Power amplifiers


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Power amplifiers

  1. 1. Power Amplifiers
  2. 2. DefinitionsIn small-signal amplifiers the main factors are: • Amplification • Linearity • GainSince large-signal, or power, amplifiers handle relatively largevoltage signals and current levels, the main factors are: • Efficiency • Maximum power capability • Impedance matching to the output device 2
  3. 3. Amplifier TypesClass A The amplifier conducts through the full 360 of the input. The Q-point is set near the middle of the load line.Class B The amplifier conducts through 180 of the input. The Q-point is set at the cutoff point.Class AB This is a compromise between the class A and B amplifiers. The amplifier conducts somewhere between 180 and 360 . The Q-point is located between the mid-point and cutoff. 3
  4. 4. Amplifier TypesClass C The amplifier conducts less than 180 of the input. The Q-point is located below the cutoff level.Class D This is an amplifier that is biased especially for digital signals. 4
  5. 5. Class A AmplifierThe output of a class A amplifierconducts for the full 360 of thecycle.The Q-point is set at the middle ofthe load line so that the AC signalcan swing a full cycle. Remember that the DC load line indicates the maximum and minimum limits set by the DC power supply. 5
  6. 6. Class B AmplifierA class B amplifier outputonly conducts for 180 orone-half of the AC inputsignal. The Q-point is at 0V on the load line, so that the AC signal can only swing for one-half cycle. 6
  7. 7. Class AB AmplifierThis amplifier is a compromise between theclass A and class B amplifier—the Q-pointis above that of the Class B but below theclass A.The output conducts between 180 and360 of the AC input signal. 7
  8. 8. Class CThe output of the class Cconducts for less than 180 of theAC cycle. The Q-point is belowcutoff. 8
  9. 9. Amplifier EfficiencyEfficiency refers to the ratio of output to input power. The lower the amountof conduction of the amplifier the higher the efficiency. 9
  10. 10. Series-Fed Class A AmplifierThis is similar to thesmall-signal amplifierexcept that it will handlehigher voltages. Thetransistor used is a high-power transistor. 10
  11. 11. Series-Fed Class A AmplifierWhen an input signal isapplied the output willvary from its dc biasoperating voltage andcurrent. A small inputsignal causes the outputvoltage to swing to amaximum of Vcc and aminimum of 0V. Thecurrent can also swingfrom 0mA to ICSAT(VCC/RC) 11
  12. 12. Series-Fed Class A AmplifierInput 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)  VCCI CQOutput Power V 2 C(rms) Po(dc)  VCE(rms)IC(rms) Po(ac)  or RC Po(dc)  I2C(rms)RcEfficiency Po(ac) %η   100 Pi(ac) 12
  13. 13. Example• Calculate the input power, output power and efficiency of the amplifier for an input voltage that results in s base current 0f 10mA if RB = 1kΩ, Rc = 20Ω, β = 25 and Vcc = 20V
  14. 14. Transformer-Coupled Class A Amplifier This circuit uses a transformer to couple to the load. This improves the efficiency of the Class A to 50%. 14
  15. 15. Transformer ActionA transformer improves the efficiency because it is able to transformthe voltage, current, and impedance Voltage Ratio V2 N 2  V1 N 1 Current Ratio I 2 N1  I1 N 2 Impedance Ratio 2 R R N  L  1  1   a2 RL R2  N2    15
  16. 16. Transformer-Coupled Class A AmplifierDC Load LineAs in all class A amplifiers the Q-point is established close to themidpoint of the DC load line. Thedc resistance is small ideally at 0Ωand a dc load line is a straightvertical line.AC Load LineThe saturation point (ICmax) is atVcc/RL and the cutoff point is atV2 (the secondary voltage of thetransformer). This increases themaximum output swing becausethe minimum and maximumvalues of IC and VCE are spreadfurther apart. 16
  17. 17. Transformer-Coupled Class A AmplifierSignal Swing and Output AC Power The voltage swing: VCE(p  p)  VCEmax  VCEmin The current swing: I c ( p  p )  I C m ax  I C m in The AC power: (VCEmax  VCEmin )(ICmax  ICmin ) Po(ac)  (max imum) 8 17
