“Transistors are electronic devices that can amplify voltages, currents and power by the application of a small signal voltage on one lead”
The transistor is another electronic device made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small change in current on one lead to produce a large change in voltage, current and power through the other two leads.
In electronic circuits, transistors have two basic functions: “switching” (digital electronics) or “amplification” (analogue electronics). Transistors are made by adding an additional semiconductor layer to a PN-junction diode. In fact bipolar transistors can be thought of as two diodes back-toback.
The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.
Bipolar transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type.
Bipolar Transistors Operating Regions
Bipolar transistors have the ability to operate within three different regions:
- Active Region – the transistor operates as an amplifier and Ic = β.Ib
- Saturation Region – the transistor is “Fully-ON” operating as a switch and Ic = I(saturation)
- Cut-off Region – the transistor is “Fully-OFF” operating as a switch and Ic = 0
Bipolar Transistor Configurations
The construction and circuit symbols for both the PNP and NPN bipolar transistor are given above with the arrow in the circuit symbol always showing the direction of “conventional current flow” between the base terminal and its emitter terminal. The direction of the arrow always points from the positive P-type region to the negative N-type region for both transistor types, exactly the same as for the standard diode symbol.