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# Power Amplifier (Class A)

## Induction of Power Amplifier

Power and Efficiency
Amplifier Classification
Basic Class A Amplifier
Transformer Coupled Class A Amplifier
Introduction
Power amplifiers are used to deliver a relatively high amount of
power, usually to a low resistance load.
Typical load values range from 300W (for transmission antennas)
to 8W (for audio speaker).
Although these load values do not cover every possibility, they
do illustrate the fact that power amplifiers usually drive low-
Typical output power rating of a power amplifier will be 1W or
higher.
Ideal power amplifier will deliver 100% of the power it draws
from the supply to load. In practice, this can never occur.
The reason for this is the fact that the components in the
amplifier will all dissipate some of the power that is being
drawn form the supply.
Amplifier Power Dissipation
P
1
= I
1
2
R
1
P
2
= I
2
2
R
2
I
CQ
R
C
R
E
R
1
R
2
V
CC
I
1
I
2
I
CC
P
C
= I
CQ
2
R
C
P
T
= I
TQ
2
R
T
P
E
= I
EQ
2
R
E
I
EQ
The total amount of power
being dissipated by the
amplifier, P
tot
, is
P
tot
=P
1
+P
2
+P
C
+P
T
+P
E

The difference between this
total value and the total power
being drawn from the supply is
the power that actually goes to

Amplifier Efficiency q
Amplifier Efficiency
A figure of merit for the power amplifier is its efficiency, q .
Efficiency ( q ) of an amplifier is defined as the ratio of ac
output power (power delivered to load) to dc input power .
By formula :

As we will see, certain amplifier configurations have much
higher efficiency ratings than others.
This is primary consideration when deciding which type of
power amplifier to use for a specific application.

Amplifier Classifications
% 100
) (
) (
% 100 = =
dc P
ac P
power input dc
power output ac
i
o
q
Amplifier Classifications
Power amplifiers are classified according to the percent of
time that collector current is nonzero.
The amount the output signal varies over one cycle of
operation for a full cycle of input signal.
v
in
v
out
A
v
Class-A
v
in
v
out
A
v
Class-B
v
in
v
out
A
v
Class-C
Efficiency Ratings
The maximum theoretical efficiency
ratings of class-A, B, and C amplifiers are:

Amplifier Maximum Theoretical
Efficiency, q
max

Class A 25%
Class B 78.5%
Class C 99%
Class A Amplifier
voutput waveform same shape vinput

waveform + t
phase shift.

The collector current is nonzero 100% of the time.
inefficient, since even with zero input signal, ICQ is
nonzero
(i.e. transistor dissipates power in the rest, or quiescent,
condition)
v
in
v
out
A
v
Basic Operation
Common-emitter (voltage-divider) configuration (RC-coupled amplifier)
R
C
+V
CC
R
E
R
1
R
2
R
L
v
in
I
CQ
I
1
I
CC
Typical Characteristic Curves
for Class-A Operation
Typical Characteristic
Previous figure shows an example of a
sinusoidal input and the resulting collector
current at the output.
The current, I
CQ
, is usually set to be in the
center of the ac load line. Why?
(DC and AC analyses discussed in previous sessions)

DC Input Power
R
C
+V
CC
R
E
R
1
R
2
R
L
v
in
I
CQ
I
1
I
CC The total dc power, P
i
(dc) , that an
amplifier draws from the power
supply :

CC CC i
I V dc P = ) (
1
I I I
CQ CC
+ =
CQ CC
I I ~ ) (
1
I I
CQ
>>
CQ CC i
I V dc P = ) (
Note that this equation is valid for most amplifier power analyses.
We can rewrite for the above equation for the ideal amplifier as
CQ CEQ i
I V dc P 2 ) ( =
AC Output Power
R
1
//R
2
v
ce
v
in
v
o
i
c
R
C
//R
L
r
C
AC output (or load) power, P
o
(ac)

Above equations can be used to
calculate the maximum possible
value of ac load power. HOW??

L
rms o
rms o rms c o
R
v
v i ac P
2
) (
) ( ) (
) ( = =
Disadvantage of using class-A amplifiers is the fact that their
efficiency ratings are so low, q
max
~ 25% .
Why?? A majority of the power that is drawn from the supply by a
class-A amplifier is used up by the amplifier itself.
Class-B Amplifier
I
C
(mA)
V
CE
V
CE(off)
= V
CC
I
C(sat)
= V
CC
/(R
C
+R
E
)
I
C
V
CE
I
C(sat)
= I
CQ
+ (V
CEQ
/r
C
)
V
CE(off)
= V
CEQ
+ I
CQ
r
C
I
C
V
CE
Q - point
L
PP
CQ CEQ
CQ CEQ
o
R
V
I V
I V
ac P
8 2
1
2 2
) (
2
= = |
.
|

\
|
|
.
|

\
|
=
% 25 % 100
2
2
1
% 100
) (
) (
= = =
CQ CEQ
CQ CEQ
dc i
ac o
I V
I V
P
P
q
Limitation
Example
Calculate the input power [P
i
(dc)], output power [P
o
(ac)],
and efficiency [q] of the amplifier circuit for an input
voltage that results in a base current of 10mA peak.
R
C
R
B
+V
CC
= 20V
I
C
V
i
25 = |
O 20
O k 1
V
o
( )
% 5 . 6 % 100
6 . 9 ) 48 . 0 )( 20 (
625 . 0 ) 20 (
2
10 250
2
250 ) 10 ( 25
20
1 1000
20
20
4 . 10 ) 20 )( 48 . 0 ( 20
48 . 0 5 . 482 ) 3 . 19 ( 25
3 . 19
1
7 . 0 20
) (
) (
) (
2
3
2
) (
) (
) (
) ( ) (
) (
= =
= = =
= O

