Download Transistor characteristics

Transistor characteristics
The characteristics of the npn transistor can be found using the circuit shown in Figure 1
IC
56 k
1 k
100 A
6V
IB
100 
VCE
VBE
Figure 1
Three characteristics are usually measured and these are shown in Figure 2:
(a) the variation of base current (IB) with base- emitter voltage (VBE),
(b) the variation of collector current (IC) with base current, and
(c) the variation of collector current with collector- emitter voltage (VCE).
IC (mA)
IC (mA)
IB (A)
40 A
30 A
(b)
20 A
(a)
(c)
VBE
IB (A)
VCE
Figure 2
Figure 2(a) shows the ‘switching on’ of the transistor. There is no significant base current
until the base-emitter voltage reaches about 0.6 V.
Figure 2(b) shows how the collector current varies when there is a change in the base
current. You can see that a change in base current (IB) of a few microamps will produce a
collector current change (IC) of a few miiliamps. This shows the amplifying action of the
transistor and also that this amplification is current-controlled rather than voltage-controlled.
The ratio of the change in collector current to the change in base current is called the current
gain of the transistor, and is written as hFE.
This usually has a value of between 100 and 200 for an npn silicon transistor.
Figure 2(c) shows the variation of collector current with collector-emitter voltage. There are
several curves, each representing a different base current.
1