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

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.

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.

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

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

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

Power Amplifiers Class C

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Class C power amplifier design

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

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

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