Friday 12 April 2013
TRANSISTOR PROBLEM 3
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If in the given circuit silicon transistor is use with
β= hfe = 30
Find minimum value of R for which the transistor is in active region
Solution
Given transistor is in active region
assume
IC = βIB , VBE = 0.7V
I1 = 0.7 – (-12 ) / 100K = 0.127mA
I2 = 12 – 0.7 / R = 11.3 /R
IB = I2 – I1
IB = 0.127mA -11.3 /R
For transistor to be in active region VBC <
0.6V
VB – VC < 0.6 V
VB= 0.7V
VC = 12- 2.2K IC
IC = βIB
IC = 30 (11.3 /R - 0.127mA )
= ( 339 / R ) –
3.81 mA
0.7V – (12- 2.2K IC ) < 0.6
0.7V – (12- 2.2K (( 339 / R ) – 3.81 mA ) ) < 0.6
0.7 – ( 20.382 – 745.8K / R ) < 0.6
-19.685 + 745.8K / R < 0.6
745.8K / R < 20.282
745.8K /20.282 < R
36.771 < R
R > 36.771KΩ
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