Class A amplifier analysis and efficiency calculation
1. INTERNATIONAL SCHOOL OF TECHNOLOGY &
SCIENCES (FOR WOMEN)
DEPARTMENT OF ELECTRONICS &
COMMUNICATION ENGINEERING
Prepared By:
Mr. K V VENKATARAMANA,
Assistant Professor.
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
II BTECH II SEMESTER
ELECTRONIC CIRCUIT ANALYSIS
UNIT IV
2. Contents:
Classification of Amplifier
Class A Power amplifier and itsAnalysis
Example
Harmonic Distortions
Class B push-pull amplifiers and itsAnalysis
Example
Complementary Symmetry push-pull Amplifier
Class AB PowerAmplifier
Class C PowerAmplifier
Thermal Stability and Heatsinks
Distortion in Amplifiers
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
3. Introduction
Power amplifiers are used to deliver a relatively high
amount ofpower, 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 doillustrate the fact that power
amplifiers usually drive low- resistance loads.
Typical output power rating of a power amplifier will
be 1W orhigher.
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.K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
4. Concept of Power Amplifier
Provide sufficient power to an output load to
drive other power device.
To deliver a large current to a small loadresistance
e.g. audio speaker;
To deliver a large voltage to alarge load resistance
e.g. switching power supply;
To provide a low output resistance inorder to avoid
loss of gain and to maintain linearity (to minimize
harmonic distortion)
To deliver power to the loadefficiently
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
6. Power Amplifier Power Dissipation
P1 = I 2R
1 1
P2 = I 2R
2 2
ICQ
RC
RE
R1
R2
VCC
I1
I2
ICC
P = I2
R
C CQ C
PT = I2
R
TQ T
P = I2
R
E EQ E
IEQ
The total amount of power
being dissipated by the
amplifier, Ptot , is
Ptot = P1 + P2 + PC + PT + P E
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.
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
7. Power Amplifier Efficiency
A figure of merit for the power amplifier is its efficiency,
.
Efficiency ( of an amplifier is defined as the
ratio of ac output power (power delivered to load) to dc
input power . B y
formula:
A s 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.
dc input power P(dc)
ac output
i
o
power
100%
P (ac)
100%
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
8. “AUDIO AMPLIFIERS” – Unit-5
“RADIO FREQUENCY AMPLIFIERS” –
Unit-6
D C POWER AMPLIFIERS
9
Power Amplifiers Classification
(Depending on Type of output load)
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
10. Power Amplifiers Classification
Class A - The transistor
conducts during the whole
cycle of sinusoidal input
signal
Class B - The transistor
conducts during one-half cycle
of input signal
Class AB - The transistor
conducts for slightly more
than half a cycle of input
signal
Class C - The transistor
conducts for less than half a
cycle of input signal
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
11. 12 K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
12. Efficiency Ratings
The maximum theoretical efficiency ratings of
class-A, B, and C amplifiers are:
Amplifier
ClassA
Class B
Class C
Maximum Theoretical
Efficiency, max
25%
78.5%
99%
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
13. Steps for Analysis of Power
Amplifiers:Draw the Circuit Diagram of relevant type
of Power Amplifer
Draw the Graphical operating point
representation
Make DC analysis (AC components as 0) &
Calculate ‘Pdc’
Make AC analysis (DC components as 0) &
Calculate ‘Pac’
Measure the Percentage Efficiency of the
amplifier (% n)
Show the Power Dissipation
Advantages and Disadvantages14 K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
14. Class A Amplifier
o u tp ut waveform same shape input waveform +
phase shift.
