3. Amplifier Power Dissipation The total amount of power being dissipated by the amplifier, P tot , is P tot = P 1 + P 2 + P C + P T + P E The difference between this total value and the total power being drawn from the supply is the power that actually goes to the load – i.e. output power .  Amplifier Efficiency 

9. Typical Characteristic Curves for Class-A Operation

11. DC Input Power The total dc power, P i (dc) , that an amplifier draws from the power supply : Note that this equation is valid for most amplifier power analyses. We can rewrite for the above equation for the ideal amplifier as

12. AC Output Power AC output (or load) power, P o (ac) Above equations can be used to calculate the maximum possible value of ac load power. HOW?? Disadvantage of using class-A amplifiers is the fact that their efficiency ratings are so low,  max  25% . Why?? A majority of the power that is drawn from the supply by a class-A amplifier is used up by the amplifier itself.  Class-B Amplifier

14. Limitation

15. Example Calculate the input power [ P i (dc) ], output power [ P o (ac) ], and efficiency [  ] of the amplifier circuit for an input voltage that results in a base current of 10mA peak.

16. Transformer-Coupled Class-A Amplifier A transformer-coupled class-A amplifier uses a transformer to couple the output signal from the amplifier to the load. The relationship between the primary and secondary values of voltage, current and impedance are summarized as: N 1 , N 2 = the number of turns in the primary and secondary V 1 , V 2 = the primary and secondary voltages I 1 , I 2 = the primary and secondary currents Z 1 , Z 2 = the primary and seconadary impedance ( Z 2 = R L )

18. Counter emf This counter emf will be present only for an instant. As the field collapses into the inductor the voltage decreases in value until it eventually reaches 0V.

19. DC Operating Characteristics The dc biasing of a transformer-coupled class-A amplifier is very similar to any other class-A amplifier with one important exception :  the value of V CEQ is designed to be as close as possible to V CC . The dc load line is very close to being a vertical line indicating that V CEQ will be approximately equal to V CC for all the values of I C . The nearly vertical load line of the transformer-coupled amplifier is caused by the extremely low dc resistance of the transformer primary. V CEQ = V CC – I CQ (R C + R E ) The value of R L is ignored in the dc analysis of the transformer-coupled class-A amplifier. The reason for this is the fact that transformer provides dc isolation between the primary and secondary. Since the load resistance is in the secondary of the transformer it dose not affect the dc analysis of the primary circuitry.

22. Maximum load power and efficiency The Power Supply for the amplifier : P S = V CC I CC Maximum peak-to-peak voltage across the primary of the transformer is approximately equal to the difference between the values of V CE(max) and V CE(min) : V PP = V CE(max) – V­ CE(min) Maximum possible peak-to-peak load voltage is found by V (P-P)max = (N 2 / N 1 )V PP The actual efficiency rating of a transformer-coupled class-A amplifier will generally be less than 40% .