Transistors
A whisper of current controlling a shout — the amplifying switch that makes every modern machine possible.
Builds on: 3.1 Diodes & LEDs1.2 Ohm's Law
The problem it solves
Suppose a sensor produces a feeble signal — microamps, far too weak to light an LED, let alone spin a motor. You need a component where a small current controls a large one. That is the transistor, invented at Bell Labs in 1947, and comfortably the most manufactured object in human history — you own trillions.
The BJT: two junctions, three legs
The classic NPN bipolar transistor is a p–n junction sandwich with three terminals: collector, base, emitter. The rule of the device:
Feed a small current into the base (it costs about 0.7 V, like a diode) and the transistor permits β times more current to flow from collector to emitter. 50 µA in, 5 mA controlled. The transistor doesn’t create the big current — the supply provides it; the base just opens the tap.
Three regions, two jobs
- Cutoff — no base current, no collector current. The switch is OFF.
- Active — I_C tracks β·I_B proportionally. This is amplifier territory: audio, radio, sensors.
- Saturation — the base asks for more than the collector circuit can supply; the transistor is fully ON, dropping only ~0.2 V. Digital electronics lives at the two extremes: cutoff = 0, saturation = 1.
To use one as a switch: collector load (LED + resistor) to the supply, emitter to ground, and a base resistor sized so the “on” signal drives it well into saturation — with, say, 5× the minimum base current. The base resistor also protects the base junction, which is a diode and would otherwise gulp unlimited current (Lesson 3.1’s rule again!).
Modern chips use MOSFETs — transistors switched by voltage on an insulated gate rather than base current — because they can be shrunk to nanometres and switched billions of times a second. A CPU is simply billions of transistor switches wired into logic. The chip in your next lesson contains about twenty-five of them, which is exactly why it’s understandable — and why it’s the perfect bridge.
⚡ Lab — Small Current, Big Current
An NPN switch driving an LED. Watch the µA-vs-mA bars — they tell the whole story.
- Raise Vin slowly. Nothing until 0.7 V — the base is a diode.
- In the active region, check Ic/Ib ≈ 100 on the readout.
- Keep raising Vin: Ic hits a ceiling. That’s saturation — the LED circuit is giving all it has.
- With Rb = 1 MΩ, can you still reach saturation? Why not?