CMOS switch operation

A CMOS switch (transmission gate) is an electronic switch made of an NMOS and a PMOS transistor connected in parallel, with opposite gate control signals. It passes analog or digital signals in both directions with low resistance when ON, and blocks them when OFF.

Circuit

              +---------+
 IN o---------|S  NMOS D|--------o OUT
         |    |    G    |   |
         |    +----|----+   |
         |         C        |
         |    +----|----+   |
         +----|S  PMOS D|---+
              |    G    |
              +----|----+
                   C'   (C' = inverted control)

The NMOS gate is driven by control C; the PMOS gate is driven by the inverse C' (through an inverter).

Operation

• C = 1 (VDD), C' = 0: both transistors are ON. The signal passes from IN to OUT with a low ON resistance (a few hundred ohms): switch closed.
• C = 0, C' = VDD: both transistors are OFF. The path is open with very high resistance: switch open.

Why both transistors?

• An NMOS alone passes low voltages well but a high level is degraded (it can pass only up to VDD − VT).
• A PMOS alone passes high voltages well but not low voltages.
• Connected in parallel, NMOS conducts for low input levels and PMOS for high input levels, so the full range 0 to VDD is passed, with ON resistance almost constant.

Applications: analog multiplexers, sample-and-hold circuits, analog switches in ADC/DAC, digital multiplexers, and latches.