THE HONG KONG POLYTECHNIC UNIVERSITY EN107/1 Department of Electronic and Information Engineering. EN107: OCL Class AB Power Amplifier Objective

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1 THE HONG KONG POLYTECHNIC UNIVERSITY EN107/1 EN107: OCL Class AB Power Amplifier Objective 1. To study the circuit performance of an OCL amplifier. 2. To study the effects of biasing on cross-over distortion of a class AB amplifier. Apparatus 1. OCL class AB amplifier 2. C.R.O. 3. Audio signal generator 4. Twin 30V/2A D.C. power supply 5. One set of headphones Introduction: An OCL amplifier is a push-pull amplifier that requires neither transformer nor output capacitor. Instead, the amplifier requires a twin D.C. power supply and a matched pair of complementary transistors at the output stage. As shown in Fig. 1, the cross-over distortion, which is an inherent characteristic of a push-pull amplifier circuit, is due to the non-linearity of the input characteristics of the output transistors at the cross-over point at which neither transistor is being turned on. The result is that a dead zone appears in the output waveform. This type of distortion can be heard as harmonic noise superimposed on the output signal, and it can be minimized or even eliminated by providing a suitable amount of bias voltage to the output transistors. A class AB amplifier is a modified form of class B push-pull amplifier where each output transistor is provided with a small amount of bias just sufficient to allow each device to conduct slightly in order to minimize the cross-over distortion.

2 THE HONG KONG POLYTECHNIC UNIVERSITY EN107/2 a) i b1 c) i b1 Cross-over point v b2 v b1 0 Dead zone θ π 2π ωt π θ i b2 i b2 b) π 0 θ θ π v i ωt Fig. 1 a) input characteristic b) input voltage waveform c) input current waveform of the push-pull transistor amplifier without bias voltage. Circuit Fig. 2 shows the circuit of the OCL amplifier used. The circuit is classified into two stages a) the pre-amplifier stage and b) the driver and the output stage. In the pre-amplifier stage, a differential amplifier is used in which negative feedback is fed to the base of Q 2 while the input signal is fed to the base of Q 1. The difference between these two voltages is amplified and fed to the input of the next stage, the driver, via the base of Q 3. The driver transistor Q 3 has the collector load composed of Q 8, R 2, R 15 and R 17 (the rubber diode). The potential drop across the rubber diode is used to bias the transistors in the output stage to operate in class AB mode.

3 THE HONG KONG POLYTECHNIC UNIVERSITY EN107/3 By turning R 17 (a rotary potentiometer) the biasing current to the output transistors can be reduced to completely cutoff (ie. Class B). For class AB operation, a moderate biasing current (or quiescent current) is required. Two Darlington pairs of complementary symmetry transistors are used in the output stage. R 7 and R 8 are inserted to provide a convenient way of observing the current waveforms and measuring the average D.C. current in Q 4, Q 6 and Q 5, Q 7. Procedure 1. Do not connect power to the OCL amplifier. 2. Reset the OCL amplifier by i. Turn R 17 counterclockwise to the end (Class B) ii. Switch SW1 to select the 15 Ω load 3. Plug the headphone into the phono socket. 4. Connect the signal generator to the I/P of the amplifier. For this experiment, the maximum I/P voltage is about 1 Vpp. However please set it to minimum at the moment. Monitor I/P with oscilloscope CH1. Monitor O/P with CH2. 5. Before connecting the D.C. power supply, select the tracking mode (series mode). The maximum output current will not exceed 1 A for this experiment. Please limit the output current accordingly (both +15V & -15V supplies) and then set the outputs to 15 V. 6. Switch off the power supply and connect it to the amplifier. 7. Now turn on the supply. 8. Set the signal generator frequency to 1 khz. Gradually increase its amplitude until a 20 Vpp voltage is seen on the output (H). 9. Observe and record voltage waveforms with oscilloscope at (B), (C), R 7, R 8 and (H). Note also the DC Levels with respect to GND (0V).

