Page 293 - Mechatronics with Experiments

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JWST499-c05
JWST499-Cetinkunt
ELECTRONIC COMPONENTS FOR MECHATRONIC SYSTEMS 279
The current through the R ,
E
V out V − 0.6
in
i = = (5.162)
E
R E R E
and the current through the collector,
i = ⋅ i B (5.163)
C
i = i + i = ( + 1) ⋅ i B (5.164)
E
C
B
( + 1)
= i C (5.165)

and
V − 0.6
in
i = i = (5.166)
E
C
+ 1 + 1 R E
as long as the transistor is not saturated, (V > V + 0.2), the current output (i )ispro-
C
E
E
portional to the base voltage. This circuit does not have any voltage gain, but it does have
current gain, and hence power gain, between input and output signals.
Note that
V = V BE + V E (5.167)
in
V = V − V BE ≥ 0.0 (5.168)
in
E
V = V CE + V E (5.169)
C
The voltage at the emitter follows the base voltage. Current through the collector and emitter
is proportional to the base voltage.
Example There are many applications where a load resistor varies and despite these
variations it is desired to provide a constant current through the load.
Let us modify the circuit of the previous example as shown in Figure 5.21. The
main additions are an input circuit supply voltage V CC1 , a resistor R , and a Zener diode
1
with breakdown voltage of V . In the output circuit, we have a load resistor R connected
L
Z
between the output supply V CC2 and the collector terminal of the transistor.

FIGURE 5.21: A transistor circuit as a
constant current-source.