Transistors are electronic devices made from semiconductors, such as silicon or germanium. They mainly function as amplifiers or switches. A bipolar transistor is composed of three layers that are called the base, emitter and collector. The base is the middle layer, and it controls the behavior of the others by behaving like a gate. Each layer has a metal lead for connection into a circuit.
An NPN bipolar transistor is so called because the outer layers are N-type semiconductors, while the base is a P-type. N stands for negative charge carriers or electrons, and P for positive charge carriers or holes.
A common emitter or CE circuit is used for amplification. A small signal introduced into the base produces a larger signal at the output. It has the emitter lead connected to ground. It is usually built with at least two resistors, with one at the base and the other at the collector.
The circuit has two loops, where one is called the base loop and the other the collector loop. The loops are found by using Kirchoff’s Law to follow the path between the supplied voltage and the transistor leads. Ohm’s Law is also used. It is V = IR, where V is the voltage, I the current and R is the resistance.
The transistor gain, or dc beta, is the ratio of the collector current IC to the base current IB, and is symbolized as Bdc, where B is the Greek letter beta. It is also called Hfe. The gain tells how much the input signal is amplified. It is a constant that depends on the transistor type.
NPN transistors may be modeled as two back-to-back diodes in what is called the Ebers-Moll model. The base-emitter behaves like a forward-biased diode, while the base-collector behaves like a reverse-biased diode. Forward biased means that the voltage is applied is in a conducting direction, while reverse-biased means that the voltage is applied against easy current flow.
Input characteristics are found by considering the base loop.
A graph of the base current IB versus VBE, which is the voltage between the base and the emitter, looks like that of an ordinary diode. The current is zero until VBE reaches 0.7 volts, where it then increases very suddenly.
The base voltage forward biases the emitter. The equation to find the voltage across the resistor RB is VBB – VBE, where VBB is the base voltage. The current IB is found using VBB – VBE / RB.
Output characteristics are found by considering the collector loop.
A graph of the collector current IC versus the collector-emitter voltage VCE shows much the same shape for different transistors, though the numbers will be different. When VCE is zero, so is IC. As VCE increases, IC will remain zero and then suddenly shoot up when the voltage reaches a certain value, much the same way as IB. Unlike IB, IC will reach a plateau and then remain basically constant as VCE increases. The graph illustrates that IC = Bdc * IB, or that a small increase in IB leads to a large increase in IC.
IB will be constant until the breakdown region of the transistor is reached. This region is where the transistor will become damaged when the voltage is too large, and is dependent on the transistor type. IB will rapidly increase when the breakdown voltage is reached.
The collector voltage reverse-biases the collector. The collector-emitter voltage is equal to the collector voltage minus the voltage across the collector resistor. It is VCE = VC – IC * RC.