Transmission gate
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A transmission gate is an electronic element. It is a good non-mechanical relay, built with CMOS technology. Sometimes known as an analog gate, analogue switch or electronic relay depending on its use. It is made by the parallel combination of an nMOS and a pMOS transistor with the input at the gate of one transistor (nMOS) being complementary to the input at the gate of the other (pMOS).
[edit] Operation
A current can flow through this element in either direction. Depending on whether or not there is a voltage 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 transistor is '0',and the complementary '1' is gate input to the pMOS,both are turned off. However when gate input to the nMOS is '1' and its complementary '0' is the gate input to the pMOS, 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.
