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EXPERIMENT 3-3 AMPLIFIER WITH VOLTAGEDIVIDER BIAS In this experiment you will become familiar with a transistor amplifier using voltage-divider bias. You'll see what factors determine its gain and distortion. EQUIPMENT VOM oscilloscope 12-VDC power supply audio signal generator 2n2222 a NPN transistor Electrolytic capacitors-15 WVDC or greater o (2) 4.7 µF ; (1) 47 µF Resistors 1/2 W ±-. 5% o 100 KΩ; 15 KΩ; (2) 5 KΩ; (3) 1 KΩ; (1) 10 Ω E3-3 Circuit for experiment 3-3. PROCEDURE 1. Build the circuit of Fig. E3-3. Measure the d-c base-to-ground voltage VB, the collector-to-ground voltage VC, and the drop across collector resistor RC. Using the voltage across Rc, determine the value of collector current IC.. Your circuit should look like the image on the next page – this will aid with the rest of the Experiment. Your values should be near to the listed ones, if not there is a problem with your circuit. Record your values. VB = 1.45VDC, ___________ Vc = 7.9VDC, ___________ VRc = 4.1VDC, ______________ IC = 0.82 mA, _________________ 68 Electronic Troubleshooting Modified for CSN’s ET 198B-005 Fall 2010 E3-3 Circuit for experiment 3-3 on the breadboard. 2. Next, with an oscilloscope connected at the outputs, adjust the signal generator until the a-c output amplitude across RL, reads 2 Vp-p. Consider the measured voltages the normal values when the amplifier is operating as it should. Make changes to various circuit components as indicated in Table E3-3, and record the changed readings. Follow the new readings with ‘ I’ if the reading increases noticeably (more than 10%), ‘D’ if the reading decreases, and S if the reading remains essentially the same as the normal value. You can determine whether Ic changes by observing the voltage across Rc or RE. If the voltage increases while the resistance remains the same, the current must have increased. If you think that the transistor is cut off or saturated, place a check in the appropriate blank in the table. Use line 1 for normal values and line 2 of the table as an example. To decrease the value of a resistor by 50%, simply bridge a resistor of equal value in parallel with it. To determine the effect of an open component, temporarily remove one end of it from the circuit. Be sure to return the circuit to its original setup before proceeding with the next check. Bipolar Transistors 69 Table E3-3 Change 1. Reference – Normal Values 2. open R 1 3. d ecr ea se R I 5 0% 4. open R 2 5. d ecr ea se R 2 5 0% 6. op en R c 7. d ec r ea se R c 5 0 % 8. open R E 9. d ecr ea se R E 5 0% 10. open R L 11. decrease RL 50% 12. open C1 13. open C2 Vc Ic vo Satura ted? D I D D open C3 QUIZ Refer to Fig. E3-3. For each symptom, choose one of the following faults as the most probable cause, and write it in the space provided. Faults a. b. c. d. e. f. g. h. i. j. k. l. R 1 open R 1 decreased R2 open R2 decreased Rc open Rc decreased RE open RE decreased C2 open C2 shorted open transistor shorted transistor Cut off? 2Vpp short C2 14. 15. VB X Symptoms 1. The a-c output voltage is lower than normal, but not zero. When you measure the voltage from collector to ground, it reads high, but the voltage across RE is normal. VB is also normal. Probable fault 2. The output voltage is lower than normal. Vc reads high, but less than Vcc, and VB reads lower than normal, but not zero. Probable fault 3. The a-c output voltage is zero and VC = VCC. VB measures normal. A resistance check from emitter to ground reads extremely high. Probable fault 4. All d-c readings are normal, but the AC signal gain is lower than normal. Probable fault 5. No a-c output voltage. Vc is equal to Vcc, and VB reads zero. Probable fault 6. Vc measures low. The voltage at the emitter is approximately equal to Vc, and VB is higher than normal. R1 measures normal. Probable fault 7. Vc is lower than normal. Output looks clipped on the negative half.cycles when the transistor is driven with a fairly large signal VB reads higher than normal. Probable fault 8. Vc = Vcc, and the voltage from emitter to ground is zero. VB measures normal. Probable fault 9. Collector-to-emitter voltage reads zero. Probable fault 10. Voltage at collector is equal to Vcc. Voltage VB is lower than normal, and voltage between base and emitter is zero. Probable fault