Power Amplifiers

Classification of Power AmplifiersPower amplifiers are classified based on the Q pointIf the operating point is chosen at the middle of the load line, it is called Class A amplifierIf the operating point is chosen at the cut-off point it is called Class B amplifierIf the operating point is chosen beyond the cut-off point it is called Class C amplifierIt conducts for 3600

Concept PreviewEfficiency is most important in power amplifiers.Poor efficiency means that much of the input power is converted to heat.A class A amplifier conducts for the entire signal cycle and has the lowest efficiency.A class B amplifier conducts for only half of the signal cycle.A class C amplifier conducts for less than half of the signal cycle.A class D amplifier switches between cutoff and saturation.

PowerAmplifierPINEfficiency =POUTPOUTPINHEAT = PIN - POUTHigh efficiency means less heat.

EfficiencyThe dc power supplied to an amplifier is PIN = VCC x IDCEfficiency = POUT/PIN x 100%The maximum efficiency for Class A amplifiers with a dc collector resistance and a separate load resistance is 25%.Class A is usually not acceptable when watts of power are required.

tICtICtICtICISATABCDThe major classes of amplifier operation

Class and efficiency quizIf POUT = 100 W and PIN = 200 W, theefficiency is _________.50%The efficiency of an ideal amplifier is__________.100%When efficiency is poor, too much of the inputis converted to ________.heatAn amplifier that conducts for the entire cycleis operating Class _______.AAn amplifier that conducts for half the cycleis operating Class _______.B

Concept PreviewClass A amplifiers operate at the center of the load line and have a large quiescent current flow.Class B amplifiers operate at cutoff and have no quiescent current flow.Class B amplifiers are usually operated in push-pull configurations.Class B amplifiers have crossover distortion.Class AB reduces crossover distortion.Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.

Class A amplifierThe Q point is chosen at the middle of load lineThis will give equal swing on either directionBoth halves of the input comes at the outputHence Class A will give (amplitude) distortionless outputIt can handle only small signalsIts efficiency is less

IcVce Ib = 60AIb = 50AIb = 20AIb = 30AIb = 40A10mA 2mA 4mA 6mA 8mA24 V0 VQAB10mA6mA4mAClass A

A large-signal amplifier can also be called a power amplifier.This class A amplifier has a large quiescent collector current.CBEVCC = 18 VRL = 12 WRB = 1.2 kWCCb = 60 IC = b x IB = 60 x 15 mA = 0.9 A

0246810121416180.20.40.60.81.01.21.4VCE in VoltsIC in AThis is a Class A amplifier.PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W

Class B amplifierThe Q point is chosen at the cut-off pointThis will give swing only on one directionOnly one half of the input comes at the outputHence Class B will give (amplitude) distorted outputIt can handle large signalsIts efficiency is highIt conducts for 1800

IcVce Ib = 60AIb = 50AIb = 20AIb = 30AIb = 40A10mA 2mA 4mA 6mA 8mA24 V0 VQ10mA0mAClass B

0246810121416180.20.40.60.81.01.21.4VCE in VoltsIC in AThis is a Class B amplifier.PC = VCE x IC = 18 V x 0 A = 0 WIts quiescent power dissipation is zero.

Class BThe collector signal is too distorted forlinear applications.

CBEE+VCCThe complementary-symmetry Class B push-pull amplifier has acceptable linearity for some applications.NPNPNP

NPNPNPClass B

CBEE+VCCSince the base-emitter junction potentialis 0.7 V, there is some crossover distortion.NPNPNP

CBEE+VCCCrossover distortion is eliminatedby applying some forward biasto the transistors (class AB).NPNPNP1.4 V

0246810121416180.20.40.60.81.01.21.4VCE in VoltsIC in AThe quiescent power dissipation is moderate for class AB.The efficiency is much better than class A.

Single-ended amplifierA bridge-tied load provides four times the outputpower for a given supply voltage and load resistance.Bridge amplifier

Class A, B, and AB quizClass A amplifiers are biased to operate nearthe ________ of the load line.centerClass B amplifiers have their Q-points at____________.cutoffThe conduction angle for class B is_________.180oTo reduce distortion, two class B transistorsare arranged in _____________.push-pullClass AB is a solution for __________distortion.crossover

Concept ReviewClass A amplifiers operate at the center of the load line and have a large quiescent current flow.Class B amplifiers operate at cutoff and have no quiescent current flow.Class B amplifiers are usually operated in push-pull configurations.Class B amplifiers have crossover distortion.Class AB reduces crossover distortion.Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.Repeat Segment

Concept PreviewClass C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency.Class C amplifiers used tuned tank circuits to reduce distortion in RF applications.Class C amplifiers cannot be used in wideband applications like audio.Class D amplifiers switch between cutoff and saturation for very high efficiency.Class D amplifiers operate at a relatively high switching frequency and often use PWM.Class D can be used in audio applications.

