DMT 2313 Electronic II Lecture VII Power Amplifiers

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

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.

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

Basic class C operation. Figure 20

Class C waveforms. Figure 21

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

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

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

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

Resonant circuit action (I) Figure 23

Resonant circuit action (II) Figure 24

Resonant circuit action (III) Figure 25

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

Maximum Output Power • Voltage developed across the tank circuit has a peak-to-peak value of approximately 2 VCC • The maximum output power Equation 28 Equation 29

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

Tuned class C amplifier with clamper bias. Figure 27

Clamper bias action (I) Figure 28

Clamper bias action (II) Figure 29

Clamper bias action (III) Figure 30