Bipolar Junction Transistors (BJTs)

A Bipolar Junction Transistor is a three terminals active semiconductor device formed by two back-to-back p-n junctions. The three terminals are labeled base, emitter and collector. Its main function is the current amplification between the collector or emitter currents and the base current.

There are 2 types of Bipolar Junction Transistor (BJT), the NPN and the PNP.

Circuit Analysis

BJT Models

To analyse a transistor circuit,

  1. Do a DC analysis by redrawing the schematic
  2. If an AC analysis is required, redraw the schematic

BJT CE Amplifier

Analyse the BJT Common Emitter Amplifier

Configuration Characteristics

Common Emitter

Common Collector

Common Base

Input

Base

Base

Emitter

Output

Collector

Emitter

Collector

Input/Output Phase relationship

180o

0o

0o

Voltage Gain

Medium

Unity

High

Current Gain

Medium

High

Unity

Input Impedance

Medium

High

Low

Output Impedance

Medium

Low

High