Download Lecture I (b) - UniMAP Portal

DMT 231/3 Electronic II
Lecture I (b)
Introduction to BJT Amplifier
food for thought
The WORLD is three days:
As for YESTERDAY, it has vanished, along
with that was in it.
As for TOMORROW, you may never see it.
As for TODAY, it is yours, so work in it.
SEM I 2008/09
LECTURE I (b): BJT REVIEW
BJT REVIEW (I)
The emitter current (iE) is the sum of the collector current (iC) and the base
current (iB)
iE  iB  iC
SEM I 2008/09
iB << iE and iC
LECTURE I (b): BJT REVIEW
BJT REVIEW (II)
• Common-emitter
current gain, β
– Range: 50 < β < 300
• Common-base current
gain, α
– Range: always slightly
less than 1
• The current
relationship between
these 2 parameters
are as follows:
iE  iC  iB
iC   iB
 iE  (1   )iB
 iC
iE  (1   )




  
iE
iC  
1  
 
But   
1 



 iC  iE
SEM I 2008/09
LECTURE I (b): BJT REVIEW
BJT REVIEW (III)
• BJT as amplifying device
– B-E junction is forward-biased
– B-C junction is reverse-biased
SEM I 2008/09
LECTURE I (b): BJT REVIEW
COMMON-EMITTER (CE) CIRCUIT
(a) with an npn transistor
(b) with a pnp transistor
(c) with a pnp transistor biased with a positive voltage source
SEM I 2008/09
LECTURE I (b): BJT REVIEW
CE CIRCUIT: DC ANALYSIS (I)
Transistor currentvoltage characteristics
of the common-emitter
circuit
SEM I 2008/09
LECTURE I (b): BJT REVIEW
CE CIRCUIT: DC ANALYSIS (II)
Common-emitter circuit with an
npn transistor
SEM I 2008/09
Common-emitter dc equivalent
circuit, with piecewise linear
parameters
LECTURE I (b): BJT REVIEW
CE CIRCUIT: DC ANALYSIS (III)
VBB  VBE (on )
IB 
RB
Usually VBE(on) = 0.7 V
I C  I B
VCC  I C RC  VCE
or V CE VCC  I C RC
SEM I 2008/09
LECTURE I (b): BJT REVIEW
DC ANALYSIS:
Load Lines & Modes of Operation (I)
Figure A
Based on Figure A, using KVL
around B-E loop:
SEM I 2008/09
Base-emitter junction characteristics
and the input load line
VBB  VBE
IB 
 I BQ
RB
LECTURE I (b): BJT REVIEW
DC ANALYSIS:
Load Lines & Modes of Operation (II)
Based on Figure A, 2 end
points of the load line are
found by setting IC = 0
VCC  I C RC  VCE
VCC  I C RC  VCE
So, VCE = VCC = 10 V
When VCE = 0,
IC = VCC/RC = 5 mA
IBQ is the value from the
previous slide = 15 µA
So, ICQ = βIBQ
If β = 200,
ICQ = 3000 µA = 3 mA
V CEQ VCC  I CQ RC
Common- emitter transistor characteristics
and the collector-emitter load line
SEM I 2008/09
So, VEQ = 4 V
LECTURE I (b): BJT REVIEW
BJT: an AMPLIFIER
• Amplification of a small ac
voltage by placing the ac
signal source in the base
circuit
• Vin is superimposed on the
DC bias voltage VBB by
connecting them in series
with base resistor RB:
I C   DC I B
• Small changes in the base
current circuit causes large
changes in collector current
circuit
SEM I 2008/09
LECTURE I (b): BJT REVIEW