Elenco Basic Electronic Experiments - Manual (Page 30)

30
EXPERIMENT #17: STANDARD TRANSISTOR BIASING CIRCUIT
Connect the circuit and press the switch while turning the variable resistor from right to left (from 0
Ω to 50kΩ). The 100kΩ
and variable 50k
Ω are a voltage divider that sets the voltage at the transistor base. If this voltage is less than 0.7V then
the transistor will be off and no current will flow through the LED. As the base voltage increases above 0.7V a small base
current starts to flow, which is amplified to produce a larger collector current that lights the LED. As the base voltage
continues to increase the transistor becomes saturated and the LED brightness will not increase further.
This circuit will normally be used with the voltage divider set so that the transistor is turned on but is not saturated. Although
this circuit does not have many applications by itself, when a small alternating current (AC) signal is applied to the base
then a larger copy of the signal will appear at the collector - a small-signal amplifier!
+9V
1k
Ω
100k Ω
3.3k Ω
50k
Ω
VARIABLE
RESISTOR