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# SIGNAL SPECTRA EXPERIMENT 1 - FINALS (for CAUAN)

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### SIGNAL SPECTRA EXPERIMENT 1 - FINALS (for CAUAN)

1. 1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite EXPERIMENT # 2 Class B Push-Pull Power AmplifierAbdon, John Kerwin M. October 13, 2011Signal Spectra and Signal Processing/ BSECE 41A1 Score: Engr. Grace Ramones Instructor1|Page
2. 2. Sample Computation:Step 4 AC collector resistance VCE=VCEQ + (ICQ)(RC + Re)= 10.273 V + 48.48mA(50Ω+5 Ω)= 12.9394VStep 6 Voltage gain (AV) of the amplifier.Step 7 Expected voltage gain (AV) based on the value of the ac collector resistance (Rc), the unbypassed emitter resistance (Re), and the ac emitter resistance (re),Step 8 Value of RE required to center the Q-point on the ac load line. VCEQ = (ICQ)(Rc + Re) VCEQ =VCC - ICQ(RE + Re + RC) VCC - ICQ(RE + Re + RC)= (ICQ)(Rc + Re) VCC (RE + Re + RC)= ( )(Rc + Re)Step 10 New dc values for ICEQ for the new value of RE2|Page
3. 3. VCEQ =20V - ( +5 + )=5.37V VCE=VCEQ + (ICQ)(RC + Re)= 5.37V + (50Ω+5 Ω)= 10.73525VStep 12 Maximum undistorted output power (PO) to the load (RL).Step 13 Power supplied by the dc voltage source (PS).Step 14 Percent efficiency (ŋ) of the amplifier.3|Page
4. 4. Objectives: 1. Determine the dc load line and locate the operating point (Q-point) on the dc load line for a large-signal class A common-emitter amplifier. 2. Determine the ac load line for a large-signal class A common-emitter amplifier. 3. Center the operating point (Q-point on the ac load for a large-signal class A common-emitter amplifier. 4. Determine the maximum ac peak-to-peak output voltage swing before peak clipping occurs and compare the measured value with the expected value for a large-signal class A common-emitter amplifier. 5. Observe nonlinear distortion of the output waveshape for a large-signal class A common-emitter amplifier. 6. Measure the large-signal voltage gain of a class A common-emitter amplifier and compare the measured and calculated values. 7. Measure the maximum undistorted output power for a class A amplifier. 8. Determine the amplifier efficiency of a class A amplifier.4|Page
5. 5. Datasheet:Materials:One digital multimeterOne function generatorOne dual-trace oscilloscopeOne dc powers supplyOne 2N3904 bipolar transistorCapacitors: two 10 µF, one 470 µFResistors: one 5 Ω, one 95 Ω, two 100 Ω, one 1 kΩ, one 2.4 kΩTheory:A power amplifier is a large-signal amplifier in the final stage of a communicationtransmitter that provides power to the antenna or in the final stage of a receiver that drivesthe speaker. When an amplifier is biased so that it operates in the linear region of thetransistor collector characteristic curve for the full 360 degrees of the input sine wavecycle, it is classified as a class A amplifier. This means that collector current flows duringthe full sine wave cycle, making class A amplifiers the least efficient of the different classesof large-signal amplifiers. In a large-signal amplifier, the input signal causes the operatingpoint (Q-point) to move over much a larger portion of the ac load line than in a small-signalamplifier. Therefore, large signal class A amplifiers require the operating point to be asclose as possible to the center of the ac load line to avoid clopping of the output waveform.In a class A amplifier, the output voltage waveform has the same shape as the input voltagewaveform, making it the most linear of the different classes of amplifiers. Most small-signalamplifiers, it is normally used in a low-power application that requires a linear amplifiersuch as an audio power amplifier or as a power amplifier in a low-power transmitter withlow-level AM or SSB modulation. In this experiment you will study a large-signal class Aamplifier.For the large –signal class A common-emitter amplifier shown in Figures 14-1 and 14-2, thedc collector-emitter voltage (VCE) can be calculated from VCE = VC – VEThe dc collector current (IC) can be determined by calculating the current in the collectorresistor (RC). Therefore,5|Page
