Introduction to the transistor
The purpose of this text is to give a basic understanding
of the behaviour of a transistor. It is made for readers
with some knowledge about electricity: voltage, current,
resistors, batteries, Ohm's law. Technical problems like
non-linearity and behaviour under
high frequencies are not talked about.
Description of the transistor
The transistor is a component with 3 electric wires coming out of it.
They are named B (base), C (collector), and E (emitter).
This is a drawing of the BC 547 transistor, four times bigger:
Such a transistor costs $0.3 in electric components stores.
Here is a classic drawing for a transistor inside electronic diagrams:
How it is used
- If one connects a tension source between the wires C and E, the
transistor will not let any current trough (fig. 1).
- But between B and E there is a shortcut. If one wants to make a
given current go trough B and E, one must use a tension source and a
resistor (fig. 2).
- If one sends a current of IB amperes between B
and E, then the resistor will allow a current of IC = ß . IB
amperes pass between C et E (fig. 3). In this case, ß is
The electronic diagrams corresponding to figures 1, 2 and 3 are figures
4, 5 and 6:
Note: For those who would like to try out these diagrams, one sole
battery of 9 Volts can replace the two batteries
(fig. 7 and 8):
Be careful for the polarity: put the positive wire and the negative
wire of the battery on the right place. The direction of the current is
very important for a transistor.
The BC 547 is a somewhat weak transistor to make a lamp light up.
Perhaps you will get better results using a stronger transistor, for
example the BD 649. Here is a drawing of it, two times bigger:
As a beginner, by making wiring errors or making the transistor
dissipate too much heat, you will probably burn a few of them. That's
The reason why one subtracts systematically 0.7 Volts from the UBE
tension is that bipolar transistors contain sort of "parasite" diode.
The tension that must be subtracted depends on the sort of
semiconductor: 0.7 Volts for silicon, 0.2 Volts for