High Power Amplifiers with BJT for Efficient Performance
Unit –II
High Power Amplifiers
-18marks
Course Outcome
Use BJT as high power amplifier
Definitions
In small-signal amplifiers the main factors
are:
•
Amplification
•
Linearity
•
Gain
Since large-signal, or
power,
amplifiers handle
relatively
large
voltage signals and
current
levels, the main factors
are:
•
Efficiency
•
Maximum power
capability
•
Impedance matching to the output
device
3
Amplifier
Types
Class
A
The amplifier conducts
through
the full
360
of the input. The Q-point is
set near the middle of the load
line.
Class
B
The amplifier conducts
through
180
of the input. The Q-point is set
at
the cutoff
point.
Class
AB
This is a
compromise
between the class A and B amplifiers. The
amplifier conducts
somewhere
between
180
and
360
. The Q-point
is
located between the mid-point and
cutoff.
4
Amplifier
Types
Class
C
The amplifier conducts
less
than
180
of the input. The
Q-point
is located
below the cutoff
level.
5
Class A
Amplifier
The output
of a class
A
amplifier
conducts
for
the full 360
of
the
cycle.
The Q-point is set
at
the middle
of
the
load line
so
that
the AC signal
can
swing a
full
cycle.
Remember
that the
DC
load line
indicates the
maximum
and
minimum
limits
set by the
DC
power
supply.
6
Class B
Amplifier
A
class B amplifier
output
only conducts
for 180
or
one-half
of the AC
input
signal.
The Q-point is
at
0V on
the
load
line,
so
that
the AC
signal can
only swing for
one-half
cycle.
7
Class AB
Amplifier
This
amplifier is a
compromise
between
the
class
A and
class B amplifier—the
Q-point
is
above that of
the Class
B
but below the
class
A.
The output conducts
between 180
and
360
of
the
AC
input
signal.
8
Class
C
The output
of
the class
C
conducts
for less
than
180
of
the
AC
cycle. The
Q-point is below
cutoff.
9
Amplifier
Efficiency
Efficiency
refers
to
the
ratio of
output
to
input
power.
The
lower
the amount
of
conduction
of
the amplifier the higher the
efficiency.
10
10
This is
similar
to the
small-signal amplifier
except that it
will
handle
higher voltages. The
transistor used is a
high-
power
transistor.
11
11
Direct Coupled Class-A Amplifier
When an input signal is
applied the output
will
vary
from
its dc bias
operating voltage and
current.
A small input
signal causes the
output
voltage to
swing
to a
maximum of
V
cc
and a
minimum of
0V.
The
current
can also
swing
from
0mA to
I
CSAT
(V
CC
/R
C
)
12
12
Direct Coupled Class-A Amplifier
C
R
V
2
C
(r
m
s
)
P
o(ac)
Input
Power
The
power into
the
amplifier
is
from
the
DC
supply.
With
no input
signal, the
DC
current
drawn
is the
collector
bias current,
I
CQ
.
P
i(dc)
V
CC
I
CQ
Output
Power
or
P
o(ac)
%η
100
P
i(ac)
Effi
c
i
e
ncy
CE(rms)
C(rms)
13
13
o
(
d
c
)
P
V
I
P
o(dc)
I
2
C(rms)
Rc
Direct Coupled Class-A Amplifier
Transformer-Coupled
Class A
Amplifier
This
circuit
uses a
transformer to couple to
the load. This
improves
the efficiency of the
Class
A to
50%.
14
14
Transformer
Action
A transformer improves the
efficiency
because
it
is
able to
transform
the
voltage,
current, and
impedance
Voltage
Ratio
V
2
N
2
V
1
N
1
Current
Ratio
I
2
N
1
I
1
N
2
Impedance
Ratio
2
L
R
a
2
2
N
1
2
N
R
R
L
R
1
15
15
Transformer-Coupled
Class A
Amplifier
DC
Load
Line
As in
all class
A
amplifiers
the
Q-
point
is
established
close to the
midpoint
of
the DC
load line.
The
dc resistance is
small ideally at
0Ω
and
a
dc
load line
is
a straight
vertical
line.
AC Load
Line
The saturation point (I
Cmax
) is
at
V
cc
/R
L
and the
cutoff
point
is at
V
2
(the secondary
voltage of
the
transformer). This increases the
maximum output
swing
because
the
minimum
and
maximum
values
of
I
C
and V
CE
are
spread
further
apart.
16
16
Transformer-Coupled
Class A
Amplifier
8
P
o
(a
c
)
(V
CEmax
V
CEmin
)(I
Cmax
I
Cmin
)
(max
imum
)
Signal
Swing
and Output AC
Power
The
voltage
swing:
V
CE(p
p)
V
CEmax
V
CEmin
The current
swing:
I
c
(
p
p
)
I
C
m
ax
I
C
m
in
The AC
power:
17
17
Transformer-Coupled
Class A
Amplifier
Efficiency
P
Q
P
i(dc)
P
o(ac)
CEmin
CEmax
V
V
CEmax
V
CEmin
2
%η
50
V
Maximum
efficiency:
Power
input
from
the
DC
source:
P
i(dc)
V
CC
I
CQ
Power
dissipated as
heat
across
the
transistor:
Note:
The larger the input and output signal,
the
lower
the heat
dissipation.
Note:
The larger
V
CEmax
and
smaller
V
CEmin
,
the
closer the efficiency
approaches
the theoretical
maximum
of
50%.
