1. Lecture VI Power Amplifiers Class A & Class AB
DMT 231/3 Electronic II
Lecture VI
Power Amplifiers
Class A & Class AB

2. Class A Power Amplifier
Large-signal amplifiers with the objective of providing power (rather than voltage) to a load.

3. Basic class A amplifier operation.
Output is shown 180° out of phase with the input (inverted).
Figure 1

4. Maximum class A output occurs when the Q-point is centered on the ac load line.
Figure 2

5. Q-point closer to cutoff.
Figure 3

6. Q-point closer to saturation.
Figure 4

7. Power Gain Ap
The ratio of the output power (power delivered to the load), PL to the input power, Pin.
Equation 1
Equation 2
Equation 3
Equation 4

8. Power Gain Ap
Equation 5
Equation 6
Equation 7

9. DC Quiescent Power PDQ
The power dissipation of a transistor with no signal input is the product of its Q-point current and voltage
Equation 8
Maintain quiescent current at least as large as the peak current requirement for the load current.  Class A power amplifier supply power to a load

10. Output Power Pout
Product of the rms load current and the rms load voltage.
Maximum peak voltage swing (CE Amplifier)
Equation 9
Equation 10
Maximum peak current swing (CE Amplifier)
Equation 11
Maximum signal power output from class A amplifier
Equation 12

11. Example: Determine the power gain of the class A power amplifier in Figure 5

12. Class B & AB Push-Pull Power Amplifier
when an amplifier is biased at cutoff so that it operates in the linear region for 180 degree of the input cycle and is in cutoff for 180 degree.

13. Class B & AB Push-Pull Power Amplifier
MORE EFFICIENT than Class A but MORE DIFFICULT to implement the circuit in order to get linear reproduction of the input waveform.
Push-pull: common type of class B or class AB amplifier circuit in which 2 transistors are used on alternating half-cycles to reproduce the input waveform at the output.

14. Basic class B amplifier operation (non-inverting).
Biased at cutoff point. ICQ=0 and VCEQ =VCE(cutoff)
Figure 6

15. Common-collector class B amplifier
Only conducts for the positive half of the cycle  to amplify the entire cycle: add a second class B amplifier that operates on the negative half of the cycle = PUSH-PULL
2 common approaches: (a) using transformer coupling (b) using 2 complementary symmetry transistors
Figure 7

16. Transformer coupled push-pull amplifiers
Q1 conducts during the positive half-cycle; Q2 conducts during the negative half-cycle. The two halves are combined by the output transformer.
Figure 8

17. Class B Push-Pull ac operation.
Figure 9

18. Illustration of crossover distortion in a class B push-pull amplifier
Illustration of crossover distortion in a class B push-pull amplifier. The transistors conduct only during portions of the input indicated by the shaded areas.
Figure 10

19. Biasing the push-pull amplifier to eliminate crossover distortion
Biasing the push-pull amplifier to eliminate crossover distortion.  modified form operation called class AB
ICQ= (VCC V )/R1
Equation 13
Figure 11

20. Eliminating crossover distortion in a transformer-coupled push-pull amplifier  The diode compensates for the base-emitter drop of the transistors and produces class AB operation.
Figure 12

21. AC Operation Ic(sat)=VCC / RL
Load lines for a complementary symmetry push-pull amplifier. Only the load lines for the npn transistor are shown.
Ic(sat)=VCC / RL
Equation 14
Figure 13

22. AC load line for Q1 of the class AB amplifier
Figure 14

23. Exercise: Determine the ideal maximum peak voltage & current
Figure 15

24. Single-ended push-pull amplifier.
The circuit operation is the same EXCEPT the bias is set to force the output emitter voltage to be VCC / 2 instead of ZERO volts used with two supplies
Figure 16

25. Exercise: Determine the maximum ideal peak values for the output voltage & current
Figure 17

26. Class B / AB Power: Maximum average output power
Pout = Iout(rms) Vout(rms)
Since Iout(rms) = Iout(peak) = Ic(sat)
and Vout(rms) = Vout(peak) = VCEQ
Pout = 0.5Ic(sat) VCEQ
Substituting VCC / 2 for VCEQ
Pout = 0.25Ic(sat) Vcc
Equation 15
Equation 16
Equation 17
Equation 18
Equation 19

27. Class B / AB Power: DC Input Power
DC Input Power comes from the VCC
PDC = ICC VCC
Since each transistors draws current for a half-cycle, the current is a half-wave signal with average value of
Thus,
Equation 20
Equation 21
Equation 22

28. Class B / AB Power: Efficiency
Efficiency is defined as the ratio of ac output power to dc input power
From Equation 19 and Equation 22,
Equation 23
Pout = 0.25Ic(sat) Vcc
Equation 24

29. Exercise: Find the maximum ac output power & dc input power of the amplifier
Figure 18