BJT, AC behavior

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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 Cal
culating
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 is a
non-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.
P.J.F. Paap
P.J.F. Paap
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BJT, DC AC signals
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BJT, DC AC signals
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BJT, characteristics
DC model ac model
DC model Vbe 0V7 Ube, Uce, Ic, Ib,
Ie Capitals ac model re 26mV/Ie 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 (?U 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 (?U 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 Input base Output
collector 2 Common Collector Input base
Output emitter or Emitter follower 3 Common
Base Input emitter Output
collector
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BJT, CE examples
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BJT, CE calculating
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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BJT, CE calculating
There is NO Rb !!
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BJT, CE example
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BJT, CE example
NO Rb1 en Rb2 in the formula !!
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BJT, CE example
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BJT, impedence reflection page 171
If you stand in base you see all resistors in the
emitter (ß1) magnified If you stand in emitter
you see all resistors in the base 1/(ß1)
magnified
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BJT, input / output resistance
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BJT, input / output resistance
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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 !
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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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