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

To analyse a transistor circuit,

1. Do a DC analysis by redrawing the schematic
• replace the BJT symbol with its DC model.
• open circuit any capacitor and short circuit any inductor.
2. If an AC analysis is required, redraw the schematic
• by replacing the BJT symbol with the small signal model.
• calculate r_e using I_E from the DC analysis and v_T=26 mV.

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	180^o	0^o	0^o
Voltage Gain	Medium	Unity	High
Current Gain	Medium	High	Unity
Input Impedance	Medium	High	Low
Output Impedance	Medium	Low	High