View
239
Download
4
Category
Tags:
Preview:
Citation preview
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
Recommended