6. 6. Figure 14–1 Large signal Class A Amplifier, DC AnalysisXMM1 R1 Rc V1 2.4kΩ 100Ω 20 V Q1 2N3904 R2 1kΩ Re 5Ω REm 95ΩFigure 14-2 Large-Signal Class A AmplifierThe ac collector resistance (RC) is equal to the parallel equivalent of the collector resistor(RC) and the load resistor (RL). Therefore,The ac load line has a slope of 1/(RC + RL) and crosses the dc load line Q-point. The ac loadline crosses the horizontal axis of the transistor collector characteristic curve plot at VCEequal to VCEQ + (ICQ)(RC + Re), where VcEQ is the collector-emitter voltage at the Q-pointand ICQ is the collector current at the Q-point.The amplifier voltage gain is measured by dividing the ac peak-to-peak output voltage (VO)by the ac peak-to-peak input voltage (Vin). The expected amplifier voltage gain for acommon emitter amplifier is calculated from6|Page
7. 7. where RC is the collector resistance, re 25 mV/IE(mA), and the Re is the unbypassedemitter resistance.In order to center the Q-point on the ac load line, you must try different values of RE untilVCEQ is equal to (ICQ)(Rc + Re), where ICQ IEQ = VE//(RE + Re), VCC - ICQ(RE + Re + RC), Rc isequal to the ac collector resistance, and RC is equal to the dc collector resistance.The amplifier output power (PO) is calculated as follows:The percent efficiency (ŋ) of a large-signal amplifier is equal to the maximum output power(PO) divided by the power supplied by the source (PS) times 100%. Therefore,where Ps = (VCC)(IS). The current at the source (IS) is determined from IS = I12 + ICQwhere I12 = VCC/(R1 + R2). Note: I12 is the current in resistors R1 and R2, neglecting the basecurrent.Procedure:Step 1 Open circuit file FIG 14–1. Bring down the multimeter enlargement and make sure that V and dc (---------) are selected. Run the simulation and record the dc base voltage (VB). Move the multimeter positive lead to node VE. Run the simulation and record the dc emitter voltage (VE). Move the multimeter positive lead to node VC. Run the simulation and record the dc collector voltage (VC). VB = 5.658V VE = 4.879V VC = 15.152VStep 2 Based on the voltages in Step 1, calculate the dc collector-emitter voltage (VCE) and the dc collector current (IC). IC = 48.48mA VCE=10.273 VStep 3 Draw the dc load line on the graph provided. Based on the calculations in Step 2, locate the operating point (Q-point) on the dc load line.7|Page
8. 8. IC(sat) 2nd AC load line 200 Center Q-point 150 1st AC load line 1st DC load line 100 1st Q-point 97.55mA 50 48.48mA 0 5 1010.27V 10.73V 12.94V 15 20 VCE(V) 5.37VStep 4 Open circuit file FIG14-2. Bring down the function generator enlargement and make sure that the following settings are selected. Sine wave , Freq = 2 kHz, Ampl = 250 mV, Offset = 0 v. Bring down the oscilloscope enlargement and make sure that the following settings are selected: Time base (Scale = 100 µa/Div, Xpos = 0 Y/T), Ch A (Scale = 200 mV/Div, Ypos = 0, AC), Ch B (Scale = 2 V/Div, Ypos = 0, AC), Trigger (Pos edge, Level = 0, Auto).Based on the value of RC and RL, calculate the ac collector resistance (RC), and then draw the ac load line through the Q-point on the graph in Step 3. VCE=VCEQ + (ICQ)(RC + Re)= 10.273 V + 48.48mA(50Ω+5 Ω)= 12.9394VQuestions: Is the operating point (Q-point) in the center of the dc load line? Is it in the center of the ac load line? No, it is neither at the center of the dc load line nor the center of the ac load line.Why is it necessary for the Q-point to be in the center of the ac load line for large signal inputs? It necessary for the Q-point to be in the center of the ac load line for large signal inputs to avoid clipping of the output wave form because in a class A amplifiers.8|Page
9. 9. Step 5 Run the simulation. Keep increasing the input signal voltage on the function generator until the output peak distortion begins to occur. Then reduce the input signal level slightly until there is no longer any output peak distortion. Pause the analysis and record the maximum undistorted ac peak-to-peak output voltage (VC) and the ac peak-to-peak input voltage (Vin) Vin = 303.852 mV Vo = 2.298VStep 6 Based on the voltages measured in Step 5, determine the voltage gain (AV) of the amplifier.Step 7 Calculate the expected voltage gain (AV) based on the value of the ac collector resistance (Rc), the unbypassed emitter resistance (Re), and the ac emitter resistance (re), where re = 25 mV / IE (mA)Questions: How did the measured amplifier voltage gain with the calculated voltage gain? There is 22.49% difference between the measured amplifier voltage gain and the calculated voltage gainWhat effect does unbypassed emitter resistance have on the amplifier voltage gain? What effect does it have on the voltage gain stability? The voltage gain is inversely proportional to the unbypassed emitter resistance When the unbypassed emitter resistance become higher and higher the voltage of an amplifier become smaller and smaller.Step 8 Calculate the value RE required to center the Q-point on the ac load line. Hint: Try different values of Re until VCEQ = (ICQ)(Rc + Re) at the new Q-point. See Theory section for details.9|Page
