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Unit –II High Power Amplifiers

-18marks. Unit –II High Power Amplifiers. Course Outcome. Use BJT as high power amplifier. Definitions. In small-signal amplifiers the main factors are: Amplification Linearity Gain

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Unit –II High Power Amplifiers

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  1. -18marks Unit –IIHigh Power Amplifiers

  2. Course Outcome Use BJT as high power amplifier

  3. Definitions • In small-signal amplifiers the main factorsare: • Amplification • Linearity • Gain • Since large-signal, or power, amplifiers handle relativelylarge voltage signals and current levels, the main factorsare: • Efficiency • Maximum powercapability • Impedance matching to the outputdevice

  4. AmplifierTypes ClassA The amplifier conducts through the full 360of the input. The Q-point is set near the middle of the loadline. ClassB The amplifier conducts through 180of the input. The Q-point is setat the cutoffpoint. ClassAB This is a compromise between the class A and B amplifiers. The amplifier conducts somewhere between 180and 360. The Q-pointis located between the mid-point andcutoff.

  5. AmplifierTypes ClassC The amplifier conducts less than 180 of the input. The Q-point is located below the cutofflevel.

  6. Class AAmplifier The output of a class Aamplifier conducts for the full 360of the cycle. The Q-point is set at the middle of the load line so that the AC signal can swing a fullcycle. Remember that the DC load line indicates the maximum andminimum limits set by the DC powersupply.

  7. Class BAmplifier A class B amplifieroutput only conducts for 180 or one-half of the AC input signal. The Q-point is at 0V onthe load line, so that the AC signal can only swing for one-halfcycle.

  8. Class ABAmplifier This amplifier is a compromise betweenthe class A and class B amplifier—the Q-point is above that of the Class B but below the classA. The output conducts between 180and 360of the AC inputsignal.

  9. ClassC The output of the class C conducts for less than 180 ofthe AC cycle. The Q-point is below cutoff.

  10. AmplifierEfficiency Efficiency refers to the ratio of output to input power. The lower the amount of conduction of the amplifier the higher theefficiency.

  11. Direct Coupled Class-A Amplifier This is similar to the small-signal amplifier except that it willhandle higher voltages. The transistor used is ahigh- powertransistor.

  12. Direct Coupled Class-A Amplifier When an input signal is applied the output will vary from its dc bias operating voltage and current. A small input signal causes theoutput voltage to swing to a maximum of Vcc and a minimum of 0V. The current can also swing from 0mA to ICSAT (VCC/RC)

  13. Direct Coupled Class-A Amplifier Input Power The power into the amplifier is from the DC supply. With no input signal, the DC current drawn is the collector bias current,ICQ. Pi(dc) VCCICQ OutputPower V2C(rms) P VI or Po(ac) o(dc) CE(rms)C(rms) R C P I2C(rms)Rc o(dc) Efficiency Po(ac) %η100 Pi(ac)

  14. Transformer-Coupled Class AAmplifier This circuit uses a transformer to couple to the load. This improves the efficiency of theClass A to50%.

  15. TransformerAction A transformer improves the efficiency because it is able totransform the voltage, current, andimpedance VoltageRatio V2 N2 V1 N1 Current Ratio I 2 N1 I1 N2 Impedance Ratio N1 2 RL R1 2  N  R a R 2 2 L

  16. Transformer-Coupled Class AAmplifier DC LoadLine As in all class A amplifiers the Q- point is established close to the midpoint of the DC load line. The dc resistance is small ideally at0Ω and a dc load line is a straight verticalline. AC LoadLine The saturation point (ICmax) is at Vcc/RL and the cutoff point is at V2 (the secondary voltage of the transformer). This increases the maximum output swing because the minimum and maximum values of IC and VCE arespread furtherapart.

  17. Transformer-Coupled Class AAmplifier Signal Swing and Output ACPower The voltageswing: VCE(pp) VCEmax VCEmin The currentswing: Ic(pp)ICmaxICmin The ACpower: (VCEmax VCEmin )(ICmax ICmin )(maximum) P o(ac) 8

  18. Transformer-Coupled Class AAmplifier Efficiency Power input from the DCsource: Pi(dc) VCCICQ Power dissipated as heat across thetransistor: Note: The larger the input and output signal,the lower the heatdissipation. PQ Pi(dc) Po(ac) Maximumefficiency: Note: The larger VCEmax and smaller VCEmin,the closer the efficiency approaches the theoretical maximum of50%. VCEmax VCEmin 2  %η 50 V • V CEmax CEmin

  19. CrossoverDistortion If the transistors Q1 and Q2 do not turn on and off at exactly the same time, then there is a gap in the outputvoltage.

  20. Class AB Push Pull Amplifier

  21. Complementary Symmetry Class AB Amplifier

  22. Class CAmplifiers A class C amplifier conducts forless than 180. In order to produce afull sine wave output, the class C uses a tuned circuit (LC tank) to provide the full AC sinewave. Class C amplifiers are used extensively in radiocommunications circuits.

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