Describe the bipolar transistor, name its three terminals, and explain how a small base current controls a larger collector current

What this dot point is asking

SEAB wants you to describe the bipolar transistor, name its three terminals, and explain how a small base current controls a much larger collector current. The central insight is that a transistor is a current-controlled valve: a tiny current into the base allows a large current to flow from collector to emitter, which lets a weak signal control a powerful load and is the basis of both switching and amplification.

The answer

The three terminals

A bipolar junction transistor has three leads:

• The base (B) is the control terminal. A small current here controls the transistor.
• The collector (C) is where the main current enters (for an npn type).
• The emitter (E) is where the main current leaves; its arrow on the symbol shows the direction of conventional current.

The most common type in the syllabus is the npn transistor, drawn with the emitter arrow pointing outwards.

Current control

The key behaviour is that a small base current $I_B$ controls a much larger collector current $I_C$. The transistor does not create energy; it acts as a valve, letting the supply push a large current through the collector and emitter in response to the small base current. This is why a transistor can let a low-power circuit (a sensor, a logic gate) control a high-power load (a motor, a lamp).

Current gain

The ratio of collector current to base current is the current gain, given the symbol $\beta$ (or $h_{FE}$):

A typical small transistor has a gain of $100$ or more, so a base current of $0.05\ \text{mA}$ can control a collector current of $5\ \text{mA}$. Rearranged, $I_C = \beta I_B$ lets you find the collector current from the base current.

The transistor as a switch

Used as a switch, the transistor has two states:

• Off: if the base-emitter voltage is below about $0.7\ \text{V}$, no base current flows, so no collector current flows. The transistor behaves like an open switch.
• On: once the base-emitter voltage reaches about $0.7\ \text{V}$, a base current flows and a large collector current is allowed through. The transistor behaves like a closed switch.

A base resistor is used to limit the base current to a safe value while still being enough to turn the transistor fully on.

Examples in context

Example 1. Turning on a motor from a logic chip. A logic gate can only supply a few milliamperes, far too little to run a motor. Its output feeds the base of a transistor through a resistor, and the transistor switches the much larger motor current from the supply. The transistor is the muscle that lets a thinking circuit move a real load.

Example 2. A touch-sensitive lamp. A tiny current leaking through a fingertip into the base of a high-gain transistor is enough to switch on a lamp drawing tens of milliamperes. The huge current gain turns an almost undetectable input into a clear, useful output, showing why the transistor is the building block of modern electronics.

Try this

• Cue. Name the three terminals of a bipolar transistor and state which one controls it. The base, collector and emitter; the base is the control terminal whose small current controls the larger collector current.

• Cue. A transistor has $I_C = 8.0\ \text{mA}$ and $I_B = 0.04\ \text{mA}$. Find its current gain. $\beta = I_C / I_B = 8.0 / 0.04 = 200$.

• Cue. Explain why a base resistor is used when switching a transistor. It limits the base current to a safe value while still being large enough to turn the transistor fully on.