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.

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.

1.

• note that the AC analysis is only valid for v_be < v_T
• short circuit any capacitor and open circuit any inductor. Short Circuit any DC power supply.

Single BJT Circuit Configuration

There are 3 configurations for the circuits with a single Bipolar Junction Transistor. They are the common emitter, common base and common collector circuits.