Lecture 4

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© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Analog Electronics

Lecture 4:Transistors

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Semiconductor material and
pn
-
junction diode

P
-
type semiconductor

N
-
type semiconductor

PN
-

Junction

Diode

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Diode and its resistive behavior


R = 0

R =
inf

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd

Transformative Resistor


Signal controlled

transformative resistor

R

Control signal

Trans
formative Res
istor

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd

BJT
Structure and Construction

The BJT has three regions called the emitter, base, and
collector. Between the regions are junctions as indicated.

npn

pnp

The base is a thin lightly
doped region compared to the
heavily doped emitter and
moderately doped collector
regions.

Base
-
emitter junction.


Base
-
collector junction

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd

BJT Operation

The heavily doped n
-
type emitter
region has a very high density of
conduction
-
band (free) electrons.


These free electrons easily
diffuse through the forward
biased BE junction into the
lightly doped and very thin p
-
type base region.


The base has a low density of
holes, which are the majority
carriers.


© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

BJT Operation

A very little free electrons recombine with holes
in base and move as valence electrons through
the base region and into the emitter region as
hole current.


The valence electrons leave the crystalline
structure of the base and become free electrons in
the metallic base lead and produce the external
base current.

Majority of free electrons move toward the reverse
-
biased BC junction and swept
across into the collector region by the attraction of the positive collector supply
voltage.


The free electrons move through the collector region, into the external circuit, and
then return into the emitter region along with the base current.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

BJT
Voltages and Currents

npn

pnp

The
conventional
current
flows
in the direction of the arrow on the
emitter terminal. The emitter
current
I
E

is the sum of the collector
current
I
C

and the small base
current
I
B
.

That
is,
I
E

=
I
C

+
I
B
.

The
voltage drop between base and emitter is V
BE

whereas the
voltage drop between collector and base is called V
CE
.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Two important parameters,
β
DC

(dc current gain) and

α
DC

are introduced and used to analyze a BJT circuit
.

DC Beta (
β
DC

)

Ratio of DC collector current
and DC base current.



β
DC =
= I
C
/I
B


DC Alpha (
α
DC

)

Ratio of DC collector current to
the DC emitter current.



α
DC =
= I
C
/I
E

BJT
Parameters

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

BJT Circuit Analysis

Currents and voltages in BJT

I
B
: dc base current

I
E
: dc emitter current

I
C
: dc source current


V
BE
: dc voltage at base
wrt
. emitter

V
C
E
: dc voltage at collector
wrt
.
emitter.

V
CB
: dc voltage at collector
wrt
. Base.

V
BE

= 0.7V

V
CE

=
V
CC



I
C

R
C

I
B

= (
V
BB



V
BE

) / R
B


V
CB

=
V
CE



V
BE



© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

BJT
Biasing

In order for a BJT to operate properly , the two
pn

junctions
must be correctly biased with external dc voltages.

npn

For the
pnp type
, the voltages
are reversed to maintain the
forward
-
reverse bias.

pnp

For the
npn

type shown, the
collector is more positive
than the base, which is more
positive than the emitter.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The collector characteristic curves
shows three mode
of operations of transistor with the variation of
collector current
I
C

w.r.t

V
CE

for a specified value of
base current
I
B
.

V
BB

is set to produce a certain
value of
I
B

and
V
CC

is zero and
V
CE

is zero.

BJT
Collector Characteristics Curve

Saturation
region

As
V
CE

is increased,
I
C

increases
until
B
.

Both BE and BC junctions are
forward biased and the
transistor is in Saturation region.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

In saturation, an increase of base current has no effect on the
collector circuit and the relation
I
C

=
b
DC
I
B

is no longer valid.

I
C(SAT)
=V
CC


V
CE(SAT
)
/R
C

BJT
Collector Characteristics Curve
-

Saturation

At this point, the transistor
current is maximum and voltage
across collector is minimum, for a
given load.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Active region

As
V
CE

is
increased furthers and exceeds 0.7V the base
-
collector
junction becomes reverse
-
biased and the transistor goes into the
active, or linear, region of its operation.

I
C

levels off and remains
essentially constant for a

given value of
I
B

as
V
CE

continues
to increase
.


the value of
I
C

is determined

only by the relationship expressed
as

BJT
Collector Characteristics Curve
-

Linear

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The collector characteristic curves illustrate the relationship of the
three transistor currents.

By setting up other values of
base current, a family of
collector curves is developed.

b
DC

is the ratio of collector
current to base current
.


It can be read from the curves.
The value of
b
DC

is nearly the
same wherever it is
read in
active region.


BJT
Collector Characteristics Curve

Cut off

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

In a BJT,
cutoff

is the condition in which there is no base
current, which results in only an extremely small leakage
current (
I
CEO
) in the collector circuit. For practical work, this
current is assumed to be zero.

In cutoff, neither the base
-
emitter
junction, nor the base
-
collector
junction are forward
-
biased.