0246810121416180.20.40.60.81.01.21.4ABCABThe class of an amplifieris determined by the biaswhich establishes the Q-point.Class C is established by reversebiasing the base-emitter junction.

Class C amplifierThe Q point is chosen at the beyond the cut-off pointThis will give only a partial swing in one directionOnly a portion of the input comes at the outputHence Class C will give (amplitude) severely distorted outputIt can handle large signalsIt conducts for less than 1800

IcVce Ib = 60AIb = 50AIb = 20AIb = 30AIb = 40A10mA 2mA 4mA 6mA 8mA24 V0 VQ`10mA0mAClass C

Class AClass BClass C

Conduction Angles& theoretical max. efficiencies:Class A = 360o 50 %*Class B = 180o 78.5 %Class AB @ 200o (between A & B)Class C @ 90o 100 %

*Class A amplifiers are seldom driven to maximum output and typically provide much less efficiency.

CBEVCCRB CCVBBClass C amplifierVBB reverse biases the base-emitter junction.Tank circuitThe transistor isoff for most ofthe input cycle and the conductionangle is small.

VBB0.7 V0 AVBE waveformIC waveformVCE waveformClass C amplifierwaveforms(with tank circuit)Low VCE when IC is flowing

CBEVCCRB CCClass C amplifier with signal biasThe base-emitterjunction rectifiesthe input signaland charges CC.Signal bias increases when the input signal increases in amplitude.

IB >> 0Three transistor operating modes:IB = 0IB > 0CutoffLinear(PC > 0)SaturationPC = 0 in both of these modes

Distortionless amplifierOut of the 3 amplifiers, Class C is unsuitable as the distortion is very heavyClass A is the best, as it gives distortionless outputBut Class A cannot handle large signals as required by the Power AmplifierThough Class B gives heavy distortion, it gives out one half of the signal perfectlyAnd Class B can handle large signals

Class A Audio AmplifierAs we have seen out of the 3 classifications, Class A is the best, as it does not give any distortionAmong the configurations, we know that CE is the best as it gives maximum power gainA CE amplifier will have high output impedanceUnfortunately for an audio amplifier, the output device is the speaker which has a low impedance

Impedance MatchingThe speaker impedance is typically about 4 Hence there is a mismatch between the high Zo of the amplifier and the low impedance of the speakerThis will result in loss of gainThis can be avoided by connecting a transformer at the output stageThe primary winding will match the high Zo of the amplifier while the secondary will match the low impedance of the speaker

Class A Audio Amplifier270 K5.6 KReRb1Rb2CeRcVcc270 KReRb1Rb2Ce

DrawbackThe drawback of this circuit is that it cannot handle large signalsIn a Class A amplifier, the operating point is chosen around the middle of the load lineIf the signal exceeds the cut-off point, the output current stops and any signal with a lower amplitude will not come at the outputSimilarly, if the signal exceeds the saturation point, the output current cannot increase any further, even if the input signal increases

IcVce Ib = 60AIb = 50AIb = 20AIb = 30AIb = 40A10mA 2mA 4mA 6mA 8mA24 V0 VQABClass A

Class B Push-Pull AmplifierTo avoid this we can use Class B which has a greater signal handling capacityBut Class B will give only one half of the signalHence we can use 2 Class B amplifiersOne for one half and one for the other halfThis type of amplifier is called Push-Pull Amplifier

Class B Push-PullVccT1T2T3TR2TR1

Push-Pull CircuitTR1 and TR2 are output transistors connected back to back, with their emitters groundedThe output transformer TR1 couples the push-pull output to the speakerIn the Push-Pull arrangement T1 conducts for one half of the signal & T2 conducts for the other halfBoth are biased in Class B and each gives one half of the signal & the combined output is coupled to the speaker

Push-Pull CircuitThe Driver Transformer TR2 gives 2 out of phase signals During one half, the +ve half forward biases T1 while the ve half reverse biases T2Thus when T1 conducts, T2 is cut-off & vice-versaThis way both the transistors conduct alternately to give the full signal output

