A series pass-transistor (or simply, pass-transistor) is a transistor configuration that is connected in series with the input and output sections of a circuit, as shown in Fig.3.37. In this diagram, we have shown an NMOS transistor in the pass-transistor mode. But, any transistor (BJT or FET) connected in this manner is called a pass-transistor. Figure 3.38 shows a pass-transistor structure using CMOS transistors. This structure is also known as transmission gate (TG). In this mode of operation, the structure acts like an analog switch. But the same structure can be used in the construction various logic gates. Such gates are called as pass-transistor logic (PTL) gates. It can be seen that TG is a bidirectional switch and can conduct in both directions.
