Data Sheet

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107. NPN Transistor Basics
Build the circuit to the left and you will see the lamp (76) light. The base
of the NPN transistor (50) is connected to the 4.5V terminal from the
battery through the 1kW resistor (42), enabling current to ow into the
base of the NPN transistor (50), which turns on the NPN transistor (50)
enabling current to ow through the lamp (76) into the collector and
out of the emitter, which turns on the lamp (76). The NPN Transistor
(50) is built by stacking three different layers of semiconductor material
together. Two layers have extra electrons added to them (a process
called “doping”) and are called N-type layers, while one layer has
electrons removed (doped with “holes” – the absence of electrons) and
are called P-type layers. This is shown in the diagram below.
Roughly speaking, the NPN transistor (50) is designed so that
conventional current can easily ow from the Base region (P-type) to
the Emitter region (N-type) when the Base is at a high enough voltage
above the Emitter. Once current begins owing from Base to Emitter,
then it becomes much easier for current to ow from the Collector to
the Emitter.
108. Voltage Divider
Build the circuit shown on the left and the lamp (76) will be on. This type
of circuit is often used to hold the voltage level at the Base of the NPN
transistor (50) at a constant level to set a particular operating point for
the NPN transistor (50).
Emitter Collector
Base