As shown in below, a transmission gate is comprised of a PMOS transistor and an NMOS transistor. When node A is high, both transistors conduct and IN and OUT are connected (bidirectional). When node A is low, both transistors are off and force a high impedance on both IN and OUT.
The common circuit symbol for transmission gate is as below:
Transmission gates are basic building blocks for logic circuitry, such as Flip-Flop, XOR gate, etc. The following diagram shows how a D-FF is built with transmission gate:
Here is a XOR gate:
Now let’s look at the sneaky leakage path caused by transmission gate in low power design. The following diagram is extracted from “Low Power Methodology Manual” book and the book can be found in: http://www.valpont.com/vlbgrefv/book-low-power-methodology-manual/
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