A transistor is an active electronic component with three leads. One lead is connected to a power source, one is the output, and the other is a gate that controls how much power is applied to the output. Transistors are typically used as amplifiers because a small signal at the gate controls a large output.

There are several types of transistors and two of the more common categories, bipolar and field-effect transistor (FET), are described in separate documents. The following discussion describes some general operating principles of transistors.

Labeling Conventions

The different types of transistors: npn and pnp (n-channel and p-channel for FETs) refer to the bias between the collector and emitter (drain and source). Npn transistors are operated with the collector biased more positive than the emitter, and pnp transitors are operated with the emitter more positive than the collector.

VB: voltage at the base
IC: current through the collector
VBE: voltage drop between the base and emitter
VCC: collector power supply
VEE: emitter power supply


The base behaves like a diode connected to the emitter, therefore the base current increases rapidly at a VBE of approximately 0.6 V. At this voltage the transistor is said to be "turned on" and current flows from the collector to the emitter.

The collector current is controlled by VBE as given by the Ebers-Moll equation:
IC = IS [exp(VBE/VT) - 1]
where IS is the maximum current of the transistor (called saturation current) and VT = kT/e. k is Boltzmann's constant, T is absolute temperature, and e is the charge of an electron. VT is approximately 25 mV at room temperature.

Further Information

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Copyright © 1996 by Brian M. Tissue

updated 2/25/96