CMOS Transmission Gate

CMOS Transmission Gate

Introduction

In electronics, pass transistor logic (PTL) describes several logic families used in the design of integrated circuits. It reduces the count of transistors used to make different logic gates, by eliminating redundant transistors. Transistors are used as switches to pass logic levels between nodes of a circuit, instead of as switches connected directly to supply voltages. This reduces the number of active devices, but has the disadvantage that the difference of the voltage between high and low logic levels decreases at each stage.

Basic Principles of Pass Transistor Logic

Basic Principles of Pass Transistor Logic

The pass transistor is driven by a periodic clock signal and acts as an access switch to either charge up or charge down the parasitic capacitance Cx, depending on the input signal Vin. Thus, two possible operations when the clock signal is active (CK = 1) are the logic "1" transfer (charging up the capacitance Cx to a logic-high level) and the logic "0" transfer (charging down the capacitance Cx to a logic-low level). In either case, the output of the depletion load nMOS inverter obviously assumes a logic-low or a logic-high level, depending upon the voltage Vx.

Applications

Pass transistor logic often uses fewer transistors, runs faster, and requires less power than the same function implemented with the same transistors in fully complementary CMOS logic.