18
18
Crossover
Distortion
If the transistors
Q
1
and
Q
2
do
not turn on and off at exactly
the same time, then
there
is a
gap
in the output
voltage.
19
19
Class AB Push Pull Amplifier
Complementary Symmetry Class AB Amplifier
Class C
Amplifiers
A
class
C amplifier conducts for
less
than
180
.
In order to
produce
a
full
sine wave output, the class C uses a
tuned
circuit
(LC tank) to
provide
the full
AC
sine
wave.
Class C amplifiers
are
used
extensively in radio
communications
circuits.
22
22
Explore the world of high-power amplifiers utilizing BJT technology, focusing on factors like linearity, efficiency, and maximum power capability. Learn about different amplifier classes - A, B, AB, and C - each with unique operating characteristics. Discover the significance of amplifier efficiency in optimizing power utilization.
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Presentation Transcript
Unit II High Power Amplifiers -18marks
Course Outcome Use BJT as high power amplifier
Definitions In small-signal amplifiers the main factorsare: Amplification Linearity Gain Since large-signal, or power, amplifiers handle relativelylarge voltage signals and current levels, the main factorsare: Efficiency Maximum powercapability Impedance matching to the outputdevice 3
AmplifierTypes ClassA The amplifier conducts through the full 360 of the input. The Q-point is set near the middle of the loadline. Class B The amplifier conducts through 180 of the input. The Q-point is setat the cutoffpoint. ClassAB This is a compromise between the class A and B amplifiers. The amplifier conducts somewhere between 180 and 360 . The Q-pointis located between the mid-point andcutoff. 4
AmplifierTypes Class C The amplifier conducts less than 180 of the input. The Q-point is located below the cutofflevel. 5
Class AAmplifier The output of a class Aamplifier conducts for the full 360 of the cycle. The Q-point is set at the middle of the load line so that the AC signal can swing a full cycle. Remember that the DC load line indicates the maximum andminimum limits set by the DC powersupply. 6
Class BAmplifier A class B amplifieroutput only conducts for 180 or one-half of the AC input signal. The Q-point is at 0V on the load line, so that the AC signal can only swing for one-half cycle. 7
Class ABAmplifier This amplifier is a compromise betweenthe class A and class B amplifier the Q-point is above that of the Class B but below the classA. The output conducts between 180 and 360 of the AC inputsignal. 8
Class C The output of the class C conducts for less than 180 of the AC cycle. The Q-point is below cutoff. 9
Amplifier Efficiency Efficiency refers to the ratio of output to input power. The lower the amount of conduction of the amplifier the higher the efficiency. 10
Direct Coupled Class-A Amplifier This is similar to the small-signal amplifier except that it willhandle higher voltages. The transistor used is ahigh- powertransistor. 11
Direct Coupled Class-A Amplifier When an input signal is applied the output will vary from its dc bias operating voltage and current. A small input signal causes theoutput voltage to swing to a maximum of Vcc and a minimum of 0V. The current can also swing from 0mA to ICSAT (VCC/RC) 12
Direct Coupled Class-A Amplifier Input Power The power into the amplifier is from the DC supply. With no input signal, the DC current drawn is the collector bias current, ICQ. Pi(dc) = =VCCICQ OutputPower V2C(rms) CE(rms) C(rms) = V o(dc) P I or Po(ac)= R C Po(dc)= I2C(rms)Rc Efficiency Po(ac) % = = 100 Pi(ac) 13
Transformer-Coupled Class AAmplifier This circuit uses a transformer to couple to the load. This improves the efficiency of theClass A to50%. 14
TransformerAction A transformer improves the efficiency because it is able to transform the voltage, current, and impedance Voltage Ratio V2 = =N2 V1 N1 Current Ratio I 2 =N1 I1 N2 Impedance Ratio N1 2 = = N R1 R L 2 = = = = a R R 2 2 L 15
Transformer-Coupled Class AAmplifier DC Load Line As in all class A amplifiers the Q- point is established close to the midpoint of the DC load line. The dc resistance is small ideally at0 and a dc load line is a straight vertical line. AC Load Line The saturation point (ICmax) is at Vcc/R L and the cutoff point is at V2 (the secondary voltage of the transformer). This increases the maximum output swing because the minimum and maximum values of IC and VCE arespread furtherapart. 16
Transformer-Coupled Class AAmplifier Signal Swing and Output ACPower The voltage swing: VCE(p p) = = VCEmax VCEmin The current swing: Ic( p p)= IC max IC min The ACpower: Po(ac)=(VCEmax VCEmin )(ICmax ICmin )(maximum) 8 17
Transformer-Coupled Class AAmplifier Efficiency Power input from the DC source: Pi(dc) = =VCCICQ Power dissipated as heat across the transistor: Note: The larger the input and output signal,the lower the heatdissipation. PQ = = Pi(dc) Po(ac) Maximumefficiency: VCEmax VCEmin 2 % = = 50 Note: The larger VCEmax and smaller VCEmin, the closer the efficiency approaches the theoretical maximum of 50%. CEmax+ V V CEmin 18
Crossover Distortion If the transistors Q1 and Q2 do not turn on and off at exactly the same time, then there is a gap in the outputvoltage. 19
Class CAmplifiers A class C amplifier conducts forless than 180 . In order to produce afull sine wave output, the class C uses a tuned circuit (LC tank) to provide the full AC sinewave. Class C amplifiers are used extensively in radiocommunications circuits. 22
