small signal model, pnp

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2 Νοε 2013 (πριν από 4 χρόνια και 2 μήνες)

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Small Signal Model

PNP Transistor

Section 4.4

BJT in the active region

Electrons cross the forward biased BE junction

and are swept reverse biased BC junction.

Large Signal Model of a BJT

Called “large” signal model

because this model is

applicable even if V
BE

changes from 300 mV to 800 mV

Experiments

Transconductance

If a signal changes the base
-
emitter
voltage by a small amount, how
much change is produced in the
collector current
?


Illustration of
Transconductance

But there is something else….

A change in V
BE

creates a change in base current!

Example 4.10

Signal Generated

By a microphone

Small Signal Equivalent Circuit

V
BE
=800 mV

β=100

I
S,Q1
=3 x 10
-
16
A

Question: If a microphone generates a 1 mV signal,

how much change is observed in the collector and

base current
?

A Simple Amplifier

Determine the output signal level if
the microphone produces a 1 mV
signal.

AC Ground


The voltage produced by a voltage
source is constant.


The small signal model is concerned
only with changes in quantities.


Therefore,
a DC voltage source must
be replaced with a ground in small
signal analysis
.

Example

Early Effect

A larger reverse bias voltage leads to a
larger BC depletion region.


The effective base width (WB) is reduced
.

The slope of the electron profile increases.

I
C

increases as VCE is increased
.

Early Effect

James M. Early

Modeling of Early Effect

What Doesn’t Change with Early
Effect
?


Modification of the Small Signal
Model

Summary

A Simple Cadence Example

Assumption

Assume that

1.
The DC at
Vout

is 0.9 V

2.
g
m
=1
mS


Gain is approximately equal to

g
m
R
C
.

Bias current is I
C
=g
m
V
t

R=(1.8V
-
0.9V)/26uA=34.6 Kohms

Gain is
-
34.6.

DC Bias of the Amplifier

Sweep the Base Voltage to
Get the IC=26 uA

Display the
Transconductance

Display Transconductance

Verify Transconductance (1)

Verify Transconductance (2)

Transconductance

VB

IC

562.5 mV

25.64
uA

563.5

mV

26.64
uA

∆VBE
=
1=ms
=
∆IC
=
=


=

=∆VBE/∆IC


1
mS

Introduce a Small Signal

Calculate Peak to Peak
Voltage

Peak to Peak Voltage=67.78
mV

67.78 mV/2=33.9

PNP Transistors

Structures of BJT Transistors

(NPN transistor)

(PNP transistor)

Voltage and Current Polarities of
NPN and PNP transistors

A “fat” voltage
between collector
and emitter voltage
places a transistor in
the active region!

A “skinny” voltage between collector
and emitter voltage places a transistor
in the active region!

Large
-
Signal Model of BJT
Transistors

(NPN)

(PNP)

Equations

Small Signal Model of BJT
Transistors

(NPN)

(PNP)