BJT
Collector Characteristics Curve

Cut off

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd



Variable
I
B

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd

BJT Switches

A BJT can be used as a switching device in logic circuits to turn on or
off current to a load. As a switch, the transistor is normally in either
cutoff (load is OFF) or saturation (load is ON).

In cutoff, the transistor
looks like an open switch.

In saturation, the transistor
looks like a closed switch.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

DC Load Line

the transistor. It is drawn by
connecting the saturation and
cutoff points
.

The transistor characteristic
curves are shown superimposed
on the load line. The region
between the saturation and
cutoff points is called the
active region.


The
DC load line

represents the circuit that is external to

Here
I
B

= 0 and
V
CE

=
V
CC

Here
V
CE

=
0 and

I
C

=
I
C
-
Sat
=
V
CC
-
V
CE(Sat)

/
R
C

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The DC Operating Point

Bias establishes the operating point (
Q
-
point) of a transistor amplifier;
the ac signal
moves
above and
below this
point.

Improper biasing can cause distortion in the output signal as the
transistor may go into the saturation and cutoff region.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The DC Operating Point

Point A,Q,B represents the Q
-
point for IB 400
m
A. 300

m
A

and 200

m
A

respectively.

Load line

The point at which the load line intersects a characteristic
curve represents the Q
-
point for that particular value of IB.

Assume a sinusoidal
Ib

is
superimposed on VBB
varying between 200uA to
400uA. It makes the collector
current varies between 20
mA

and 40
mA
.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The DC Operating Point

A signal that swings
outside the active
region will be clipped.

For example, the
bias has established
a low
Q
-

point. As a
result, the signal is
will be clipped
because it is too
close to cutoff.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Voltage
-
Divider Bias

A practical way to establish a
Q
-
point is to form a voltage
-
divider from
V
CC
.

Summary

R
1

and
R
2

are selected to establish
V
B
. If
the divider is stiff,
I
B

is small compared
to
I
2
. Then,

Determine the base voltage for the
circuit.

4.62 V

I
2

I
B

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

DC Load Line

What is the saturation current and
the cutoff voltage for the circuit?
Assume
V
CE

= 0.2 V in saturation.

4.48 mA

15 V

Is the transistor saturated?

I
C

=
b

I
B

= 200 (10.45
m
A) =


2.09 mA

Since
I
C

<
I
SAT
, it is not saturated.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Data Sheets

Data sheets give manufacturer’s specifications for maximum operating
conditions, thermal, and electrical characteristics. For example, an
electrical characteristic is
b
DC
, which is given as
h
FE
. The 2N3904 shows
a range of
b
’s on the data sheet from 100 to 300 for
I
C

= 10 mA.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Data Sheets

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

DC and AC Quantities

The text uses capital letters for both AC and DC currents and voltages
with
rms

values assumed unless stated otherwise.

DC Quantities

use upper case roman subscripts. Example:
V
CE
.
(The second letter in the subscript indicates the reference point.)

AC Quantities

and time varying signals use lower case italic
subscripts. Example:
V
ce
.

Internal transistor resistances

are indicated as lower case
quantities with a prime and an appropriate subscript. Example:
r
e

.

External resistances

are indicated as capital
R

with either a
capital or lower case subscript depending on if it is a DC or ac
resistance. Examples:
R
C

and
R
c
.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

BJT Amplifiers

A BJT amplifies AC signals by converting some of the DC power from
the power supplies to AC signal power. An ac signal at the input is
superimposed in the dc bias by the capacitive coupling. The output ac
signal is inverted and rides on a dc level of
V
CE
.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The FET

The idea for a field
-
effect transistor (FET) was first
proposed by Julius Lilienthal, a physicist and inventor. In
1930 he was granted a U.S. patent for the device.

His ideas were later refined and
developed into the FET. Materials
were not available at the time to
build his device. A practical FET
was not constructed until the
1950’s. Today FETs are the most
widely used components in
integrated circuits.


© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The JFET

The JFET (or Junction Field Effect Transistor) is a normally
ON device. For the
n
-
channel device illustrated, when the
drain is positive with respect to the source and there is no
gate
-
source voltage, there is current in the channel.

When a negative gate voltage is
applied to the FET, the electric
field causes the channel to
narrow, which in turn causes
current to decrease.

D

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The JFET

As in the base of bipolar transistors, there are two types of
JFETs:
n
-
channel and
p
-
channel. The dc voltages are
opposite polarities for each type.

The symbol for an
n
-
channel JFET is
shown, along with the proper polarities of
the applied dc voltages. For an
n
-
channel
device, the gate is always operated with a
negative (or zero) voltage with respect to
the source.

D

Gate

Source

Drain

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd

The JFET

There are three regions in the characteristic curve for a JFET
as illustrated for the case when
V
GS

= 0 V.