Class D AmplifierDuring the +ve half cycle Q1 gets Forward Bias and it conductsDuring the -ve half cycle Q2 gets Forward Bias and it conductsThus both the transistors conduct alternatelyThe amplifier works for 3600No distortion100% efficiency

During the first half T1 conductsIc flows from the centre-tapping through T1 to groundThis half is coupled to the speaker through TR1Working of Push-Pull Circuit

During the second half T2 conductsIc flows from the centre-tapping through T2 to groundThis half is coupled to the speaker through TR1Working of Push-Pull Circuit

Drawbacks Though this circuit functions well it has a few drawbacksTransformer coupling affects the quality of outputPhase shifting circuit is a mustBoth these drawbacks can be avoided if we use one pair of PNP and NPN transistors at the output

T1T2VccComplementary Symmetry Amplifier

Complementary Symmetry AmplifierThis circuit uses one NPN transistor & one PNP transistor at the output stage

During the +ve half, T1(NPN) base gets forward bias & it conducts while T2 (PNP) gets reverse biased and does not conduct

This gives one half of the signal at the speaker coupled to the emitter

Complementary Symmetry AmplifierDuring the other half, T2 gets forward bias and conducts while T1 gets reverse biased and does not conduct

Thus T1 & T2 conduct alternately giving a distortionless output

This circuit does not require a phase shifter

Cross over distortionClass B Push-Pull amplifier has one limitationAs the phase of the signal changes from +ve to ve (or vice-versa) one transistor stops conducting while the other begins conductingBut the transistor cannot conduct instantaneously as it requires a minimum Vbe before it starts conductingThus as the signal crosses over zero, a distortion occursThis is called Cross over distortion

Cross over distortion Vbe-Vbe

Class AB amplifierThis circuit overcomes cross-over distortionBiasing is done such that even if there is no input signal, a small current keeps the output transistor conductingThis circuit uses 2 diodes whose characteristics matches with that of the BE junction of the output transistorsBiasing resistors R1 & R2 are also identical values

T1T2VccClass AB amplifierR1R2D1D2

Symmetrical componentsSince R1 & D1 are identical to R2 & D2, the diode junction as well as the output point will be at half the supply voltageBecause of symmetry both T1 & T2 will conduct equallyEven when there is no input signal, there will be a current Icq = (I/2 Vcc 0.6) / R1This will keep the output transistors conducting

Elimination of cross-over distortionNormally, during cross-over there will not be any output till the non-conducting transistor gets the minimum VbeThis causes distortionThis has been eliminated here, since the 0.6 V across the diodes keep the transistors on and gives a continuous output signal without producing cross-over distortion

Thermal stabilityIn addition, the two diodes also provide thermal stabilityThey prevent the output transistors going to Thermal Run AwayWhen the output current is high, heat dissipation is moreThe increase in temperature produces more charge carrier in the BE junction of T1 & T2

This increases Ib & hence IcThis in turn increases the power dissipation & hence the heatThis chain goes on till too much current flows and destroys the transistorsThis is called Thermal Run AwayThis is arrested by the diodes in the output circuit

When the charge carriers increase in the B-E junction of T1 & T2, a similar increase takes place in D1 & D2, due to matching characteristicsThis increase in the diode current, produces more drop across R1 & R2 and brings down the forward bias at the base of T1 & T2Thus the 2 diodes prevent cross-over distortion as well as provide thermal stability

A switch-mode amplifier uses a rectangular input signal to drive the transistor rapidly between cutoff and saturation. The efficiency is very high.CBERB They are alsocalled Class Damplifiers.

If the switching frequency is a good deal higherthan the signal frequency, a Class D amplifier iscapable of linear amplification. Pulse-widthmodulation and a low-pass filter are often used.

PWMLPFThe low-pass filter rejects the switching frequency.

Class C and D quizClass C amplifiers use _______ circuits torestore sinusoidal signals.tankThe base-emitter junction in a class Camplifier is ________ biased.reverseThe theoretical maximum efficiency for class C is ___________.100%Class D amplifiers are also known as__________ amplifiers.switch-modeClass D amplifiers employ a varying duty-cycle known as _________.PWM

Concept ReviewClass C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency.Class C amplifiers used tuned tank circuits to reduce distortion in RF applications.Class C amplifiers cannot be used in wideband applications like audio.Class D amplifiers switch between cutoff and saturation for very high efficiency.Class D amplifiers operate at a relatively high switching frequency and often use PWM.Class D can be used in audio applications.

Repeat Segment