Introduction to BJT Amplifier BJT (Review). Still remember about BJT? The emitter current (i E ) is...
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Transcript of Introduction to BJT Amplifier BJT (Review). Still remember about BJT? The emitter current (i E ) is...
Introduction to BJT Amplifier
BJT (Review)
Still remember about BJT?
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
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
transistorsymbols
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
)(
)(
Refresh..
BJT as amplifying device B-E junction is forward-biased B-C junction is reverse-biased
BIASING OF BJT
Remember…! for normal operation
emitter-base junction is always forward-biased
AND
collector-base junction is always reverse-biased
FORWARD BIASING E/B JUNCTION
REVERSE BIASING C/B JUNCTION
BIASING NPN TRANSISTOR
Common-Emitter Circuit
(a) with an npn transistor
(b) with a pnp transistor
(c) with a pnp transistor biased with a positive voltage source
DC Analysis - Common-Emitter Circuit
Transistor current-voltage characteristics of the common-emitter circuit
DC Analysis - Common-Emitter Circuit
Common-emitter circuit with an npn transistor
Common-emitter dc equivalent circuit, with piecewise linear parameters
DC Analysis - Common-Emitter Circuit
B
BEBBB R
onVVI
)(
Look for calculation examples in Neamen (Chapter 3), Example 3.3 & 3.4
Usually VBE(on) = 0.7 V
CCCCCE
CECCCC
BC
RIVV
VRIV
II
or
Common-emitter dc equivalent circuit
DC Analysis - Load Line & Modes of Operation
Base-emitter junction characteristics and the input load line
BQB
BEBBB I
R
VVI
Base on Figure A, using KVL around B-E loop:
Figure A
Base on Figure A, 2 end points of the load line are found by setting IC = 0
So, VCE = VCC = 10 VWhen VCE = 0,IC = VCC/RC = 5 mA
IBQ is the value from the previous slide = 15 µASo, ICQ = βIBQ
If β = 200, ICQ = 3000 µA = 3 mA
So, VEQ = 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
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
END