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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