Transistors

In one line

A transistor lets a small signal control a large one. Used as a switch, it turns things on and off with no moving parts; used as an amplifier, it makes a weak signal bigger. Billions of them, etched microscopically, make a processor.

How it works

A transistor has three terminals. A small current or voltage on the control terminal (the base of a BJT, the gate of a MOSFET) decides how much current flows through the other two. Two modes of use:

• Switch — control fully on or fully off. A microcontroller pin can only supply a trickle of current, so it switches a transistor that in turn drives a motor, pump, buzzer or bright LED.
• Amplifier — control held in between, so a small wiggle in the input becomes a large wiggle at the output. This is how millivolt biosignals get boosted to a level a converter can read.

Where it shows up in digital health

• Every chip — the microcontroller in a glucose meter, the processor in an ultrasound — is millions-to-billions of transistor switches.
• Driving actuators: a transistor (or a MOSFET / H-bridge of them) lets a low-power controller switch an infusion-pump motor or a valve.
• Front-end amplification of ECG, EMG and pulse signals starts with transistors (usually packaged as an op-amp).

Watch for

A MOSFET used as a power switch must be sized for the current and kept cool — a hot, under-rated switch is a common failure point in motor and heater circuits.

Key takeaways

• A transistor lets a small signal control a large one — as a switch or an amplifier.
• As a switch, it lets a low-power controller pin drive motors, pumps and bright LEDs.
• As an amplifier, it boosts millivolt biosignals (usually packaged as an op-amp).
• Billions of transistor switches are every processor; size power switches for current and heat.