4 THE HONG KONG POLYTECHNIC UNIVERSITY EN107/4 Notes: When measuring waveforms across R 7 & R 8, use ONE probe only. The other probe must be disconnected from the circuit. (B) DC voltage = V (C) DC voltage = V (R 7 ) (R 8 ) (H) DC voltage = 0.08 V

5 THE HONG KONG POLYTECHNIC UNIVERSITY EN107/5 10. Monitor O/P again. Check if you can identify the cross-over distortion. 11. Gradually reduce V in until a significant cross-over distortion is seen in V out and record the waveform (~ 2Vpp). 12. Reduce V in to minimum before switching SW1. Now switch to headphone and put on the headphone. Slowly increase V in until the volume is at a comfortable level. 13. Turn R 17 to and from between Class B and Class AB. Check if you can sense the cross-over distortion. Caution: keep the supply current below 0.5 A. 14. Repeat (13) with different volume levels (be careful do not turn on too loud) and various frequencies. Comment on the sensitivity of the human ear to cross-over distortion. Caution: remember to reduce V in before changing the input frequency. Input frequency Comment 20 Hz 7.25 Vpp, distortion heard 100 Hz 7.19 Vpp, distortion heard 500 Hz 2.25 Vpp, distortion heard 2 khz 2.19 Vpp, distortion heard 10 khz 1.63 Vpp, cannot tell distortion from signal 15 khz Inaudible

6 THE HONG KONG POLYTECHNIC UNIVERSITY EN107/6 15. Reset the OCL amplifier as in (2). Setup the signal generator as in (8). 16. Monitor both V in and V out with oscilloscope. a) Calculate the voltage gain A v of the circuit where A V V = V out in V in = Vpp V out = 19.9 Vpp A v = b) Measure the average D.C. supply currents. Derive it from the D.C. voltage across R 7 and R 8. Voltage across R 7 = V Voltage across R 8 = V current = A current = A c) Calculate the output power P out where P out V 2 out (. rms.) = P out = 3.3 W R L d) Calculate the efficiency η of the output stage where Pout Pout η = = η = 0.54 P V I dc.. dc.. dc Reduce V in to minimum. Turn R 17 such that the supply current (quiescent current) is about 0.1 A. 18. Adjust V in to give 20 V pp output. Repeat (16). V in = Vpp V out = 20.2 Vpp A v = Voltage across R 7 = V Voltage across R 8 = V current = A current = A P out = 3.4 W η = Vary input frequencies from 100 Hz to 40 khz and plot the gain-frequency response of the circuit using db and log frequency. Determine the bandwidth of the amplifier.

7 THE HONG KONG POLYTECHNIC UNIVERSITY EN107/7 Frequency V in V out A v A v in decibel 100 Hz Hz Hz Hz Hz khz khz khz khz khz khz khz khz khz Bandwidth = 12 khz 20. Record the serial number of the OCL amplifier. Serial no. 14

8 Problems for further study: THE HONG KONG POLYTECHNIC UNIVERSITY EN107/8 1. Explain why rubber diode was used to provide base emitter bias for the output transistors instead of a resistor. 2. Describe how this amplifier can be modified to work with a single D.C. power supply and compare the gain bandwidth of these two circuits. PREAMP DRIVER OUTPUT C1 220u R3 1k C4 R B R21 13k Q3 Q2SA1015 R1 D1 1N5819 V2 15V RUBBER DIODE C17 470u Q4 QTIP31C R V F I/P GND R23 1k R16 1.5k C3 3.3u C8 10n C16 220u Q2SC1815 Q1 R12 10k C2 100u Q2SC1815 Q2 R11 2.2k R9 6.8k 150p R24 C14 47u R R25 33k 33k 560 C5 100u R18 6.8k R4 2.2k R R22 13k C Tune R17 to vary the quiescent current. D2 1N5819 R15 4.3k Q8 QTIP31C R2 1.1k R17 1K V3 15V Q5 QTIP32C C18 470u Q6 Q2N3055 R5 510 H SW1 R6 510 Q7 QMJ2955 R8 1 G -15V SPDT 1 2 R R19 15 F1 Fuse J1 PhoneJack Fig. 2 Schematic diagram of OCL amplifier circuit ---END---