1. Power Amplifiers Power Amplifiers are used in the transmitter
There are different classes for the power amplifier
Class A, Class B, Class AB, Class C, Class D
and Class E
Those amplifier differs in their angle of conduction, efficiency, linearity and the in the amount of distortion they introduce to the system

2. Power Amplifiers 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.

3. Power Amplifiers 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.
Class C
The amplifier conducts less than 180 of the input. The Q-point is located below the cutoff level.

4. Power Amplifiers Class D
This is an amplifier that is biased especially for digital signals.

5. Class C power amplifiers
Class C amplifier is obtained if the output current conduction angle is less than 180
Class C have a greater efficiency than both class A and class B
Class C also have larger distortion compared with the A and B
Class C amplifier is used when there is no variation in signal amplitude and the output circuit contains a tuned circuit to filter out the harmonics
Class C amplifier is used for applications similar to FM Modulation

6. Class C power amplifiers
The output of the class C conducts for less than 180 of the AC cycle
The Q-point is below cutoff as shown below

7. Power Amplifiers Class C

8. Class C power amplifier
The circuit shown to the left will be considered in analysis of the class C amplifier

9. Class C power amplifier
The amplifier efficiency can be computed from the following equation
The output power is the AC power flowing in the load resistance
The input power is the supply power which can be determined from the multiplication of the supply voltage and the average collector current
The main step in the analysis is to compute the average collector current which is explained in the next slide

10. Class C power amplifier
According to the figure shown to the left, the collector current is given by
Where ID is given by

11. Class C power amplifier
The direct collector current is determined by its average value which is given by
It is desired to find the current as a function of the angles θ1 and θ2 rather than t1 and t2
This can be achieved by using the following mapping and

12. Class C power amplifier
The average collector current can be rewritten as
By evaluating this equation we may have the following expression
To simplify the notation, the conduction angle will be defined as
ICav

13. Class C power amplifier
By substituting the conduction angle expression into the average collector current equation we get
Now the supplied input power can be written as
If the output RLC circuit is a narrow band filter tuned to the fundamental frequency of the current pulses, then the output power will be

14. Class C power amplifier
Where I1 is the amplitude of the fundamental current components which is determined by the trigonometric Fourier series as shown below
By solving the previous equation we may have
The output power now can be written as

15. Class C power amplifier
The efficiency of the amplifier can be written as
A plot of the efficiency as a a function of the conduction angle is shown below

16. Class C power amplifier design
There are four important design parameters are of great importance for PA design in general
These parameters are
The output power
Transistor power dissipation
Maximum collector to emitter voltage VCEmax
The maximum transistor output current Ip

17. Class C power amplifier design
The maximum collector current is given by
Since
The collector current can be rewritten as
The maximum current in terms of the output current can be written as
Note that the value of the collector voltage VCC can be written as

18. Class C power amplifier design
Now the maximum collector current can be rewritten as
A normalized peak collector current is defined as

19. Class C power amplifier design
A plot of the normalized peak current versus the conduction angle is shown below

20. Class C power amplifier design
The power dissipated in the transistor is given by
Note the value of Ip can be expressed as
From we can conclude that
If the value of Ip is substituted in the PT equation then

21. Class C power amplifier design
The power dissipated in the transistor is given by
Or PT can be rewritten as

22. Class C power amplifier design
A normalized plot of PT/PO versus theta is shown below

23. Class C power amplifier design example
Example: Design a class C amplifier that will deliver 5-W average power to a 50 Ω load at a frequency of 1 MHz using a transistor with a safe power dissipation rating of 0.5 W
Solution:
The average output power is given by Or

24. Class C power amplifier design example
Solution:
Since the allowable power dissipation is
The maximum conduction angle can be found from the graph shown in slide 19 or by solving the PT/PO equation
The value of the normalized current corresponds to this angle is refer to the figure in slide 16
The peak collector current is given by

25. Class C power amplifier design
An alternate design procedure for class C amplifiers is
Select the power supply
Select the transistor
Determine the maximum output power without exceeding the transistor ratings
The transistor then can be driven to its maximum allowed value of output current
Determine the value of the load resistance that twill result in the maximum current according to

26. Class C power amplifier design
Now the transistor power equation can be modified as
The normalized transistor power dissipation is given by
Where

27. Class C power amplifier design

28. Class C power amplifier design example 2
Example 2: Determine the maximum output power and the conduction angle of a class C amplifier using a transistor with maximum power dissipation rating of 4 W and a maximum output current of 1.5 A. The supply voltage is 48 V
Solution:
The normalized maximum transistor dissipation is given by

29. Class C power amplifier design example 2
Solution:
The conduction angle for maximum normalized transistor power P΄T is found to be as
If we refer back (PT/PO vs θ ) plot we find that the value of PT/PO which corresponds to this angle is
The output power now can be found as

30. Class C power amplifier design example 2
Solution:
Finally the value of the load resistance that results in this output power is given by