Bipolar Junction Transistors (NPN and PNP) and how to test them using digital multimeter

Bipolar junction transistors were the ones we started using long back for our hobby electronics projects. In those days we had PNP type of germanium transistors and NPN type of silicon transistors. Now germanium transistors are seldom seen and we have both NPN and PNP types of silicon transistors. The other varieties were unijunction transistors (UJT) and field effect transistors (FET) and so on. NPN and PNP designations are depending on the components of the transistors. N-type material has extra electrons  while P-type have holes or regions deficient in electrons. The different types are made by adding small amount of impurities to silicon by a process known as doping. NPN transistor has N-type material on either sides while the central portion is P-type. The composition of PNP transistor is the other way round.

The three leads of the transistor are known as collector, base and emitter. Base is almost analogous to the control grid in the vacuum tubes of yesteryears. Current gain or amplification occurs in transistors between the base and collector currents, with common emitter configuration. It is technically termed as hFE, which stands for Hybrid parameter Forward current gain, common Emitter. Another term is β of the transistor. It is measured by dividing the collector current by the base current in a common emitter configuration. Most digital multimeters have an option to check hFE of transistors.

The transistor has to be inserted in the NPN or PNP slots according to the pin-out diagram and the selector knob turned to the hFE option to read the value on the digital display. Here you can see that BC547B which is an NPN transistor has been inserted in the socket for NPN transistors and the selector knob has been turned to hFE on the dial. Digital display shows the hFE value as 256, which is the current gain of this device as measured by the digital multimeter.

BC557B is a common PNP transistor which comes in a TO-92 plastic package. TO-92 stands for Transistor Outline Package, Case Style 92, defined by the Joint Electron Device Engineering Council. Maximum collector-emitter voltage is rated as 45 V, emitter base voltage as 5 V and power dissipation as 500 mW. Typical current gain or hFE is 150, though it has a range upto 450. It is a general purpose PNP transistor. Corresponding NPN transistor is BC547B, which also comes in the TO-92 plastic package.