Comparison of A, B & C Power Amplifiers
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This document describes three classes of transistor amplifier operation: Class A, B, and C. Class A operation has one device conducting over the entire AC cycle with a conduction angle of 360 degrees. Class B has two devices each conducting for half the cycle with a conduction angle of 180 degrees. Class C has very brief conduction over a small portion of the cycle with a conduction angle less than 180 degrees. Class A has the highest linearity and lowest distortion but also the lowest maximum efficiency of 25%. Class B has a higher maximum efficiency of 78.5% while Class C can reach 100% efficiency but has the poorest linearity and highest distortion.
Comparison of A, B & C Power Amplifiers
- 1. 1. Circuit Diagram • Class A
- 2. 1. Circuit Diagram • Class B
- 3. 1. Circuit Diagram • Class C
- 4. Collector current waveforms for transistors operating in (a) class A, (b) class B, (c) class C amplifier stages. 2. Output Waveforms
- 5. 3. OPERATING MODE – CLASS A Time Iout One device conducts for the whole of the a.c. cycle. Conduction angle = 360 °. .
- 6. Time Iout Two devices, each conducting for half of the a.c. cycle. Conduction angle = 180 °. 3. OPERATING MODE – CLASS B
- 7. Time Iout One device conducts a small portion of the a.c. cycle. Conduction angle << 180 °. 3. OPERATING MODE – CLASS C
- 8. 4. Efficiency – Class A • The maximum possible overall efficiency with resistive load is 25%. The maximum possible collector efficiency with resistive load is 50%. In case an output transformer is used, both of these efficiencies are 50%.
- 9. 4. Efficiency – Class B • In comparison to class A amplifiers average current is less, power dissipation is less. So overall efficiency is increased. The theoretical efficiency in class B operation is about 78.5% while it is only 50% in class A operation.
- 10. 4. Efficiency – Class C • Most of the dc power is converted into ac load because transistor and coil losses are small. When the conduction angle is 1800 , efficiency is 78% - the theoretical maximum efficiency of class B amplifier. As the conduction angle decreases, efficiency increases. It has a maximum efficiency of 100%.
- 11. 5. Maximum Peak to Peak Voltage • Class A MPP < VCC • Class B MPP = VCC • Class C MPP = 2(VCC)
- 12. 6. Linearity & Distortion • Class A :- has the highest linearity & lowest distortion among all the other classes • Class B :- Has lower linearity than the Class A amplifiers & has small to moderate distortion. • Class C :- Has the poorest linearity & highest distortion among the classes.
- 13. 7. USES • Class A – Low power Amplifiers where efficiency is not important • Class B – Output power Amplifiers • Class C – Tuned RF Amplifiers but cannot be used as audio amplifiers due to high distortion
- 14. THANK YOU