12 - Talking Electronics

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306Principles of Electronics12Transistor AudioPower Amplifiers12.1 Transistor Audio Power Amplifier12.2 Small-Signal and Large-SignalAmplifiers12.3 Output Power of Amplifier12.4 Difference Between Voltage andPower Amplifiers12.5 Performance Quantities of PowerAmplifiers12.6 Classification of Power Amplifiers12.7 Expression for Collector Efficiency12.8 Maximum CollectorEfficiency of Series – Fed Class AAmplifier12.9 Maximum CollectorEfficiency of Transformer CoupledClass A Power Amplifier12.10 Important Points About Class APower Amplifier12.11 Thermal Runaway12.12 Heat Sink12.13 Mathematical Analysis12.14 Stages Of A Practical PowerAmplifier12.15 Driver Stage12.16 Output Stage12.17 Push-Pull Amplifier12.18 Maximum Efficiency for Class BPower Amplifier12.19 CTIONApractical amplifier always consists of a number of stages that amplify a weak signal untilsufficient power is available to operate a loudspeaker or other output device. The first few stages inthis multistage amplifier have the function of only voltage amplification. However, the last stage is designedto provide maximum power. This final stage is knownas power stage.The term audio means the range of frequencieswhich our ears can hear. The range of human hearingextends from 20 Hz to 20 kHz. Therefore, audio amplifiers amplify electrical signals that have a frequencyrange corresponding to the range of human hearing i.e.20 Hz to 20 kHz. Fig. 12.1 shows the block diagram ofan audio amplifier. The early stages build up the voltage level of the signal while the last stage builds uppower to a level sufficient to operate the loudspeaker.In this chapter, we shall talk about the final stage in amultistage amplifier—the power amplifier.

Transistor Audio Power Amplifiers307Fig. 12.112.1 Transistor Audio Power AmplifierA transistor amplifier which raises the power level of thesignals that have audio frequency range is known as transistor audio power amplifier.In general, the last stage of a multistage amplifier is thepower stage. The power amplifier differs from all theprevious stages in that here a concentrated effort is made toobtain maximum output power. A transistor that is suitablefor power amplification is generally called a powertransistor. It differs from other transistors mostly in size ; itis considerably larger to provide for handling the greatamount of power. Audio power amplifiers are used to delivera large amount of power to a low resistance load. Typicalload values range from 300Ω (for transmission antennas) to8Ω (for loudspeakers). Although these load values do notcover every possibility, they do illustrate the fact that audiopower amplifiers usually drive low-resistance loads. Thetypical power output rating of a power amplifier is 1W ormore.Transistor Audio Power Amplifiers12.2 Small-Signal and Large-Signal AmplifiersThe input signal to a multistage amplifier is generally small (a few mV from a cassette or CD or a few μVfrom an antenna). Therefore, the first few stages of a multistage amplifier handle small signals andhave the function of only voltage amplification. However, the last stage handles a large signal and itsjob is to produce a large amount of power in order to operate the output device (e.g. speaker).(i) Small-signal amplifiers. Those amplifiers which handle small input a.c. signals (a few μVor a few mV) are called small-signal amplifiers. Voltage amplifiers generally fall in this class. Thesmall-signal amplifiers are designed to operate over the linear portion of the output characteristics.Therefore, the transistor parameters such as current gain, input impedance, output impedance etc. donot change as the amplitude of the signal changes. Such amplifiers amplify the signal with little or nodistortion.(ii) Large-signal amplifiers. Those amplifiers which handle large input a.c. signals (a fewvolts) are called large-signal amplifiers. Power amplifiers fall in this class. The large-signal amplifiers are designed to provide a large amount of a.c. power output so that they can operate the outputdevice e.g. a speaker. The main features of a large-signal amplifier or power amplifier are the circuit’spower efficiency, the maximum amount of power that the circuit is capable of handling and the impedance matching to the output device. It may be noted that all large-signal amplifiers are not neces-

