How a transistor works

How a transistor works

A transistor may be used to switch or to amplify. The image to the right represents a typical transistor in a circuit. Its three components are the "base", "emitter" and "collector" which correspond to regions of the mixed semiconductors from which the transistor is made. Current may flow from the emitter to the collector [or vice-versa, depending on the type of transistor] depending on the voltage applied to the base, but only if this voltage exceeds a certain value: [Typically of the order of 0.2 - 0.6 volts] this is depicted in the graph below at A and is referred to as Vbe.

Transistor as a switch

It can be seen from the graph that once the base voltage reaches a certain level, shown at B, no more current will flow and the output will be held at a fixed voltage. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off, [apart from a small value due to leakage currents] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Transistor as an amplifier

A varying base voltage, Vin, as long as it exceeds Vbe, controls current through the transistor and thus influences the output voltage Vout. The slope of the graph is such that small swings in Vin will produce large changes in Vout.

This occurs because the base voltage controls how much of the power supply voltage Vcc causes current through the transistor itself, and how much of it causes current through a load driven by Vout. It is important that the operating parameters of the transistor are chosen and the circuit designed such that as far as possible the transistor operates within a linear portion of the graph, such as that shown between A and B, otherwise the output signal will suffer distortion.

Footnotes

ee also

*Digital logic

External links

* http://userpages.wittenberg.edu/bshelburne/Comp150/LogicGatesCircuits.html


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