A Low-Power Audio AmplifierThe object of this experiment is to assemble and test a low-power audio amplifier thatmight be used to drive headp hones. Because an op amp is current limited to less than 25 mA,it cannot be used to directly driv e a lo w impedance load. A solution to this problem is toaddcurrentboostersbetweentheop-ampoutputandtheloadasshowninFigure1. BJT’sQ1and Q2are connected as a complementary common-collector amplifier to supply loadcurren t. Q1supplies positiv e load cu rrent and Q2supplies n egative load current. The opamp is connected as a unit y-gain voltage follower. A t wo-resistor feedback net work can beadded to the op amp to increase the gain of the circuit for any specific ap plica tion .Figure 1: Op amp w ith com plem entary comm o n-collector stage.Assemble the circuit with R1=10kΩ. You may use any op amp that is availablein the laboratory. A TL071, TL081, or LF351 is recomm en ded. The transistors shouldbe a complem ent pair such as the 2N4401/2N4403 or the 2N3904/2N3906.Becausethelaboratory only has 0.25 W resistors, the va lue of R2must be calculated so that it does notbecome a sm okin g resistor at the maximum po wer output. Calculate the value of R2so thatit dissipates 0.25 W when the output voltage is a sine wa ve with a peak value of 10 V.1. Assemble the circuit.2. Driv e the circuit with a sine w a ve to produce a peak output v oltage of 1Vat a frequencyof 100 Hz.3. Observ e and record the waveforms at the output of the op amp and across the loadresistor R2.4. Increase the input v oltage to produce a peak output voltage of 8V and repeat themeasurements.5. In your report, explain the cause of an y observ ed distortion.1A method of reducing the distortion at the output is to take the feedback from the outputof the comm on-collector stage rather than from the output of the op amp as show n in Figure2. Assemble this circuit and repeat the abo ve measurements. Note that the distortion inthe w av efo rm has been mo ved from the output to the input of the common-collector stage.You shou ld explain this in your report.Figure 2: Circuit with feedback tak en at the load.Althou gh the circuit in Figure 2 is an improvement over the circuit in Figure 2, distortionmeasu rem ents wou ld rev eal some residual distortion caused b y the two BJT ’s turning onand off as the output signal swings positive and negative. An improv ement is to add biasv o ltages that will bias the BJT ’s at or near the threshold of conduction in order to decreasethe distortion seen at the base inputs of the transistors. A possible circuit is shown in Figure3. D iodes D1and D2are biased on b y resistors R3and R4.WithR3= R4, calculate thevalue of the resistors which will bias the two diodes at 1mA. Assume a typical diode v oltageof 0.65 V. Assemble the circuit using 1N4148 diodes.P ow er should be applied to the circuit in Figure 3 with caution. If the diode v oltages arelarger than the threshold v oltag es for Q1and Q2, the bias current in the BJT’s can becomeexcessiv e and yo u will hav e smoking transistors. You should monitor the current output ofthe power supply as the dc voltages are increased. If yo u see a rapid increase in curren t, itwill be necessary to connect a resistor between the bases of the BJT’s to decrease the biasv o ltage across the bases. The value of this resistor m ust be tweaked to obtain an acceptablecomp rom ise. The threshold vo ltage of the 1N4148 diode should be slightly less than that ofthe BJT ’s so that this should not be a problem. When you ha ve an operating circuit, repeatthe measurements made for the circuit in Figure 1. The distortion observed at the outpu t ofthe op am p should be decreased comp ared to that observ ed for the circuit in Figure 2. Youshould explain this in y ou r report.In real wo rld applications of circuits such as these, the class of operation specifies ho wthe tw o transistors are biased. If both transistors are cut off quiescently, the circuit is saidto be biased class-C. Such is the case for the circuit in Figure 2. If a bias is added bet weenthe BJT bases such that the transistors are biased just at the threshold of cond uction, thecircuit is said to be biased class-B. If neither transistor cuts off wh en po wer is delivered to2Figure 3: Circuit with bias diodes.the load, the circuit is said to be biased class-A. It can be shown that a class-A amplifiermust ha ve a quiescen t bias current in the output transistors that is equal to one-half thepeak load curren t. For high-power amplifiers, this current can be quite large, causing highquiescent power dissipation in the transistors. A compro m ise is to bias both transistorsjust above the threshold of conduction. The amplifier is then said to be biased class-AB.Thisisthemostcommonlyusedmethodusedinaudiopoweramplifiers. Th e bias voltageis usually adjustable via a poten tiometer. Thermal runaway can be a problem with thesecircuits unless the bias v o ltage is varied with the temperature of the transistors so as to k eepthe quiescen t bias curren t constant. There are standard circuits used to achieve this thatuse a diode or a BJT attac hed to the heat sinks to sense the temperature and adjust thebias v o lta ge accordingly. It is anticipated that the circuit as sho w n in Figure 3 will be biasedclass-B just below class-AB so that thermal runa way will not be a prob lem with the