308Principles of Electronicssarily power amplifiers but it is safe to say that most are. In general, where amount of power involvedis 1W or more, the amplifier is termed as power amplifier.12.3 Output Power of AmplifierAn amplifier converts d.c. power drawn from d.c. supply VCC into a.c. output power. The outputpower is always less than the input power because losses occur in the various resistors present in thecircuit. For example, consider the R-C coupled amplifier circuit shown in Fig. 12.2. The currents are22flowing through various resistors causing I R loss. Thus power loss in R1 is I1 R1, power loss in RC is22IC RC, power loss in RE is IE RE and so on. All these losses appear as heat. Therefore, losses occuringin an amplifier not only decrease the efficiency but they also increase the temperature of the circuit.Fig. 12.22When load RL is connected to the amplifier, A.C. output power, PO VLRLwhere VL r.m.s. value of load voltageExample 12.1. If in Fig. 12.2; R1 10 kΩ ; R2 2.2 kΩ ; RC 3.6 kΩ ; RE 1.1. kΩ and VCC 10 V, find the d.c. power drawn from the supply by the amplifier.Solution. The current I1 flowing through R1 also flows through R2 (a reasonable assumptionbecause IB is small).VCC10V10VI1 R R 10 kΩ 2.2 kΩ 12.2 kΩ 0.82 mA12D.C. voltage across R2, V2 I1 R2 0.82 mA 2.2 kΩ 1.8VD.C. voltage across RE, VE V2 – VBE 1.8V – 0.7V 1.1VD.C. emitter current, IE VE/RE 1.1V/1.1 kΩ 1 mA IC j IE 1 mATotal d.c current IT drawn from the supply isIT IC I1 1 mA 0. 82 mA 1.82 mA D.C. power drawn from the supply isPdc VCC IT 10V 1.82 mA 18.2 mW

Transistor Audio Power Amplifiers309Example 12.2. Determine the a.c. load power for the circuit shown in Fig. 12.3.Fig. 12.3Solution. The reading of a.c. voltmeter is 10.6V. Since a.c. voltmeters read r.m.s. voltage, wehave,22VL(10.6) A.C. output power, PO 561.8 mWRL200 ΩExample 12.3. In an RC coupled power amplifier, the a.c. voltage across load RL ( 100 Ω ) hasa peak- to-peak value of 18V. Find the maximum possible a.c. load power.Solution. The peak-to-peak voltage, VPP 18V. Therefore, peak voltage (or maximum voltage) VPP/2 and the r.m.s value, VL VPP/2 2 .2 HerePO (max) 22VL(V / 2 2)V PP PPRLRL8 RLVPP 18V and RL 100Ω2 PO (max) (18V )–3 405 10 W 405 mW(8 100) Ω12.4 Difference Between Voltage and Power AmplifiersThe distinction between voltage and power amplifiers is somewhat artificial since useful power (i.e.product of voltage and current) is always developed in the load resistance through which currentflows. The difference between the two types is really one of degree; it is a question of how muchvoltage and how much power. A voltage amplifier is designed to achieve maximum voltage amplification. It is, however, not important to raise the power level. On the other hand, a power amplifier isdesigned to obtain maximum output power.1. Voltage amplifier. The voltage gain of an amplifier is given by :RAv β CRinIn order to achieve high voltage amplification, the following features are incorporated in suchamplifiers :

310Principles of Electronics(i) The transistor with high β ( 100) is used in the circuit. In other words, those transistors areemployed which have thin base.(ii) The input resistance Rin of the transistor is sought to be quite low as compared to thecollector load RC.(iii) A relatively high load RC is used in the collector. To permit this condition, voltage amplifiersare always operated at low collector currents (j 1 mA). If the collector current is small, we can uselarge RC in the collector circuit.2. Power amplifier. A power amplifier is required to deliver a large amount of power and as suchit has to handle large current. In order to achieve high power amplification, the following features areincorporated in such amplifiers :(i) The size of power transistor is made considerably larger in order to dissipate the heat produced in the transistor during operation.(ii) The base is made thicker to handle large currents. In other words, transistors with comparatively smaller β are used.(iii) Transformer coupling is used for impedance matching.The comparison between voltage and power amplifiers is given below in the tabular form :S. No.1.2.3.4.5.6.7.ParticularVoltage amplifierPower amplifierβRCCouplingInput voltageCollector currentPower outputOutput impedanceHigh ( 100)High (4 – 10 kΩ)usually R C couplinglow (a few mV)low (j 1 mA)lowHigh (j 12 kΩ)low (5 to 20)low (5 to 20 Ω)Invariably transformer couplingHigh ( 2 4 V)High ( 100 mA)highlow (200 Ω)Example 12.4. A power amplifier operated from 12V battery gives an output of 2W. Find themaximum collector current in the circuit.Solution.Let IC be the maximum collector current.Power battery voltage collector currentor2 12 IC2 1 IC A 166.7 mA126This example shows that a power amplifier handles large power as well as large current.Example 12.5. A voltage amplifier operated from a 12 V battery has a collector load of 4 kΩ.Find the maximum collector current in the circuit.Solution.The maximum collector current will flow when the whole battery voltage is dropped across RC.12 V Max. collector current battery voltage 3 mA4 kΩcollector loadThis example shows that a voltage amplifier handles small current.Example 12.6. A power amplifier supplies 50 W to an 8-ohm speaker. Find (i) a.c. outputvoltage (ii) a.c. output current.