Between
A

and
B

is the
Ohmic

region
, where current and voltage
are related by Ohm’s law.

From
B

to
C

is the
active
(or
constant
-
current
)

region

where
current is essentially independent
of
V
DS
.

Beyond
C

is the
breakdown
region
. Operation here can
damage the FET.

Ohmic region

Active region

(constant current)

(pinch
-
off voltage)

Breakdown

V
GS

= 0

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd

The JFET

When
V
GS

is set to different values, the relationship between
V
DS

and
I
D

develops a family of characteristic curves for the
device.

D


Notice
that
V
p

is
positive and has the
same magnitude as
V
GS(off)
.

Pinch
-
off Voltage

I
DSS

V
GS(off)
.


© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The JFET

A plot of
V
GS

to
I
D

is called the transfer or
transconductance

characteristic curve
. The transfer curve is a is a plot of the
output current (
I
D
) to the input voltage (
V
GS
).

The transfer curve is based on the
equation

By substitution, you can find other
points on the curve for plotting the
universal curve.

I
DSS

2

0.3
V
GS(off)

0.5
V
GS(off)

I
DSS

4

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The input resistance of a JFET is given by:

Example


JFET Input Resistance

Compare the input resistance of a 2N5485 at 25
o
C and at 100
o
C.
The specification sheet shows that for
V
GS

=

20 V,
I
GSS



1 nA at 25
o
C and 0.2
m
A at 100
o
C.

At 25
o
C,

where

I
GSS

is the current into the reverse biased gate.

20 G
W
!

JFETs have very high input resistance, but it drops when the temperature
increases.

At 100
o
C,

100 M
W

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd


JFET Biasing

Self
-
bias is simple and effective, so it is the most common
biasing method for JFETs. With self bias, the gate is
essentially at 0 V.

An
n
-
channel JFET is illustrated. The current
in
R
S

develops the necessary reverse bias that
forces the gate to be less than the source.

Assume the resistors are as shown and the
drain current is 3.0 mA. What is
V
GS
?

= +12 V

1.5 k
W

330
W

1.0 M
W

V
G

= 0 V;
V
S

= (3.0 mA)(330
W
) = 0.99 V

V
GS
= 0


0.99 V =



0.99 V

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

You can use the transfer curve to obtain a reasonable value
for the source resistor in a self
-
biased circuit.

What value of
R
S

should you use
to set the
Q

point as shown?

375
W


JFET Biasing

Q

The
Q

point is approximately at
I
D

= 4.0 mA and
V
GS

=

1.25 V.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

Voltage
-
divider biasing is a combination of a voltage
-
divider
and a source resistor to keep the source more positive than
the gate.


JFET Biasing

V
G

is set by the voltage
-
divider and is independent
of
V
S
.
V
S

must be larger than
V
G

in order to
maintain the gate at a negative voltage with
respect to the source.

Voltage
-
divider bias helps stabilize the bias for
variations between transistors.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The metal oxide semiconductor FET uses an insulated gate
to isolate the gate from the channel. Two types are the
enhancement mode (E
-
MOSFET) and the depletion mode
(D
-
MOSFET).


The MOSFET

An E
-
MOSFET has no
channel until it is induced by
a voltage applied to the gate,
so it operates only in
enhancement mode. An
n
-
channel type is illustrated
here; a positive gate voltage
induces the channel.


E
-
MOSFET

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
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Electronic Devices, 9th edition

Thomas L. Floyd

The D
-
MOSFET has a channel that can is controlled by the
gate voltage. For an
n
-
channel type, a negative voltage
depletes the channel; and a positive voltage enhances the
channel.


The MOSFET

A D
-
MOSFET can
operate in either
mode, depending on
the gate voltage.


D
-
MOSFET

operating in D
-
mode

operating in E
-
mode

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

MOSFET symbols are shown. Notice the broken line
representing the E
-
MOSFET that has an induced channel.
The
n

channel has an i
n
ward pointing arrow.

Summary


The MOSFET

n

channel
p

channel

D

D

G

G

S

S

E
-
MOSFETs

n

channel
p

channel

D

D

G

G

S

S

D
-
MOSFETs

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The transfer curve for a
MOSFET
has the same parabolic
shape as the JFET but the position is shifted along the
x
-
axis.
The transfer curve for an
n
-
channel E
-
MOSFET is entirely in
the first quadrant as shown.

Summary


The MOSFET

The curve starts at
V
GS(th)
, which is a
nonzero voltage that is required to have
channel conduction. The equation for
the drain current is

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd

The
D
-
MOSFET can be operated in either mode. For the
n
-
channel device illustrated, operation to the left of the
y
-
axis
means it is in depletion mode; operation to the right means is
in enhancement mode.


The MOSFET

As with the JFET,
I
D

is zero at
V
GS(off)
.
When VGS is 0, the drain current is
IDSS, which for this device is
not

the
maximum current. The equation for
drain current is