1. DMT 231/3 Electronic II Power Amplifiers Class C
Lecture VII
Power Amplifiers
Class C

2. Class C Amplifier
More EFFICIENT than either class A or push-pull class B and class AB
More output power can be obtained from class C operation
Output amplitude is a NONLINEAR function of the input.
Class C amplifiers are not used for linear amplification
Generally used in RF applications e.g. Oscillators.

3. Basic class C amplifier operation (non-inverting).
Figure 19

4. Basic class C operation.
Figure 20

5. Class C waveforms.
Figure 21

6. Power Dissipation LOW power dissipation
 the transistor is ON for only a small percentage of the input cycle.
T: time between pulses (period of the ac input voltage
The transistor is ON for a short time, ton

7. Power Dissipation The power dissipation during the ON time
The power dissipation AVERAGED over the entire cycle
Equation 25
Equation 26

8. Tuned class C amplifier.
Collector voltage (output) is not a replica of the input Resistively loaded class C amplifier alone is of no value in linear applications Necessary to use a class C amplifier with a parallel resonant circuit (tank) The tank circuit has HIGH IMPEDANCE only near the resonant frequency  The gain is LARGE only at this frequency

9. Tuned class C amplifier.
The resonant frequency
Equation 27
Figure 22

10. Resonant circuit action (I)
Figure 23

11. Resonant circuit action (II)
Figure 24

12. Resonant circuit action (III)
Figure 25

13. Tank circuit oscillations. Vr is the voltage across the tank circuit.
Figure 26

14. The maximum output power
Voltage developed across the tank circuit has a peak-to-peak value of approximately 2VCC
The maximum output power
Equation 28
Equation 29

15. Maximum Output Power Total power, Efficiency,
Equation 30
Equation 31 if
The class C efficiency closely approaches 1 (100 %)

16. Tuned class C amplifier with clamper bias.
Figure 27

17. Clamper bias action (I)
Figure 28

18. Clamper bias action (II)
Figure 29

19. Clamper bias action (III)
Figure 30