Transmission gate
A transmission gate is an analog gate similar to a relay that can conduct in both directions or block by a control signal with almost any voltage potential. It is a CMOS-based switch using a p-channel MOSFET that passes a strong 1 but a poor 0, and a n-channel MOSFET that passes a strong 0 but a poor 1. Both the PMOS and NMOS work simultaneously.
Structure
In principle, a transmission gate is made up of two field-effect transistors, in which – in contrast to traditional discrete field-effect transistors – the substrate terminal is not connected internally to the source terminal. The two transistors, an n-channel MOSFET and a p-channel MOSFET, are connected in parallel with the drain and source terminals of the two transistors connected together. Their gate terminals are connected to each other by a NOT gate, to form the control terminal.Unlike with discrete FETs, the substrate terminal is not connected to the source connection. Instead, the substrate terminals are connected to the respective supply potential in order to ensure that the parasitic substrate diode is always reversely biased and so does not affect signal flow. The substrate terminal of the p-channel MOSFET is thus connected to the positive supply potential, and the substrate terminal of the n-channel MOSFET connected to the negative supply potential.
Function
When the control input is a logic zero, the gate of the n-channel MOSFET is also at a negative supply voltage potential. The gate terminal of the p-channel MOSFET is caused by the inverter, to the positive supply voltage potential. Regardless of on which switching terminal of the transmission gate a voltage is applied, the gate-source voltage of the n-channel MOSFETs is always negative, and the p-channel MOSFETs is always positive. Accordingly, neither of the two transistors will conduct and the transmission gate turns off.When the control input is a logic one, the gate terminal of the n-channel MOSFETs is located at a positive supply voltage potential. By the inverter, the gate terminal of the p-channel MOSFETs is now at a negative supply voltage potential. As the substrate terminal of the transistors is not connected to the source terminal, the drain and source terminals are almost equal and the transistors start conducting at a voltage difference between the gate terminal and one of these conducts.
One of the switching terminals of the transmission gate is raised to a voltage near the negative supply voltage, a positive gate-source voltage will occur at the N-channel MOSFET, and the transistor begins to conduct, and the transmission gate conducts. The voltage at one of the switching terminals of the transmission gate is now raised continuously up to the positive supply voltage potential, so the gate-source voltage is reduced on the n-channel MOSFET, and this begins to turn off. At the same time, the p-channel MOSFET has a negative gate-source voltage builds up, whereby this transistor starts to conduct and the transmission gate switches.
Thereby it is achieved that the transmission gate passes over the entire voltage range. The transition resistance of the transmission gate varies depending upon the voltage to be switched, and corresponds to a superposition of the resistance curves of the two transistors.