Power amplifiers
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Power amplifiers

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Power amplifiers Power amplifiers Presentation Transcript

  • Power Amplifiers
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • Class CThe output of the class Cconducts for less than 180 of theAC cycle. The Q-point is belowcutoff. 8
  • Amplifier EfficiencyEfficiency refers to the ratio of output to input power. The lower the amountof conduction of the amplifier the higher the efficiency. 9
  • Series-Fed Class A AmplifierThis is similar to thesmall-signal amplifierexcept that it will handlehigher voltages. Thetransistor used is a high-power transistor. 10
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • Example• Calculate the efficiency of a transformer coupled class A amplifier for a supply of 12V and output of 6V
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • Harmonic DistortionAccording to Fourieranalysis, if a signal is notpurely sinusoidal, then itcontains harmonics. 28
  • 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
  • Power Transistor Derating CurvePower transistors dissipatea lot of power in heat. Thiscan be destructive to theamplifier as well as tosurrounding components. 30
  • 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
  • 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