Page 198 - Physics Form 5 KSSM_Neat

Transistors are widely used in digital circuits like Info
computers. What are the characteristics of transistor
circuits? How are npn transistors and pnp transistors For pnp transistors, the polarity of
connected in a circuit? the battery for both base circuit and
collector circuit has to be reversed.
A transistor circuit consist of two main parts Hence the directions of flow of the
which are a base circuit and a collector circuit. base current, I , collector current,
B
KEMENTERIAN PENDIDIKAN MALAYSIA
Figure 5.13 shows an npn transistor circuit and a pnp I and emitter current, I are also
E
C
reversed.
transistor circuits.

L L
2 2
I I
L C + L C –
1 R 1 R
B I B I
B 6 V B 6 V
– +
V V
BE BE
I I
E E
+ – – +
(a) npn transistor (b) pnp transistor
Figure 5.13 Transistor circuits

• When the switch is open, bulb L does not light up as the circuit is incomplete and the base
1
current, I is zero.
B
• Bulb L does not light up although the collector circuit is complete because the transistor is
2
not turned on and the collector current, I is zero.
c
• When the switch is closed, bulb L is dim because R has a high resistance and the base
B
1
current, I is very small. Bulb L lights up very brightly because the collector current, I is
c
B
2
large compared with the base current, I .
B
• A small base current can produce a minimum base voltage, V to turn on the collector
BE
circuit.
• The base current, I can control the flow of the collector current, I . This enables the
c
B
transistor to act as a switch.
• The resistance at the base circuit, R is large to limit the base current, I so that the
B
B
transistor will not become too hot and burn.

188 LS 5.3.1