In this lecture, we will examine a simple switch circuit called the CMOS transmission gate (TG) or pass gate, and present a new class of logic circuits which use the TGs as their basic building blocks. The CMOS transmission gate consists of one nMOS and one pMOS transistor, connected in parallel. The gate voltages applied to these two transistors are also set to be complementary signals. As such, the CMOS TG operates as a bidirectional switch between the nodes A and B which is controlled by signal C. If the control signal C is logic-high, i.e., equal to VDD, then both transistors are turned on and provide a low-resistance current path between the nodes A and B. If, on the other hand, the control signal C is low, then both transistors will be off, and the path between the nodes A and B will be an open circuit. This condition is also called the high-impedance state. Combinational
MOS Logic Circuits Note that the substrate terminal of the nMOS transistor is connected to ground and the substrate terminal of the pMOS transistor is connected to VDD. Thus, we must take into account the substrate-bias effect for both transistors, depending on the bias conditions.