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Transmission gate
 
 
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A transmission gate is an electronic
Electronics

Electronics refers to the flow of charge through nonmetal electrical conductor , whereas electrical refers to the flow of charge through metal electrical conductor....
 element. It is a good non-mechanical relay
Relay

A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an magnet to open or close one or many sets of contacts....
, built with CMOS
CMOS

Complementary metal?oxide?semiconductor , is a major class of integrated circuits. CMOS technology is used in microprocessors, microcontrollers, Static Random Access Memory, and other digital logic circuits....
 technology. Sometimes known as an analog gate
Gate

A gate is a point of entry to a space enclosed by walls, or an opening in a fence. Gates may prevent or control entry or exit, or they may be merely decorative....
, analogue switch
Analogue switch

The analogue switch, also called the bilateral switch, is an electronics component that behaves in a similar way to a relay, but has no moving parts....
 or electronic relay depending on its use. It is made by the parallel combination of an nMOS
NMOS logic

nMOS logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 and a pMOS
PMOS logic

pMOS logic uses p-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. pMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 transistor with the input at the gate of one transistor being complementary to the input at the gate of the other.

rrent can flow through this element in either direction.






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Cmos Transmission Gate
A transmission gate is an electronic
Electronics

Electronics refers to the flow of charge through nonmetal electrical conductor , whereas electrical refers to the flow of charge through metal electrical conductor....
 element. It is a good non-mechanical relay
Relay

A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an magnet to open or close one or many sets of contacts....
, built with CMOS
CMOS

Complementary metal?oxide?semiconductor , is a major class of integrated circuits. CMOS technology is used in microprocessors, microcontrollers, Static Random Access Memory, and other digital logic circuits....
 technology. Sometimes known as an analog gate
Gate

A gate is a point of entry to a space enclosed by walls, or an opening in a fence. Gates may prevent or control entry or exit, or they may be merely decorative....
, analogue switch
Analogue switch

The analogue switch, also called the bilateral switch, is an electronics component that behaves in a similar way to a relay, but has no moving parts....
 or electronic relay depending on its use. It is made by the parallel combination of an nMOS
NMOS logic

nMOS logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 and a pMOS
PMOS logic

pMOS logic uses p-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. pMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 transistor with the input at the gate of one transistor being complementary to the input at the gate of the other.

Operation

A current can flow through this element in either direction. Depending on whether or not there is a voltage
Voltage

Electrical tension is the potential difference between two points of an electrical or electronic circuit, expressed in volts. It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor....
 on the gate, the connection between the input and output is either low-resistance or high-resistance, respectively. Ron = 100 ohm and Roff > 5 megohm.

The operation can also be understood this way: when the gate input to the nMOS
NMOS logic

nMOS logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 transistor is '0',and the complementary '1' is gate input to the pMOS
PMOS logic

pMOS logic uses p-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. pMOS transistors have three modes of operation: cut-off, triode, and saturation ....
, both are turned off. However when gate input to the nMOS
NMOS logic

nMOS logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 is '1' and its complementary '0' is the gate input to the pMOS
PMOS logic

pMOS logic uses p-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. pMOS transistors have three modes of operation: cut-off, triode, and saturation ....
, both are turned on and passes any signal '1' or '0' equally well without degradation. The use of transmission gates eliminates the undesirable threshold voltage effects which give rise to loss of logic levels in pass transistors.

The above logic was invented as a solution to problems of earlier cmos logics.It provides with a less no. of transistors needed for the implementation of a particular logic.

This logic can be used to design multiplexers.

Design

It would seem that a transmission gate could be constructed using simply a single pMOS
PMOS logic

pMOS logic uses p-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. pMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 or nMOS transistor. If only an individual nMOS
NMOS logic

nMOS logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 transistor were to be used, and there was a high voltage out the OUT and a low voltage on the IN and we are trying to transmit the zero to the OUT, then the nMOS
NMOS logic

nMOS logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 will drain some of the voltage but not all of it leaving the OUT somewhere in the 'no mans' voltage region of digital circuits. Adding the pMOS
PMOS logic

pMOS logic uses p-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. pMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 gate in parallel allows all the voltage to drain after the nMOS
NMOS logic

nMOS logic uses n-type metal-oxide-semiconductor field effect transistors to implement logic gates and other digital circuits. nMOS transistors have three modes of operation: cut-off, triode, and saturation ....
 shuts off before all the voltage is drained. This also solves the problem when transmitting a high voltage to OUT.¹

Sources

  • - Mark Lundstrom (Lecture 24)