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

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

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

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

Q-point closer to cutoff. Figure 3

Q-point closer to saturation. Figure 4

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

Power Gain Ap Equation 5 Equation 6 Equation 7

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

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

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

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.

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.

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

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

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

Class B Push-Pull ac operation. Figure 9

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

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 - 0.7V )/R1 Equation 13 Figure 11

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

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

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

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

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

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

Class B / AB Power: Maximum average output power Pout = Iout(rms) Vout(rms) Since Iout(rms) = 0.7071Iout(peak) = 0.7071Ic(sat) and Vout(rms) = 0.7071Vout(peak) = 0.7071VCEQ 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

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

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

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