Tab Electronics Guide to Understanding Electricity ... - Sciences Club

156 Chapter Six
Transistor Principles
Figure 6-2 shows an NPN transistor connected in a simple circuit to
illustrate basic transistor operation. A PNP transistor would operate in
exactly the same manner, only the voltage polarities would have to be
changed. Note that the emitter lead is connected to circuit common (the
most negative potential in the circuit) through the emitter resistor (R e
),
the base lead is connected to a potentiometer (P1), and the collector lead
is connected to 30 volts, through the collector resistor (R c
).
A transistor actually consists of two diode junctions: the base-to-emitter
junction and the base-to-collector junction. Assume that P1 is adjusted
to provide 1.7 volts to the base (note how P1 could provide any voltage to
the base from 0 to 30 volts). The 1.7-volt potential applied to the P-material
base, in reference to the N-material emitter at 0 volts (circuit common),
creates a forward-biased diode and causes current to flow from emitter to
base. However, because of a phenomenon known as transistor action, an
additional current will also flow from emitter to collector.
To understand transistor action, we have to consider several conditions
occurring simultaneously within the transistor. First, notice that the
base-to-collector junction is reverse-biased. The collector is at a much
Figure 6-2
Simple circuit to
illustrate basic
transistor operation.
30 Vdc
R C 10 k
Collector voltage
P1
N
P
N
Collector
Depletion area
Base
Emitter
R E 1 k
Emitter voltage