12 - Talking Electronics

1 306 principles of Electronics Audio Power amplifier and Large-SignalAmplifiers Power of amplifier Between Voltage andPower Amplifiers Quantities of PowerAmplifiers of Power Amplifiers for Collector Efficiency CollectorEfficiency of Series Fed Class CollectorEfficiency of Transformer CoupledClass A Power Points About Class APower Of A Practical Efficiency for Class BPower practical amplifier always consists of a num-ber of stages that amplify a weak signal untilsufficient power is available to operate a loud-speaker or other output device.

2 The first few stages inthis multistage amplifier have the function of only volt-age amplification. However, the last stage is designedto provide maximum power. This final stage is knownas power term audio means the range of frequencieswhich our ears can hear. The range of human hearingextends from 20 Hz to 20 kHz. Therefore, audio am-plifiers amplify electrical signals that have a frequencyrange corresponding to the range of human hearing Hz to 20 kHz. Fig. shows the block diagram ofan audio amplifier .

3 The early stages build up the volt-age level of the signal while the last stage builds uppower to a level sufficient to operate the this chapter, we shall talk about the final stage in amultistage amplifier the power AudioPower Amplifiers12 Transistor Audio Power Amplifiers 307 Fig. Transistor Audio Power AmplifierA transistor amplifier which raises the power level of thesignals that have audio frequency range is known as tran-sistor audio power general, the last stage of a multistage amplifier is thepower stage.

4 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 300 (for transmission antennas) to8 (for loudspeakers).

5 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 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 Vfrom 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 ( speaker).

6 (i) Small-signal amplifiers. Those amplifiers which handle small input signals (a few Vor 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 , 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.

7 Those amplifiers which handle large input signals (a fewvolts) are called large-signal amplifiers. Power amplifiers fall in this class. The large-signal amplifi-ers are designed to provide a large amount of power output so that they can operate the outputdevice 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 im-pedance matching to the output device. It may be noted that all large-signal amplifiers are not neces-Transistor Audio Power Amplifiers308 principles of Electronicssarily power amplifiers but it is safe to say that most are.

8 In general, where amount of power involvedis 1W or more, the amplifier is termed as power Output Power of AmplifierAn amplifier converts power drawn from supply VCC into 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. The currents areflowing through various resistors causing I2R loss. Thus power loss in R1 is I12 R1, power loss in RC isIC2 RC, power loss in RE is IE2 RE and so on.

9 All these losses appear as heat. Therefore, losses occuringin an amplifier not only decrease the efficiency but they also increase the temperature of the load RL is connected to the amplifier , output power, PO = 2 LLVR whereVL= value of load voltageExample If in Fig. ; R1 = 10 k ; R2 = k ; RC = k ; RE = k and VCC= + 10 V, find the power drawn from the supply by the The current I1 flowing through R1 also flows through R2 (a reasonable assumptionbecause IB is small).I1=1210V10V=10 k + k CCVRR=+ = voltage across R2, V2=I1 R2 = mA k = voltage across RE, VE=V2 VBE = = emitter current, IE=VE/RE = k = 1 mA ICjIE = 1 mATotal current IT drawn from the supply isIT=IC + I1 = 1 mA + 0.

10 82 mA = mA power drawn from the supply isPdc=VCC IT = 10V mA = mWTransistor Audio Power Amplifiers 309 Example Determine the load power for the circuit shown in Fig. The reading of voltmeter is Since voltmeters read voltage, wehave, output power, PO=22( )200 LLVR= = mWExample In an RC coupled power amplifier , the voltage across load RL (= 100 ) hasa peak- to-peak value of 18V. Find the maximum possible load The peak-to-peak voltage, VPP = 18V. Therefore, peak voltage (or maximum voltage) =VPP/2 and the value, VL = VPP/22.