Transistor Audio Power AmplifiersSolution.(i) (ii)311P V 2/Ra.c. output voltage, V PR 50 8 20 Va.c. output current, I V/R 20/8 2.5 A12.5 Performance Quantities of Power AmplifiersAs mentioned previously, the prime objective for a power amplifier is to obtain maximum outputpower. Since a transistor, like any other electronic device has voltage, current and power dissipationlimits, therefore, the criteria for a power amplifier are : collector efficiency, distortion and powerdissipation capability.(i) Collector efficiency. The main criterion for a power amplifier is not the power gain ratherit is the maximum a.c. power output. Now, an amplifier converts d.c. power from supplyinto a.c. power output. Therefore, the ability of a power amplifier to convert d.c. powerfrom supply into a.c. output power is a measure of its effectiveness. This is known ascollector efficiency and may be defined as under :The ratio of a.c. output power to the zero signal power (i.e. d.c. power) supplied by the batteryof a power amplifier is known as collector efficiency.Collector efficiency means as to how well an amplifier converts d.c. power from the battery intoa.c. output power. For instance, if the d.c. power supplied by the battery is 10W and a.c. output poweris 2W, then collector efficiency is 20%. The greater the collector efficiency, the larger is the a.c.power output. It is obvious that for power amplifiers, maximum collector efficiency is the desiredgoal.(ii) Distortion. The change of output waveshape from the input wave shape of anamplifier is known as distortion.A transistor like other electronic devices, isessentially a non-linear device. Therefore, whenever a signal is applied to the input of the transistor, the output signal is not exactly like the inputsignal i.e. distortion occurs. Distortion is not aproblem for small signals (i.e. voltage amplifiers)since transistor is a linear device for small variations about the operating point. However, a poweramplifier handles large signals and, therefore, theproblem of distortion immediately arises. For thecomparison of two power amplifiers, the one whichhas the less distortion is the better. We shall discuss the method of reducing distortion in amplifiers in the chapter of negative feedback in amplifiers.Power Dissipation Channels in a(iii) Power dissipation capability. The abilMicrofabricated Atomic Clockity of a power transistor to dissipate heatis known as power dissipation capability.As stated before, a power transistor handles large currents and heats up during operation. As anytemperature change influences the operation of transistor, therefore, the transistor must dissipate thisheat to its surroundings. To achieve this, generally a heat sink (a metal case) is attached to a power

312Principles of Electronicstransistor case. The increased surface area allows heat to escape easily and keeps the case temperature of the transistor within permissible limits.12.6 Classification of Power AmplifiersTransistor power amplifiers handle large signals. Many of them are driven so hard by the input largesignal that collector current is either cut-off or is in the saturation region during a large portion of theinput cycle. Therefore, such amplifiers are generally classified according to their mode of operationi.e. the portion of the input cycle during which the collector current is expected to flow. On this basis,they are classified as :(i) class A power amplifier (ii) class B power amplifier (iii) class C power amplifier(i) Class A power amplifier. If the collector current flows at all times during the full cycle ofthe signal, the power amplifier is known as class A power amplifier.Fig. 12.4Obviously, for this to happen, the power amplifier must be biased in such a way that no part of thesignal is cut off. Fig. 12.4 (i) shows circuit of class A power amplifier. Note that collector has atransformer as the load which is most common for all classes of power amplifiers. The use of transformer permits impedance matching, resulting in the transference of maximum power to the load e.g.loudspeaker.Fig. 12.4 (ii) shows the class A operation in terms of a.c. load line. The operating point Q is soselected that collector current flows at all times throughout the full cycle of the applied signal. As theoutput wave shape is exactly similar to the input wave shape, therefore, such amplifiers have leastdistortion. However, they have the disadvantage of low power output and low collector efficiency(about 35%).(ii) Class B power amplifier. If the collector current flows only during the positive half-cycleof the input signal, it is called a class B power amplifier.In class B operation, the transistor bias is so adjusted that zero signal collector current is zero i.e.no biasing circuit is needed at all. During the positive half-cycle of the signal, the input circuit isforward biased and hence collector current flows. However, during the negative half-cycle of thesignal, the input circuit is reverse biased and no collector current flows. Fig. 12.5 shows the class B

