PHY2003 (Practical Electronics II)
Homework
Worksheet 9
Bipolar Junction Transistors
Introduction
PHY2003 does not require a knowledge of the semiconductor physics underlying how transistors
work, they are treated as nonlinear three-terminal devices. Figure 1 defines the current conventions
used in PHY2003. The rules-of-thumb for analysing bipolar junction transistor (BJT) circuits at
low-frequencies are:
NPN: V C > V B = V E + 0.6 V
(9.1)
PNP: V C < V B = V E ! 0.6 V
" NPN: V B < V E + 0.6 V %
if #
& then IC = 0 else IC = hFE IB
$ PNP: V B > V E ! 0.6 V '
typically 50 < hFE < 500 therefore
(9.2)
IE ! " IC .
(9.3)
Required Reading
Bipolar Transistors – Storey (1998) §7.1–7.7 pp. 234–295 / (2006) §8.1–8.8 pp. 221–284.
Another source of the required information are the WWW references listed under:
<http://newton.ex.ac.uk/teaching/CDHW/Electronics2/ElectronicsResources.html#xistors>
+12V
VC
VB
NPN
IB
VE
PNP
IC
VB
IE
VE
R1
R2
C1
X1
IB
1M
3K3
10N
BC107
R1
3
C1
IE
V1
R2
2
V3
X1
IC
VC
Figure 9.1 Current conventions
Circuit 9.1 Common Emitter Amplifier
1
PHY2003 (Practical Electronics II)
Homework
Worksheet 9
12V
R1
C1
R3
4
V1
2
C1
R1
R2
R3
R4
X1
0µ1
33K
10K
2K7
1K0
BC107
V4
X1
3
R2
R4
Circuit 9.2 Series Feedback Amplifier
Exercise 9.1
Analyse circuit 9.1 using typical values from the BC107 datasheet and find:
(a) the quiescent voltage at node 3,
(b) the DC impedance of node 2 and hence the low-frequency –3 dB point
(c) the small-signal voltage gain v3 v1 at 1 kHz and 10 kHz.
Answers:
(a) 6.4 V (assuming HFE = 150) (b) 2.2 k!, 7.2 kHz (c) –60 and –400
Exercise 9.2
Analyse circuit 9.2 using typical values from the BC107 datasheet and find:
(a) the quiescent voltage at node 3
(b) the quiescent voltage at node 4
(c) the small-signal gain v 4 v1 at 1kHz
Answers:
(a) 2.1 V (b) 6.3 V (b) –60 and –400 (c) v 4 v1 = !2.7
© Copyright CDH Williams
University of Exeter 2006
CW061019/5
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