10. 10. VCEQ = (ICQ)(Rc + Re) VCEQ =VCC - ICQ(RE + Re + RC) VCC - ICQ(RE + Re + RC)= (ICQ)(Rc + Re) VCC (RE + Re + RC)= ( )(Rc + Re)Question: Did you need to increase RE to center the Q-point on the ac load line? Explain why. No, the RE was reduced to to center the Q-point of the ac load line. Moreover, it was reduced to to make VCEQ = ICQStep 9 Change RE to the value calculated in Step 8 and repeat the procedure in Step 5. Record the maximum undistorted ac peak-to-peak output voltage (Vo) and the ac peak-to-peak input voltage (Vin) for this centered Q-point. Vin = 547.839 mV Vo = 4.311 VQuestion: How did the maximum undistorted peak-to-peak output voltage measured in Step 9, for the centered Q-point, compare with the maximum undistorted peak-to-peak output voltage measured in Step 5, for the original Q-point that was not centered? Compare with the maximum undistorted peak-to-peak output voltage measured in Step 9, the maximum undistorted peak-to-peak output voltage measured in step 5 is much higher.Step 10 Calculate the new dc values for ICEQ for the new value of RE Locate the new dc load line and the new Q-point on the graph in Step 3. Draw the new ac load line through the new Q-point. VCEQ =20V - ( +5 + )=5.37V VCE=VCEQ + (ICQ)(RC + Re)= 5.37V + (50Ω+5 Ω)= 10.73525V10 | P a g e
11. 11. Questions: What effect did the new Q-point have on the location of the new ac load line? It changes the location or position of the ac load line.What was the location of the new Q-point on the new load line? The Q-point is now located at the center of the ac load line.Step 11 Based on the new centered Q-point and the ac load line, estimate what maximum ac peak-to-peak output voltage (VO) should be before output clipping occurs. Vo = 4.31 VQuestion: How did the maximum undistorted peak-to-peak output voltage measured in Step 9 for the centered Q-point, compared with the expected maximum estimated in Step 11? The maximum undistorted peak-to-peak output voltage measured in Step 9 for the centered Q-point is approximately the same as expected maximum estimated in Step 11.Step 12 Based on the maximum undistorted ac-peak-to-peak output voltage (Vo) measured in Step 9, calculate the maximum undistorted output power (PO) to the load (RL).Step 13 Based on the supply voltage (VCC), the new collector current at the new operating point (ICQ), and the bias resistor current (I12), calculate the power supplied by the dc voltage source (PS).11 | P a g e
12. 12. Step 14 Based on the power supplied by the dc voltage source (PS) and the maximum undistorted output power (PO) calculated in Step 12, calculate the percent efficiency (ŋ) of the amplifier.Question: is the efficiency of a class A amplifier high or low? Explain why. The efficiency of class A is low. Class A have low efficiency because the collector current flows during the full sine wave.12 | P a g e
13. 13. Conclusion: After performing the experiment, I conclude that class A amplifiers conductsat full sine wave. The quiescent point or the operating point of the dc load line is located atthe intersection of the dc collector-emitter voltage and the dc collector current. Meanwhile,the ac load line crosses the quiescent point and the horizontal axis of the transistorcollector characteristic curve plot at the dc collector-emitter voltage. There is incidencethat the quiescent point is not at the center of the ac load line that causes clipping in theoutput wave form. Adjusting the emitter resistance affects the locations of the quiescentpoint. Furthermore, the peak-to-peak undistorted input and output voltages aremuch higher when the quiescent point is located at the center of the ac load line. Finally,the location of the ac load line changes as the quiescent point changes.13 | P a g e