Saturday 13 April 2013
TRANSISTOR PROBLEM 4
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Find the value of R1,R2 and the range
of RL
for the transistor to remain in active region. Assume for both the transistor β = 100.
Zener voltage = 4V
Load current = 2mA
Zener current = 5mA
VEB1 = VEB2 = 0.7V
Solution
For active region IC = βIB
But IC = IL
IB = IL / β
IB = 2mA / 100 = 0.02mA
IE = IC + IB
IE = 2mA + 0.02mA =
2.02mA
Let the voltage across R1
be VR1
Applying KVL to the transistor Q2
VR1 = 12 - 0.7 - 4
= 7.3V
R1= VR1 / ( IZ
+ IB )
= 7.3 / (5 + 0.02 )mA
R1
=1.45 KΩ
Applying KCL between nodes A and B
12 – VR2
–VEB1-VR1 =0
VR2 =
4V
R2 = VR2
/ IE
=4 / 2.02mA
R2 = 1.98KΩ
Applying KVL to the above loop
VR1 –VBC1-VRL
= 0
VBC1 = VR1 –VRL
= 7.3 –( 2mA )RL
For transistor to be in active region VBC1 <
0.6
7.3 –( 2mA )RL < 0.6
6.7 < ( 2mA )RL
6.7 / 2mA < RL
3.35KΩ < RL
RL < 3.35KΩ
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