The collector current is nonzero 100% of thetime.
inefficient, since even with zero input signal, ICQ is
nonzero (i.e. transistor dissipates power in
the rest, or quiescent, condition)
vin vout
Av
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
15. Class A Amplifier Classification
Series FED/Direct Coupled Transformer Coupled
RC
RE
R1
R2
RL
v in
ICQ
I1
+VCC
ICC
i C
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
16. Series FED/Direct Coupled Class A Power
Amplifier Analysis
Common-emitter (voltage-divider) configuration (RC-
coupled amplifier)
RC
RE
R1
R2
RL
vin
ICQ
I1
+VCC
ICC
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
17. Typical Characteristic Curves for Class A
Operation
Configuration : No inductor @ transformer are used
(a) Common-emitter amplifier,
(b) dc load line (the Q point is at centre of the load line)
(c)instantaneous power dissipation versus time in the
transistor
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
18. DC Input Power
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 CCi
P(dc) V I
I I ICC CQ 1
I ICC CQ
(I I )CQ 1
i CC CQ
P(dc) V I
Note : That this equation is valid for most amplifier power analyses. We can
rewrite for the above equation for the ideal amplifier as
Pi(dc) VCC ICQ
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
19. AC Output Power
R1//R2
vcevin
vo
ic
RC//RLrC
Po(ac) ic(rms)vo(rms)
2
Vm
Vo(rms) 2
Im
Io(rms)
V m
V p p
2 2
I m
I p p
2
Vpp
V maxV min
2
I pp
Im ax I m in
22 2
o
P (ac)
Vm
*
Im
Vm *Im
2 8
o
Vpp
*
Ipp
P (ac) 2 2
Vpp * Ipp
8
o
P(ac)
(VmaxVmin)*(Im axImin)
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
21. Advantages:
1) Circuit is Simple to Design
2) Less no. of Components
Dis-Advantages:
1) Load Resistor is directly connected to collector. This causes
considerable wastage of Power
2) Power Dissipation is More
3) Efficiency is very poor, due to large power Dissipation
21
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
22. Transformer Coupled Amplifier
•
•
•
•
• The theoretical maximum efficiency of a basic
RC-coupled class-A amplifier is limited to 25%.
In practical circuit, the efficiency is less
than 25%. Used for output power of
about 1 W only.
Transformer coupling can increase the
maximum efficiency to 50%
Disadvantage of transformer coupling –
expensive & bulky.K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
25. DC Input Power
The total dc power, Pi(dc) , that an
amplifier draws from the power supply :
CC CCi
P(dc) V I
I I (I I )
CC CQ CQ 1
CC CQi
P(dc) V I
Note : That this equation is valid for most amplifier power analyses. We can
rewrite for the above equation for the ideal amplifier as
Pi(dc) VCC ICQ
i C
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
26. AC Output Power
2
V pp
V maxV min
i C
Po(ac) i1(rms)v1(rms) Po(ac) i2(rms)v2(rms)
At Primary side A.C power generated:
L
L
R'
V2
1rmsR' 1rmsPo(ac)i1(rms)v1(rms)I2
22 2
1111 Im* mVIm mV
o * P(ac)
2
V p p
V m
2
Ip p
I m
2
I pp
Im ax I m in
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
27. 2 8
o
8
o
P(ac)
(VmaxVmin)*(Im axImin)
We consider an idea Transformer, means Power delivered on primary side isequal
to the secondary side load, and the values of currents and voltages are similar.
Vpp
*
Ipp
P (ac) 2 2
Vpp * Ipp
Pi ( d c ) V C C I C Q
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
29. Power
Dissipation:
29
pdpDCpAC
In case of Class A amplifiers, Power Dissipation is maximum when no input
signal is applied.
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
30. 31
Advantages:
1) Efficiency be somewhat high compared to series fed Class A
2)Impedance matching is required for maximum power transfer is
possible
Dis-Advantages:
1) Due to Transformer, the circuit becomes bulkier, heavier and
costlier compared to series fed type
2) Frequency response of the circuit is poor
3) Circuit is complicated to design and implementation compared to
series fed type
31. Class A
advantages:•No cross over distortion
•No switching distortion
•Lower harmonic distortion in the voltage amplifier
•Lower harmonic distortion in the current amplifier
•No signal dependent distortion from the power supply
•Constant and low output impedance
•Simpler design
Class A dis-advantages:
• Low Efficiency
• Power Dissipation more
• Wastage of Power
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
32. Example
Calculate the input power [Pi(dc)], output power
[Po(ac)], and efficiency [] of the amplifier circuit for
an input voltage that results in a base current of
10mA peak.
