The bipolar junction transistor (commonly called just transistor) is a three terminal component, and one of the most used components in electronics. Many Integrated Circuits are based on transistors as well. It is used both as amplifier and as switch.

The transistor terminals are called Base, Emitter, and Collector. In general one of the terminals is used as input, one as output, and one as common (shared by both input and output part of the circuit). In most applications the base terminal is used as input and the emitter is the common that is somehow connected to the circuit ground (e.g. via a resistor or directly).

A transistor can be used as an electronic switch or as an amplifier, however in both applications it operates as a current amplifier (the larger output current is controlled by a smaller input current). By applying a current through resistors connected to the transistor we can translate the output current to an output voltage (thanks to Ohm’s law). The current amplification factor (gain) is often referred to as hfe or β (check a transistor datasheet).

Usage as switch

When a transistor is used as a switch, it is operated in two regions of its characteristic. A transistor switch can be, just like a regular switch, on or off. There are no intermediate states of ‘semi conductance’. The regions of the characteristic as switch are commonly known as the Saturation Region (i.e. switch closed) and the Cut-off Region.

Cut-off Region
When the transistor input or base current (Ib) is zero, the resulting output current or collector current (Ic) will also be zero (remember, it’s a current amplifier). When there is no collector current there will be no current flowing through the load, so no voltage difference across the load resulting in the maximum collector-emitter voltage (Vce). The transistor (and the load) is switched “Fully-Off”.

Saturation Region
Here the transistor will be controlled such that the maximum amount of base current is applied, resulting in maximum achievable collector (output) current (Ic,max) resulting in the minimum collector-emitter voltage drop. This in turn results in the maximum voltage difference across the load. And as such maximum efficiency. The transistor (and the load) is switched “Fully-On”.

Usage as amplifier

When used as amplifier, it is operated in a state of ‘semi conductance’. The output current is proportional to the input current. This mode is used in traditional audio output circuits, and for other signal amplifier purposes.

A bit of transistor physics

A Bipolar Junction Transistor (BJT) has three terminals called Base (B), Collector (C) and Emitter (E), each of which is connected to a doped semiconductor material region (either N-type or P-type). Bipolar transistors consist of either a P-N-P or an N-P-N semiconductor “sandwich” structure. A bipolar junction transistor uses both electron and hole charge carriers. In contrast, unipolar transistors, such as field-effect  transistors (FET), only use one kind of charge carrier.

Bipolar transistors work as current-controlled current regulators (as opposed to the FET which is voltage controlled). In other words, transistors restrict the amount of current passed according to a smaller, controlling current. The main current that is controlled goes from collector to emitter, or from emitter to collector, depending on the type of transistor it is (NPN or PNP, respectively). The small current that controls the main current goes from base to emitter, or from emitter to base, once again depending on the kind of transistor it is (NPN or PNP, respectively).

The polarity of NPN and PNP transistors is different, though for most applications a NPN transistor is needed.

Basic circuits

On the Instructables website you can find some basic circuits both for application as amplifier as well as switch.

Most important specifications

  • hfe or β: the DC current gain (usually between 50 and 500. If you need higher, using a Darlington transistor is an option)
  • Vbe,saturation: the base-emitter saturation voltage (usually around 0.7 V)
  • Ic,max: the maximum collector current that can be switched by the transistor (A)
  • Vce,max:the maximum collector-emitter voltage that the transistor can handle when switched off (So when Ib = 0 A).