Digital Electronics: Principles, Devices and Applications

160 Digital Electronics+VDDQ7Q8Q5Q6Y=(A+B).(C+D)CQ 3 Q 4DAQ1Q2BFigure 5.44Two-wide, two-input OR-AND-INVERT gate.5.5.1.9 Transmission GateThe transmission gate, also called the bilateral switch, is exclusive to CMOS logic and does not havea counterpart in the TTL and ECL families. It is essentially a single-pole, single-throw (SPST) switch.The opening and closing operations can be controlled by externally applied logic levels. Figure 5.45(a)shows the circuit symbol. If a logic ‘0’ at the control input corresponds to an open switch, then alogic ‘1’ corresponds to a closed switch, and vice versa. The internal schematic of a transmission gateis nothing but a parallel connection of an N-channel MOSFET and a P-channel MOSFET with thecontrol input applied to the gates, as shown in Fig. 5.45(b). Control inputs to the gate terminals of twoMOSFETs are the complement of each other. This is ensured by an inbuilt inverter.When the control input is HIGH (logic ‘1’), both devices are conducting and the switch is closed.When the control input is LOW (logic ‘0’), both devices are open and therefore the switch is open. Itmay be mentioned here that there is no discrimination between input and output terminals. Either ofthe two can be treated as the input terminal for the purpose of applying input. This is made possibleby the symmetry of the two MOSFETs.It may also be mentioned here that the ON-resistance of a conducting MOSFET depends upondrain and source voltages. In the case of an N-channel MOSFET, if the source voltage is close toV DD , there is an increase in ON-resistance, leading to an increased voltage drop across the switch.A similar phenomenon is observed when the source voltage of a P-channel MOSFET is close to