A BC547 transistor is a negative-positive-negative (NPN) transistor that is used for many purposes. Together with other electronic components, such as resistors, coils, and capacitors, it can be used as the active component for switches and amplifiers. Like all other NPN transistors, this type has an emitter terminal, a base or control terminal, and a collector terminal. In a typical configuration, the current flowing from the base to the emitter controls the collector current. A short vertical line, which is the base, can indicate the transistor schematic for an NPN transistor, and the emitter, which is a diagonal line connecting to the base, is an arrowhead pointing away from the base.
There are various types of transistors, and the BC547 is a bipolar junction transistor (BJT). There are also transistors that have one junction, such as the junction field-effect transistor, or no junctions at all, such as the metal oxide field-effect transistor (MOSFET). During the design and manufacture of transistors, the characteristics can be predefined and achieved. The negative (N)-type material inside an NPN transistor has an excess of electrons, while the positive (P)-type material has a lack of electrons, both due to a contamination process called doping.
The BC547 transistor comes in one package. When several are placed in a single package, it is usually referred to as a transistor array. Arrays are commonly used in digital switching. Eight transistors may be placed in one package to make layout much easier, for example.
To make use of a transistor as an audio preamplifier, a direct current (DC) source is needed, such as a 12-volt (V) power supply. In a common emitter configuration, the negative side of the power supply is alternating current (AC)-coupled to the emitter via a capacitor. There is also a small resistance connecting the power supply to the emitter. The power supply is then connected to the collector via a resistor, which may be referred to as a limiting resistor. When the collector-to-emitter current flows, there will be a voltage drop in the limiting resistor, and in the idle state, the collector voltage is typically 6 V.
Transistor circuit design requires a thorough understanding of current-voltage ratings of various components, such as transistors and resistors. One goal is to keep the components from burning up, while another is to make the circuit work. Saving electricity is also important, such as in the case of battery-operated devices.