFF purpose, there's no need to use transmission gate, as the path is a current path and the current flows thru the current mirror will not be affected by the device size of NMOS or PMOS. So ppl use as small as possible, also to reduce charge injection. When in comes to ADC design or switch cap design, there's always charge injection and ppl add dummy to resolve it. if you use transmission gate, the clocking input will not be perfect and you need to have two pairs of dummy to resolve charge injection, also harder to control clocking. So ppl seldom use in analog design or RF design
 

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