Transistor Audio Power Amplifiers313operation in terms of a.c. load line. Obviously, the operating point Q shall be located at collector cutoff voltage. It is easy to see that output from a class B amplifier is amplified half-wave rectification.In a class B amplifier, the negative half-cycle of the signal is cut off and hence a severe distortionoccurs. However, class B amplifiers provide higher power output and collector efficiency (50 60%). Such amplifiers are mostly used for power amplification in push-pull arrangement. In such anarrangement, 2 transistors are used in class B operation. One transistor amplifies the positive halfcycle of the signal while the other amplifies the negative half-cycle.Fig. 12.5(iii) Class C power amplifier. If the collector current flows for less than half-cycle of the inputsignal, it is called class C power amplifier.In class C amplifier, the base is given some negative bias so that collector current does not flowjust when the positive half-cycle of the signal starts. Such amplifiers are never used for power amplification. However, they are used as tuned amplifiers i.e. to amplify a narrow band of frequencies nearthe resonant frequency.12.7 Expression for Collector EfficiencyFor comparing power amplifiers, collector efficiency is the main criterion. The greater the collectorefficiency, the better is the power amplifier.Now,Collector efficiency, η a.c. power outputd.c. power inputPo Pdcwhere*Pdc VCC ICPo Vce Icwhere V is the r.m.s. value of signal output voltage and I is the r.m.s. value of output signalceccurrent. In terms of peak-to-peak values (which are often convenient values in load-line work), the a.c.power output can be expressed as : * Note that d.c. input power to the collector circuit of power amplifier is the product of collector supply VCC(and not the collector-emitter voltage) and the average (i.e. d.c.) collector current IC.

Principles of Electronics314* Po [(0.5 0.707) vce (p p)] [(0.5 0.707) ic (p p)]vce( p p) ic( p p) 8vce ( p p ) ic ( p p )Collector η 8 VCC I C 12.8. Maximum Collector Efficiency of Series-Fed Class AAmplifierFig. 12.6 (i) shows a **series – fed class A amplifier. This circuit is seldom used for power amplification due to its poor collector efficiency. Nevertheless, it will help the reader to understand the classA operation. The d.c. load line of the circuit is shown in Fig. 12.6 (ii). When an ac signal is appliedto the amplifier, the output current and voltage will vary about the operating point Q. In order toachieve the maximum symmetrical swing of current and voltage (to achieve maximum output power),the Q point should be located at the centre of the dc load line. In that case, operating point is IC VCC/2RC and VCE VCC/2 .Fig. 12.6Maximum vce (p p) VCCMaximum ic (p p) VCC/RCMax. ac output power,2vce ( p p) ic ( p p)V VCC / RCV CC CC888 RCPo (max) 2V V D.C. power supplied, Pdc VCC IC VCC CC CC2 RC 2 RC * Po(max)Maximum collector η Pdc 100 2/ 8 RCVCC2VCC / 2 RC 100 25% peak-to-peak value r.m.s. value 1 2 2 0.5 0.707 peak-to-peak value** Note that the input to this circuit is a large signal and that transistor used is a power transistor.

Transistor Audio Power Amplifiers315Thus the maximum collector efficiency of a class A series-fed amplifier is 25%. In actual practice,the collector efficiency is far less than this value.Example 12.7. Calculate the (i) output power (ii) input power and (iii) collector efficiency of theamplifier circuit shown in Fig. 12.7 (i). It is given that input voltage results in a base current of10 mA peak.Fig. 12.7Solution. First draw the d.c. load line by locating the two end points viz., IC (sat) VCC/RC 20 V/20 Ω 1 A 1000 mA and VCE VCC 20 V as shown in Fig. 12.7 (ii). The operating point Qof the circuit can be located as under :VCC VBE20 0.7IB 19.3 mA RB1 kΩ IC β IB 25 (19.3 mA) 482 mAAlsoVCE VCC IC RC 20 V (482 mA) (20 Ω) 10.4 VThe operating point Q (10.4 V, 482 mA) is shown on the d.c. load line.(i) ic-(peak) β ib (peak) 25 (10 mA) 250 mA2 3 2ic ( peak )(250 10 ) 20 0.625 WRC 22 3(ii) Pdc VCC IC (20 V) (482 10 ) 9.6 WP (ac) 100 0.625 100 6.5 %(iii) Collector η oPdc9.6 Po (ac) 12.9. Maximum Collector Efficiency of Transformer CoupledClass A Power AmplifierIn class A power amplifier, the load can be either connected directly in the collector or it can betransformer coupled. The latter method is often preferred for two main reasons. First, transformercoupling permits impedance matching and secondly it keeps the d.c. power loss small because of thesmall resistance of the transformer primary winding.Fig. 12.8 (i) shows the transformer coupled class A power amplifier. In order to determinemaximum collector efficiency, refer to the output characteristics shown in Fig. 12.8 (ii). Under zerosignal conditions, the effective resistance in the collector circui

Transistor Audio Power Amplifiers 307 Fig. 12.1 12.1 Transistor Audio Power Amplifier A transistor amplifier which raises the power level of the signals that have audio frequency range is known as tran-sistor audio power amplifier. In general, the last stage of a multistage amplifier is

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