Basic Electronic Theory 03

Harold Hall



Basically, these are simple amplifying devices. They can though be made to perform a multitude of functions by means of the external components connected to them. Reference to a catalogue will bring up a vast number of transistors, but even so, they will all fit into two different forms known as PNP and NPN types. Explaining their differences is though beyond what this article is attempting to achieve.


Sk 7 includes the symbols for the two types showing that they both have the same three connections, Base, emitter and collector and as far as this article is concerned can be considered to work identically. The input current is applied to the Base which then causes a much larger current to flow between the Emitter and the Collector.


Workshop Data


If the collector was connected directly to the power rail the collector voltage would be unable to vary and there would be no output. This is overcome by the dropper resistance between the power rail and the collector.


The level of current entering the Base is though critical and normally would be more than the input signal would provide, because of this a standing current has to be provided on which the signal is superimposed. The two additional resistors shown on Sk 8 are one method of providing this extra current.


The input can be a switched input such as the inputs to the gates, earlier,a gradual change in input, typically to control the acceleration of a motor, or at audio frequencies and above.


The wide range of uses for the transistor make it impossible to go into more detail. However, the above should help the viewer analyse an existing circuit.

The voltage across this will vary with the changes in current that the transistor dictates and as a result the collector voltage will vary in line with the input but at a greater amplitude. Also, an increasing input current will result in a greater current though the transistor and the resistor, resulting in an increase voltage across the resistor. As this is connected to the fixed voltage power rail the voltage at the collector will reduce. Therefore, whilst the output will be amplified and follow the input signal it will be inverted, see Sk 8.