Analog Switches#

An analog switch lets a digital signal turn an analog one on and off. Where a logic gate can only handle clean highs and lows, an analog switch passes whatever voltage sits on it — an audio signal, a sensor output, a reference — while a logic-level control decides whether the path is open or closed. It is the fundamental primitive for routing analog signals under digital control.

The Transmission Gate#

The building block is the CMOS transmission gate: an NMOS and a PMOS transistor wired in parallel, driven by complementary control signals. When enabled, the pair conducts in either direction and passes any voltage between the supply rails; when disabled, the path is open. Because the two transistor types cover opposite ends of the voltage range, together they pass a signal cleanly across the whole rail-to-rail span, which a single transistor could not.

This one cell is what the glue-logic parts are made of: a 4066 is four transmission gates, and an analog multiplexer like the 4051 is a tree of them selected by an address. It is also the switch inside a sample-and-hold.

What Makes It Non-Ideal#

An analog switch is not a perfect relay, and its imperfections shape how it is used:

• On-resistance (Ron) — a closed switch has tens to hundreds of ohms, and that resistance varies a little with the signal level, which can distort the signal into a low-impedance load. Feeding high-impedance loads keeps it negligible.
• Rail limits — it can only pass voltages between its own supplies; anything outside is clipped or conducts through the protection structures.
• Charge injection — opening the switch dumps a small packet of charge onto the signal, a brief offset that matters most in precision and sample-and-hold circuits.

For switching power rather than signals, this is the wrong tool — that is a job for a MOSFET, relay, or solid-state relay. The analog switch’s domain is small-signal routing, controlled by logic.