This document discusses power amplifiers and provides information on several key topics:
- Power amplifiers are classified based on the percentage of time the collector current is non-zero, with classes including A, B, and C.
- Class A amplifiers have the lowest theoretical maximum efficiency of 25% since the collector current is always on.
- The document provides equations to calculate input power, output power, and efficiency for different amplifier classes and configurations.
- A transformer-coupled class A amplifier is described that uses a transformer to couple the output signal from the amplifier to the load.
2. Induction of Power Amplifier
Power and Efficiency
Amplifier Classification
Basic Class A Amplifier
Transformer Coupled Class A Amplifier
3. 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-resistance loads.
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.
4. P1 = I1
2R1
P2 = I2
2R2
ICQ
RC
RE
R1
R2
VCC
I1
I2
ICC
PC = ICQ
2
RC
PT = ITQ
2
RT
PE = IEQ
2
RE
IEQ
The total amount of power
being dissipated by the
amplifier, Ptot , is
Ptot = P1 + P2 + PC + PT + PE
The difference between this
total value and the total power
being drawn from the supply is
the power that actually goes to
the load – i.e. output power.
Amplifier Efficiency h
5. A figure of merit for the power amplifier is its efficiency, h .
Efficiency ( h ) 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
h
6. 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.
vin vout
Av Class-A
vin vout
Av Class-B
vin vout
Av Class-C
7. The maximum theoretical efficiency ratings
of class-A, B, and C amplifiers are:
Amplifier Maximum Theoretical
Efficiency, hmax
Class A 25%
Class B 78.5%
Class C 99%
8. output waveform same shape input waveform +
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)
vin vout
Av
11. Previous figure shows an example of a
sinusoidal input and the resulting collector
current at the output.
The current, ICQ , is usually set to be in the
center of the ac load line. Why?
(DC and AC analyses discussed in previous
sessions)
12. RC
+VCC
RE
R1
R2
RL
vin
ICQ
I1
ICC
The total dc power, Pi(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
ICQ
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
)
(
13. R1//R2
vce
vin
vo
ic
RC
//RL
rC
AC output (or load) power, Po(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, hmax 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
14. IC
(mA)
VCE
VCE(off) = VCC
IC(sat) = VCC/(RC+RE)
DC Load Line
IC
VCE
IC(sat) = ICQ + (VCEQ/rC)
VCE(off) = VCEQ + ICQrC
ac load line
IC
VCE
Q - point
ac load line
dc load line
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
h
15.
16. Calculate the input power [Pi(dc)], output power [Po(ac)],
and efficiency [h] of the amplifier circuit for an input
voltage that results in a base current of 10mA peak.
RC
RB
+VCC
= 20V
IC
Vi
25
20
k
1
Vo
)
%
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
)
(
)
(
)
(
)
(
)
(
)
(
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
h
17. Input
+VCC
RE
R1
R2
RL
N1
:N2
Z2 = RL
Z1
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
N1, N2 = the number of turns in the primary and secondary
V1, V2 = the primary and secondary voltages
I1, I2 = the primary and secondary currents
Z1, Z2 = the primary and seconadary impedance ( Z2 = RL )
18. 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.