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BJT, AC behavior
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AGENDA
BJT
AC behaviour
DC & AC signals
Characteristics
DC input characteristics
AC input characteristics
How to draw and ac circuit
Configurations
Common Emitter
Examples
Calculating
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BJT
Impedance reflection p 171
Input/output resistance
Source resistiance
CE example
Common Collector
Example
Calculations
Common Base
Example
Calculations
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BJT, AC behavior
Signal operation can be seen as a small
variation on the DC bias of a circuit. If
the signals are very small to the DC bias,
transistor parameters can be considered
as constant. While the BJT
isPaap
a nonP.J.F.
lineair behavior component this
appoximation is only valid for very
small variations on the DC-bias point.
The first thing to do is to explain the AC
model of a BJT, afterwards we make
applications an calculations on a
complete AC circuit.
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BJT, DC & AC signals
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BJT, DC & AC signals
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BJT, characteristics
DC model; Vbe = 0V7
ac model; re = 26mV/Ie
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DC model ac model
Ube, Uce, Ic, Ib, Ie Capitals
ube, uce, ic, ib, ie Low cases
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BJT, DC input characteristics
Vbe = 0V7
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BJT, AC input characteristics
re = 26mV/Ic
The dynamic resistor can be
calculated by the DC current Ic
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BJT, characteristics
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BJT, how to draw an ac circuit 1/4
1 Kill all DC sources (ΔU = 0 V)
replacing each dc voltage source with a short circuit
and
each dc current source with an open circuit
2 Replace all Capacitors ( xc = 1/2.pi.f )
3 Use the ac model of the active devices
T model for BJT
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BJT, how to draw an ac circuit 2/4
1 Kill all DC sources
(ΔU = 0 V)
replacing each dc
voltage source
with a short
circuit
and
each dc current
source with an
open circuit
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BJT, how to draw an ac circuit 3/4
2 Replace all
Capacitors
( xc = 1/2.pi.f )
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BJT, how to draw an ac circuit 4/4
3 Use the ac model of the active devices,
T model for BJT
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BJT, configurations
1 Common Emitter
2 Common Collector
or Emitter follower
3 Common Base
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Input = base
Input = base
Output = collector
Output = emitter
Input = emitter Output = collector
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BJT, CE examples
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BJT, CE calculating
Au 
ix  something
ix  something different
Law of Bollen
here ix = ib
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BJT, CE calculating

Ib does NOT flow through Rb

When Rb changes in value,
base current will NOT change

Through re,
base current & collector current

If input voltage increases,
output voltage decreases
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Au 
ix  something
ix  something different
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BJT, CE calculating
ix  something
Au 
ix  something different
  ib Rc Rload
U out
Au 

U in
1    ib re
 Rc Rload
Au 
re
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There is NO Rb !!
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BJT, CE example
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BJT, CE example
U out
  ib Rc
 ic Rc
Au 


U in
ie re
1    ib re
 Rc
Au 
re
NO Rb1 en Rb2
in the formula !!
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BJT, CE example
 Rc
Au 
re
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 Rc
Au 
re  Re
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BJT, impedence reflection page 171
If you stand in base you see all resistors in the
emitter (ß+1) magnified
If you stand in emitter you see all resistors in the
base 1/(ß+1) magnified
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BJT, input / output resistance
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BJT, input / output resistance
 Rc
Au 
re  Re
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rin  Rb1 Rb 2   re  Re 
rout  Rc
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BJT, source resistance
Rs causes an
attenuation in
combination
with the input
resistance of
the circuit.
Then the signal
is amplificated !
uout
us
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
ub
us

uout
ub
rin
 Rc


rin  Rs
re
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BJT, CE example
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BJT, CC example
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BJT, CC
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BJT, CC
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BJT
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BJT, CB
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BJT, CB
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