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
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
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 about 100.
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
As a beginner, by making wiring errors or making the transistor
dissipate too much heat, you will probably burn a few of them.
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