+VCC = 20V
IC
Vi
25
RC
20
RB
1k
Vo
2 2
100% 6.5%
C
C
c(sat)
Pi(dc)
Pi(dc) VCC ICQ (20V )(0.48A) 9.6W
(20) 0.625W
250103
A2
C ( peak )
R
I2
Po(ac)
20V
1000mA 1A
R 20
VCC
I
19.3mA
20V 0.7V
1kRB
VCC VBE
IBQ
Po(ac)
VCE (cutoff ) VCC 20V
IC( peak ) Ib( peak ) 25(10mA peak) 250mA peak
ICQ IB 25(19.3mA) 482.5mA 0.48A
VCEQ VCC ICRC 20V (0.48A)(20) 10.4V
33. Class B Power
Amplifier Consists of Push-Pull & complementary pair electronic devices
One conducts for one half cycle of the input signal and the other
conducts for another half of the input signal
When the input is zero, both devices are off, the bias currents are
zero and the output is zero.
Ideal voltage gain is unity
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
34. For input larger than zero, A turn ON and supplies current to the
load.
For input less than zero, B turn ON and sinks current from the
load
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
35. Class-B transformer coupled push
pull amplifier
36
Driver Transformer
Load Transformer
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
36. DC Input Power
CC dc CC m
P(dc) V I
2
V I
•DC biasing point is on the X-axis
so the Vceq=VCC. And Iceq
becomes 0. hence Q-point (Vcc,0).
• Each Transformer output is an Half-rectified
Waveform. Then peak value of the output current is the average d.c current
I1 (dc) Im
I 2 (dc) Im
Im
2Im
I (dc) Im
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
37. AC Output Power
Po(ac) i1(rms)v1(rms) Po(ac) i2(rms)v2(rms)
At Primary side A.C power generated:
L
L
R'
V2
1rmsR' 1rmsPo(ac)i1(rms)v1(rms)I2
R
VI R
L
m L
o '
2
m
2 '
222
Vm Im Vm*Im
2 2
* P(ac)
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
39. Power
Dissipation:pdpDCpAC
pd 2
VccImVm*Im
2
In case of Class A amplifiers, Power Dissipation is maximum when no input
signal is applied. But in Case of Class B, when input is 0 then Vm=0. Hence
Power dissipation is very less compared to Class A.
39
40. 41
Advantages:
1) Efficiency is much higher than Class A operation.
2) When there is no input signal, the power dissipation is zero.
3) Harmonics get cancelled. This reduces the harmonic distortion.
4) Ripples present in supply voltage also get eliminated.
5) Due to the Transformer impedence Matching is possible.
Dis-Advantages:
1) Frequency response of the circuit is poor
2) Center Tap transformers are necessary.
3) Transformers make the circuit bulky and hence costlier
43. 44
Advantages:
1) As the circuit is transformer less, its weight, size and cost are less.
2)Due to the common collector configuration, impedance matching
is possible.
3)Frequency response of the circuit is improves due to transformer
less Class B amplifier circuit
Dis-Advantages:
1) Circuit needs two separate voltage supplies
2) Output is distorted to cross-over distortion
51. Collector Currents & Output Current
iCpiCn iL
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
52. Comparison of Amplifiers
Class A B C AB
Operating 360˚ 180˚ Less that 180˚ 180˚ to
360˚ Cycle
Position of Q- Center of Load On X-axis Below X-axis In between
X-
Point Line axis to
center
of Load line
Efficiency Poor, Better, High In
between 25% to 50% 78.5% 78.5% to
50%
Distortion No Present Highest Present
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS
53. Amplifier Distortion :
Amplifier Distortion can take on many forms
such as Amplitude, Frequency and Phase
Distortion due to Clipping
Distortion in Amplifiers
K V VENKATARAMANA‖ ECA ‖ Dept of ECE ‖ ISTS