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.
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
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.