  18. 18. Transformer-Coupled Class A Amplifier Efficiency Power input from the DC source: Pi(dc)  VCCI CQ Power dissipated as heat across the transistor: PQ  Pi(dc)  Po(ac) Note: The larger the input and output signal, the lower the heat dissipation. Maximum efficiency: 2 V  VCEmin  Note: The larger VCEmax and smaller VCEmin, the %η  50 CEmax V   closer the efficiency approaches the theoretical  CEmax  VCEmin  maximum of 50%. 18
  19. 19. Example• Calculate the efficiency of a transformer coupled class A amplifier for a supply of 12V and output of 6V
  20. 20. Class B AmplifierIn class B, the transistor isbiased just off. The AC signalturns the transistor on.The transistor only conductswhen it is turned on by one-half of the AC cycle.In order to get a full AC cycleout of a class B amplifier, youneed two transistors: • An npn transistor that provides the negative half of the AC cycle • A pnp transistor that provides the positive half. 20
  21. 21. Class B Amplifier: EfficiencyThe maximum efficiency of a class B is 78.5%.. Po(ac ) %   100 Pi(dc ) 2 VCC maximumPo(dc)  2R LFor maximum power, VL=VCC  2VCC  2V 2 CC maximumPi(dc)  VCC (maximumI dc )  VCC   πR   πR   L  L 21
  22. 22. Transformer-Coupled Push-Pull Class B AmplifierThe center-tappedtransformer on the inputproduces oppositepolarity signals to thetwo transistor inputs.The center-tappedtransformer on theoutput combines the twohalves of the ACwaveform together. 22
  23. 23. Class B Amplifier Push-Pull Operation • During the positive half-cycle of the AC input, transistor Q1 (npn) is conducting and Q2 (pnp) is off. • During the negative half-cycle of the AC input, transistor Q2 (pnp) is conducting and Q1 (npn) is off.Each transistor produces one-half of an AC cycle. The transformer combines thetwo outputs to form a full AC cycle. 23
  24. 24. Crossover DistortionIf the transistors Q1 and Q2 donot turn on and off at exactlythe same time, then there is agap in the output voltage. 24
  25. 25. Quasi-Complementary Push-Pull Amplifier A Darlington pair and a feedback pair combination perform the push-pull operation. This increases the output power capability. 25
  26. 26. Amplifier DistortionIf the output of an amplifier is not a complete AC sine wave,then it is distorting the output. The amplifier is non-linear.This distortion can be analyzed using Fourier analysis. InFourier analysis, any distorted periodic waveform can bebroken down into frequency components. Thesecomponents are harmonics of the fundamental frequency. 26
  27. 27. HarmonicsHarmonics are integer multiples of a fundamental frequency.If the fundamental frequency is 5kHz: 1st harmonic 1 x 5kHz 2nd harmonic 2 x 5kHz 3rd harmonic 3 x 5kHz 4th harmonic 4 x 5kHz etc.Note that the 1st and 3rd harmonics are called odd harmonics and the2nd and 4th are called even harmonics. 27
  28. 28. Harmonic DistortionAccording to Fourieranalysis, if a signal is notpurely sinusoidal, then itcontains harmonics. 28
  29. 29. Harmonic Distortion CalculationsHarmonic distortion (D) can be calculated: An % nth harmonicdistortion  %Dn   100 A1 where An is the amplitude of the fundamental frequency An is the amplitude of the highest harmonicThe total harmonic distortion (THD) is determined by: % THD  D 2  D 2  D 2    100 2 3 3 29
  30. 30. Power Transistor Derating CurvePower transistors dissipatea lot of power in heat. Thiscan be destructive to theamplifier as well as tosurrounding components. 30
  31. 31. Class C AmplifiersA class C amplifier conducts for lessthan 180. In order to produce a fullsine wave output, the class C uses atuned circuit (LC tank) to providethe full AC sine wave.Class C amplifiers are usedextensively in radio communicationscircuits. 31
  32. 32. Class D AmplifierA class D amplifier amplifiespulses, and requires a pulsedinput.There are many circuits thatcan convert a sinusoidalwaveform to a pulse, as wellas circuits that convert apulse to a sine wave. Thiscircuit has applications indigital circuitry. 32