ECE 3050A – Fall 2004 Page 1
QUIZ NO. 1 - SOLUTION
(Average score = 9.1/10 of those taking the quiz.)
The npn BJT transistor shown has the parameter of
β
F = 100
Assume that VBEQ = 0.6V.
a.) Assume the BJT transistor is in the forward active region
and find the value of RE that gives a collector current of 1mA.
b.) What value of RC will cause the base-collector junction to
have zero volts (i.e., enter the saturation region for a BJT)?
Solution
(a.) The first step is to find a Thevenin equivalent circuit seen
from the base to ground. This circuit is shown below where
VBB =
10·R1
R1+R2 = 5V (2) and RB = R1|| R2 = 50kΩ (2)
R
C
R
E
R
B
=50kΩ
I
C
I
B
I
E
V
C
V
B
V
E
F04Q01S1
V
BB
=5V
V
CC
=10V
A collector current of 1mA implies that the base current, IB = 10µA. The equation for the
base current can be found from the base-emitter voltage loop and used to solve for RE.
VBB = IBRB + VBEQ + IB (1+
β
F)RE → (1+
β
F)RE =
VBB -0.6
IB - RB
101RE = 4.4V
10µA - 50K = 440K – 50K = 390K → RE = 390K
101 = 3.86kΩ (2)
(b.) Since VBC = 0, VB = VC. Thus, we need to first find VB.
VB = VBB - IB RB = 5V – 10µA(50K) = 4.5V (2)
∴ RC =
VCC -VC
IC = 10-4.5
1mA = 5.5kΩ (2)
R
C
10V
R
E
R
2
=
100kΩ
R
1
=
100kΩ
I
C
I
B
I
E
V
C
V
B
V
E
F04Q01P1