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Transistor TJBModelo pequenos sinais
Modelo alta frequência
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Modelagem do Transistor TBJ
Resposta ac do TBJ para pequenos sinais;
Modelos utilizados de representação do TBJ.
Amplitude do sinal de entrada: técnica e modelo
de pequenos sinais ou grandes sinais
Modelos utilizados:
modelo re
modelo híbrido equivalente
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Ideia básica - modelo
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The Transistor as an Amplifier
(a) Conceptual circuit to illustrate the operation of the transistor of an amplifier. (b) The circuit of (a) with the
signal source vbe eliminated for dc (bias) analysis.
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The Small-Signal Model
Parameters of the BJT
The Collector Current and the
Transconductance
The Base Current and the Input Resistence at the Base
The Emitter Current and the Input Resistance at the Emitter
be
c
T
C
IiBE
Cm
v
i
V
I
v
ig
CC
mB
T
b
beb
gI
V
i
vrr
mmE
T
e
bee
ggI
V
i
vr
1
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Modelagem TBJ
Um modelo é a combinação de elementos de
circuito, apropriadamente escolhidos, que
aproximam melhor o funcionamento real de
um dispositivo semicondutor sob condições de
operação específicas.
Uma vez determinado o circuito equivalente, o
símbolo gráfico do dispositivo pode ser
substituído, no desenho esquemático, por este
circuito (modelo), e os métodos básicos de
análise de circuitos.
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Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source
representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance
re rather than the base resistance r featured in the hybrid- model.
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Idem Coletor-Comum
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Ganho de corrente – divisor de corrente
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Ganho de corrente – forma simplificada:
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Quadro resumo
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Example to Show Wave Forms (=100)
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Signal waveforms in the circuit of former Example
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(a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.
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Distortion due to Cutoff or
Nonlinearity
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Augmenting the T-Model to Account for the Early effect for the small-signal operation of
the BJT.
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Relationships Between the Small-
Signal Model Parameters of the BJTModel Parameters in Terms of DC Bias Currents:
In terms of gm
In terms of re
Relationships between and :
T
Cm
V
Ig
C
T
E
Te
I
V
I
Vr
C
Tb
I
Vrr
C
Ao
I
Vr
m
eg
r
m
bg
rr
e
mr
g
err 1
e
mrr
g11
1 1
1
11
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Graphical construction for the determination of the dc base current in the shown circuit.
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Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE in the shown circuit.
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Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB
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Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which is too
close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to the
saturation region, thus limiting the negative swing of vCE.
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Note: Early effect has been
neglacted ( VA )
Exercice: What’s the Q-point if
a) = 500
b)
c) = 75 and RC = 56 k
0V 5V 10V 15VV
400uA
300uA
200uA
100uA
0A
I = 1 uA
I = 2 uA
I = 3 uA
I = 4 uA
I = 5 uA
Q-point
Load Line
I = 2.7 uA
CC
IC
B
B
B
B
B
B
12V
314 uA
RC
RE
R1
R2 36 k
18 k
22k
16 k
V = +12 VCC
Q1
= 75
Load line for the four resistor bias circuit
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ceV
be
v 1.65 VA 206
v 8 mV
180 phase shift between
input and output signals
0V 2V 4V 6V 8V 10V 12V
Collecter-emitter Voltage
4.0mA
3.0mA
2.0mA
1.0mA
0A
Collector
Current
t
t
V = 0.692 V
V = 0.708 V
Load Line
V (t)
Q-point
v (t)
V = 0.700 V
I = 20 uA
I = 10 uA
I = 30 uA
V = 0.717 V
I = 15 uA
ce
BE
B
be
B
B
B
BE
BE
BE
Load line Q-point and signals for the BJT amplifier
Load Line Q-Point and signals for the BJT
amplifier
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R
VVVII BEEECC
REF
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Analysis Circuits Step-by-StepDC Analysis
1. ;
2. Find the Q-point using large-signal model
AC Analysis
3. ;
;
4. BJT small-signal model
5. Analyze the circuit
6. Combine DC AC results
A B
A B
A B
A B
A B C
A B L
A B
A B
E
A B
A B
A B
A B
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The common-emitter amplifier. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its model.
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The common-emitter amplifier with a resistance Re in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T
model (c) The circuit in (b) with ro eliminated.
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The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model.
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The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T
model. (c) The circuit in (b) redrawn to show that ro is in parallel with RL. (d) Circuit for determining Ro.
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Acoplamento RC
Acoplamento direto
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Acoplamento por transformador
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Resposta em baixas frequências
Efeito Cs
1
2 ( )Ls
s i s
fR R C
Efeito CC
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Resposta em baixas frequências
Efeito CE
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Exemplo:
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Q1
BC337AP
R1
10kΩ
R2
40kΩ
R3
1kΩ
R4
1kΩ
R5
1kΩ
R6
1kΩ
C1
10µF
C2
20µF
C3
1µF
VCC
20.0V
XFG1
XBP1
IN OUT
XSC1
A B
Ext Trig+
+
_
_ + _
Simulação BC 337
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Figure 5.73 Analysis of the low-frequency response of the CE amplifier: (a) amplifier circuit with dc sources removed; (b) the effect of CC1 is
determined with CE and CC2 assumed to be acting as perfect short circuits;
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Figure 5.73 (Continued) (c) the effect of CE is determined with CC1 and CC2 assumed to be acting as perfect short circuits; (d) the effect of CC2 is
determined with CC1 and CE assumed to be acting as perfect short circuits;
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Capacitores que influenciam o comportamento em
alta frequência
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Efeito da capacitância Miller
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Modelo de Giacoletto ou π-Híbrido
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Modelo π-Híbrido altas frequências
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The unity-gain frequency ( fT ) : the frequency at
which drops to one
o
β
ββ
s1
β
π π μ
1
r (C C )
T o ββ m
T
π μ
g
C C
Figure 17.23 - Common-emitter current gain versus frequency for the BJT
dB
20
40
60
20 log o
f
fT
0
Frequency (Hz - Log Scale)
109
108
107
106
105
f
- 3 dB
- 20 dB/decade
Finding the short-circuit current gain of the BJT
Cm
r
ro
g vm
Cib
ic
+
-
vbe be
im 0
+ -vbe
Common-emitter current gain versus
frequency for the BJT
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Figure 5.72 Determining the high-frequency response of the CE amplifier: (a) equivalent circuit; (b) the circuit of (a) simplified at both the input
side and the output side; (c) equivalent circuit with Cm replaced at the input side with the equivalent capacitance Ceq; (d) sketch of the frequency-
response plot, which is that of a low-pass STC circuit.
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Exemplo
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