= =
= = =
= =
= =
O
= =
= O = =
~ = = =
=
O

dc i
ac o
CQ CC dc i
C
peak C
ac o
C
CC
sat c
B
P
P
W A V I V P
W
A
R
I
P
peak mA peak mA I I
V V V
A mA
V
R
V
I
V A V R I V V
A mA mA I I
mA
k
V V
R
V V
I
peak b peak C
CC cutoff CE
C C CC CEQ
CQ
B
BE CC
BQ
q
|
|
Transformer-Coupled Class-A Amplifier
Input
+V
CC
R
E
R
1
R
2
R
L
N
1
:N
2
Z
2
= R
L
Z
1
A transformer-coupled class-A amplifier
uses a transformer to couple the output
signal from the amplifier to the load.
The relationship between the primary
and secondary values of voltage, current
and impedance are summarized as:
L
R
Z
Z
Z
N
N
I
I
V
V
N
N
1
2
1
2
2
1
1
2
2
1
2
1
= =
|
|
.
|

\
|
= =
N
1
, N
2
= the number of turns in the primary and secondary
V
1
, V
2
= the primary and secondary voltages
I
1
, I
2
= the primary and secondary currents
Z
1
, Z
2
= the primary and seconadary impedance ( Z
2
= R
L
)
Transformer-Coupled Class-A Amplifier
An important characteristic of the transformer
is the ability to produce a counter emf, or kick
emf.
When an inductor experiences a rapid change in
supply voltage, it will produce a voltage with a
polarity that is opposite to the original voltage
polarity.
The counter emf is caused by the
electromagnetic field that surrounds the inductor.
Counter emf
10V
+
-
+
-
10V
SW1
10V
+
-
+
-
10V
This counter emf will be present only for an instant.
As the field collapses into the inductor the voltage
decreases in value until it eventually reaches 0V.
DC Operating Characteristics
The dc biasing of a transformer-coupled class-A amplifier is very similar to any
other class-A amplifier with one important exception :
the value of V
CEQ
is designed to be as close as possible to V
CC
.
Input
+V
CC
R
E
R
1
R
2
R
L
N
1
:N
2
Z
2
= R
L
Z
1
V
CE
I
C
I
B
= 0mA
The dc load line is very close to being a vertical line
indicating that V
CEQ
will be approximately equal to
V
CC
for all the values of I
C
.
The nearly vertical load line of the transformer-
coupled amplifier is caused by the extremely low dc
resistance of the transformer primary.
V
CEQ
= V
CC
I
CQ
(R
C
+ R
E
)
The value of R
L
is ignored in the dc analysis of the
transformer-coupled class-A amplifier. The reason for
this is the fact that transformer provides dc isolation
between the primary and secondary. Since the load
resistance is in the secondary of the transformer it
dose not affect the dc analysis of the primary circuitry.
AC Operating Characteristics
Input
+V
CC
R
E
R
1
R
2
R
L
N
1
:N
2
Z
2
= R
L
Z
1
V
CE
I
C
I
B
= 0mA
I
C(max)
= ??
~ V
CEQ
~ V
CC
~ 2V
CC
Q-point
1. Determine the maximum possible change in V
CE

Since V
CE
cannot change by an amount
greater than (V
CEQ
0V), v
ce
= V
CEQ
.
2. Determine the corresponding change in I
C

Find the value of Z
1
for the transformer: Z
1
=
(N
1
/N
2
)
2
Z
2
and i
c
= v
ce
/ Z
1

3. Plot a line that passes through the Q-point and
the value of I
C(max)
.
I
C(max)
= I
CQ
+ i
c

4. Locate the two points where the load line passes
through the lies representing the minimum and
maximum values of I
B
. These two points are then
used to find the maximum and minimum values of
I
C
and V
CE

Input
+V
CC
R
E
R
1
R
2
R
L
N
1
:N
2
Z
2
= R
L
Z
1
R
1
//R
2
v
ce
v
in
i
c
Z
1
v
o
V
CE
I
C
I
B
= 0mA
I
C(max)
= ??
~ V
CEQ
~ V
CC
~ 2V
CC
Q-point
I
CQ
The Power Supply for the amplifier : P
S
=V
CC
I
CC

Maximum peak-to-peak voltage across the primary of the transformer
is approximately equal to the difference between the values of V
CE(max)

and V
CE(min)
: V
PP
=V
CE(max)
V
CE(min)

is found by V
(P-P)max
=(N
2
/ N
1
)V
PP

The actual efficiency rating of a transformer-coupled class-A amplifier
will generally be less than 40%.
N
1
: N
2
R
L
V
PP
V
(P-P) max
There are several reasons for the
difference between the practical and
theoretical efficiency ratings for the
amplifier :
1. The derivation of the q = 50% value assumes
that V
CEQ
= V
CC
. In practice, V
CEQ
will
always be some value that is less the V
CC
.
2. The transformer is subject to various power
losses. Among these losses are couple loss
and hysteresis loss. These transformer power
losses are not considered in the derivation of
the q = 50% value.
One of the primary advantages of using the
transformer-coupled class-A amplifier is the
increased efficiency over the RC-coupled class-A
circuit.
Another advantage is the fact that the
transformer-coupled amplifier is easily converted
into a type of amplifier that is used extensively in
communications :- the tuned amplifier.
A tuned amplifier is a circuit that is designed to
have a specific value of power gain over a specific
range of frequency.
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