Introduction to BJT Amplifier Bipolar Junction Transistor (Review) 1
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Introduction to BJT Amplifier
Bipolar Junction Transistor
(Review)
1
n-type & p-type semiconductors
Si = 14
n-type:
- to increase the no. of conduction-band electrons in intrinsic silicon (such as As, P, Bi, Sb) P = 15
- majority carrier: electrons
p-type:
- to increase the no. of holes in intrinsic silicon (such as B, In, Ga) B = 5
- majority carrier: holes
2
What is pn junction?
Still remember this?
What’s the different between these two figures? Which one is forward-biased and reverse-biased?
3
Remember these symbols?
Still remember about BJT?
Which one is NPN, PNP?
What is C,B,E…?
4
BJT Current
CBE III
The emitter current ( iE ) is the sum of the collector current (iC) and the base current (iB)
iB << iE and iC OTHER PRAMETERS &
EQUATIONS? CBE iii 5
BJT
Basic structure and schematic symbol
E C
B
E
B
C
n npE C
B
E C
B
E
B
C
p pnE C
B
npn type pnp type
approximate equivalents
transistor symbols
6
Refresh…
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:
EC
EC
CE
BE
BC
BCE
ii
ii
ii
ii
ii
iii
1
1
1
1
But
)(
)(
7
Refresh...
BJT as amplifying device B-E junction is forward-biased
B-C junction is reverse-biased
8
Biasing of BJT
Remember…! for normal operation
emitter-base junction is always forward-biased
AND
collector-base junction is always reverse-biased
9
Common-Emitter Circuit
(a) with an npn transistor
(b) with a pnp transistor
(c) with a pnp transistor biased with a positive voltage source
10
DC Analysis: Common-Emitter Circuit
Transistor
current-voltage
characteristics
of the common-
emitter circuit
11
DC Analysis: Common-Emitter Circuit
Common-emitter circuit with an
npn transistor
Common-emitter dc equivalent
circuit, with piecewise linear
parameters 12
DC Analysis: Common-Emitter Circuit
B
BEBBB
R
onVVI
)(
Look for calculation examples in Neamen (Chapter 5), Example 5.3 & 5.4
Usually VBE(on) = 0.7 V
CCCCCE
CECCCC
BC
RIVV
VRIV
II
or
Common-emitter dc equivalent circuit
13
DC Analysis: Load Line & Modes of Operation
Base-emitter junction characteristics
and the input load line BQ
B
BEBBB I
R
VVI
Base on Figure A, using KVL
around B-E loop:
Figure A
14
Base on Figure A, 2 end
points of the load line are
found by setting IC = 0
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
So, VCEQ = 4 V
DC Analysis: Load Line & Modes of Operation
CECCCC VRIV
CCQCCCEQ RIVV
Common- emitter transistor characteristics
and the collector-emitter load line
CECCCC VRIV
15
BJT as 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:
• Small changes in the base
current circuit causes large
changes in collector current
circuit
BDCC II
16
BJT as an Amplifier (cont’)
(a)A bipolar inverter circuit to be used as a time-varying amplifier
(b) The voltage transfer characteristic 17
Self- Reading
Textbook: Donald A. Neamen, ‘MICROELECTRONICS Circuit Analysis & Design’,3rd Edition’, McGraw Hill International Edition, 2007
Chapter 5:The Bipolar Junction Transistor
Page: 287-296, 303-309.
18
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