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How Does a BJT Switch and Amplify Signals?

  • Basic
  • Article
  • Ages 14+
  • 4 min read
A small black BJT transistor with a rounded body and three metal leads stands on a test board.

A BJT is a semiconductor component with three pins: collector, base and emitter. A small current at the base pin steers the main current running between collector and emitter. That lets it work as an electronic switch that turns a load on and off, or amplify a weak signal into a stronger one.

Contents
The three pins of a BJT: one control pin and two for the main current.
  • Base pin
  • Collector pin
  • Emitter pin

What you will learn

  • Name the three pins of a BJT correctly.
  • Explain the two modes in which a BJT acts as a switch.
  • Describe how a BJT amplifies a signal.

Before you start

It helps to know what current, voltage and a diode are.

What pins and types does a BJT have?

This introduction presents bipolar junction (BJT) and metal-oxide field-effect (MOSFET) transistors as two basic types, then focuses on the BJT. Its three pins are called collector (C), base (B) and emitter (E).

The BJT comes in two versions: NPN and PNP. Their symbols differ in the direction of the emitter arrow; in active or saturation mode, conventional main current flows from emitter to collector in a PNP, opposite to an NPN.

Only the arrow direction on the emitter tells NPN from PNP.
  • Arrow pointing out, NPN
  • Arrow pointing in, PNP

How does a BJT work as a switch?

A transistor is like a water valve, a mechanism for controlling the flow rate. In an NPN example, the base pin is like a handle that adjusts how many electrons flow from emitter to collector. In active or saturation mode, conventional main current flows from collector to emitter in an NPN and from emitter to collector in a PNP.

In the ideal cutoff model, collector and emitter currents are zero; a real device still has small leakage. In a simplified switch model, a saturated BJT acts almost like a short circuit between collector and emitter, though a real device still has a small voltage between those pins. Driven in either of these modes, the transistor creates the binary on/off effect of a switch.

Cutoff is nearly open; saturation is a conducting state with low collector-emitter voltage.
  • Off (cutoff)
  • Conducting (saturation)
  • Load switched on

How does a BJT amplify a signal?

Transistors can amplify electrical signals, producing an output with a similar shape but higher power. In forward active mode, collector current magnitude is approximately proportional to base current; gain depends on the device and operating conditions. The output can be inverted depending on the circuit.

Transistors are non-linear devices, unlike resistors, which keep a linear relationship between voltage and current. So the voltages on each pin tell you which mode it is in.

A small signal controls a stronger signal; depending on the circuit, the output can be inverted.
  • Small signal at the base
  • Transistor
  • Stronger signal

Real-world example

With an adult, look at a circuit that drives an indicator light. The microcontroller supplies a signal to the base, while the transistor switches the part of the circuit with the light. A series resistor at the base limits current; its value must suit the transistor and control signal.

Common misunderstandings

A transistor is only a switch. Transistors can amplify electrical signals, turning a low-power signal into a stronger one.

A BJT always acts like a fixed resistor. A BJT is non-linear; its state depends on its operating region and base current.

Frequently asked questions

How many pins does a BJT have and what are they called?

A BJT has three pins: collector, base and emitter.

What is the difference between NPN and PNP?

Their symbols differ in the direction of the emitter arrow; in active or saturation mode, conventional main current flows from emitter to collector in a PNP, opposite to an NPN.

When is a transistor a switch?

When it is driven in cutoff or saturation mode, the transistor creates the on/off effect of a switch.

What does the base pin do?

In an NPN example, the base adjusts how many electrons flow from emitter to collector; conventional main current in a PNP flows from emitter to collector in active or saturation mode.

Summary

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

Transistor
A semiconductor component used as an electronic switch or to amplify signals.
Bipolar junction transistorTransistor lưỡng cực
A transistor with collector, base and emitter pins, controlled by current at the base.
BaseChân base
The base pin controls BJT operation; it can be pictured like the handle of a water valve.
Collector and emitterCollector và emitter
The two main pins of a BJT; the main current runs between them when the transistor conducts.
Cutoff and saturationChế độ ngắt và bão hòa
The two switch states of a transistor: off or conducting with a low collector-emitter voltage.

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Sources

These sources are written for older readers. If they feel hard, read them with an adult.