LABORATORY MODULE. Analog Electronics. Semester 2 (2005/2006)

Transcription

1 LABORATORY MODULE ENT 162 Analog Electronics Semester 2 (2005/2006) EXPERIMENT 5 : The Class A Common-Emitter Power Amplifier Name Matrix No. : : PUSAT PENGAJIAN KEJURUTERAAN MEKATRONIK KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA (KUKUM) Page 1 of 8

2 EXPERIMENT 5 The Class A Common-Emitter Power Amplifier 1. OBJECTIVE: 1.1 To demonstrate the operation and characteristics of a Class A Common Emitter Power Amplifier. 2. PARTS AND EQUIPMENT: 2.1 Resistor 1/4W 220 Ω - 1 Pc 2.2 Resistor 1/4W 560 Ω - 1 Pc 2.3 Resistor 1/4W 1 kω - 1 Pc 2.4 Resistor 1/4W 4.7 kω - 1 Pc 2.5 Resistor 1/4W 10 kω - 1 Pc 2.6 Resistor 1/4W 100 kω - 1 Pc 2.7 Potentiometer 5 kω - 1 Pc 2.8 Capacitor 25V 2.2 µf - 2 Pcs 2.9 Capacitor 25V 100 µf - 1 Pc N3904 NPN Silicon Transistor - 1 Pc V DC Power Supply - 1 Unit 2.12 Signal Generator - 1 Unit 2.13 Dual Trace Oscilloscope - 1 Unit 2.14 Breadboard - 1 Unit 2.15 Multimeter - 1 Unit 2.16 Jumper Wire 3. INTRODUCTION: The Class A Amplifier is biased such that Collector current always flows during the entire cycle of the input waveform. The Amplifier s Q point should be biased at the center of the ac load line so that the output signal can have the maximum possible swing in both directions. Clipping will occur simultaneously on both peaks of the output signal if the Amplifier is overdriven. If the Q point is not centered on the ac load line, output waveform clipping will occur first, either at saturation or at cutoff. Page 2 of 8

3 Useful formulas: R2 1. V B = [ R1 + R2 ] V CC - Dc Base voltage 2. V E = V B - V BE - Dc Emitter voltage VE 3. I CQ = RE - Collector current 4. V CEQ = V CC I CQ (R C + R E ) - Dc Collector-Emitter voltage VCEQ 5. I C(sat) = I CQ + RC - Collector saturation current 6. V ce(cutoff) = V CEQ + I CQ R C - Collector-emitter cutoff voltage 7. I C(sat) = 2I CQ - Centered Q point 8. V CE(cutoff) =2V CEQ - Centered Q point 9. R C = V I CEQ CQ - Centered Q point 10. P o(rms) = 2 [ Vo( rms ) ] R L - RMS output (load) power 11. P DC = V CC I CQ - Dc Power Supplied to Amplifier 12. %η = Po ( rms) PDC X 100% - Amplifier percent efficiency 4. PROCEDURE: 4.1 Wire the circuit shown in Figure 5.1. Do not connect the Signal Generator and the Power Supply yet. 4.2 Check all connection. Apply only the 15 V supply voltage to breadboard. With a multimeter, individually measure the transistor dc Base and Emitter voltages with respect to Ground, as well as V CEQ. Record your results in Table 5.1. Base on the resistor values in Figure 5.1, determine the expected values of these three voltages, assuming a Base-Emitter voltage drop of 0.7V and compare them with the measured values in Table 5.1. Page 3 of 8

4 Figure 5.1: Experiment Setup 4.3 Measure the quiescent dc Collector current, I CQ and compare it with the expected value (Equation 3). Record your values in Table Connect Channel 1 of your Oscilloscope at point I, V in and Channel 2 to point O, V out. Adjust your Oscilloscope to the following approximate settings: Channel 1: 10mV / division, ac coupling Channel 2: 1V / division, ac coupling Time Base: 0.2ms / division 4.5 From Equation 9, determine the value of the load resistor, R L that gives a centered Q point on the ac load line. Adjust the 5kΩ Potentiometer load as closely as you can to that value. 4.6 Connect the Signal Generator to the breadboard and adjust the sine wave output level of the Generator at 30mV peak-to-peak at a frequency of 5 khz. If your Signal Generator cannot be adjusted to 30mV peak-to-peak, any value higher than this can be used as long as the output signal is not clipped. Then slowly increase the output level of the Signal Generator. Page 4 of 8

5 4.7 Reduce the input signal level to zero and replace the Potentiometer with a 220Ω resistor. Slowly increase the input signal level. Confirm the situation by drawing the ac load line with the Q point on the graph page provided. 4.8 Reduce the input signal level to zero and replace the 220Ω resistor with a 100kΩ resistor. Slowly increase the input signal level. Confirm the situation by drawing the ac load line with the Q point on the graph page provided. 4.9 Replace the 100kΩ resistor with the 5kΩ Potentiometer. As in Step 4.5, again adjust the 5kΩ Potentiometer to the resistance that centers the Q point on the ac load line Carefully increase the peak-to-peak input signal just before both output peak clip off. With your multimeter, measure the RMS voltage across the load resistor, V o(rms). Calculate the RMS output power of the Amplifier (Equation 10). Record these results in Table Using Equation 11, calculate the dc Power Supplied. Record this value in Table Calculate the Percent Efficiency (%η) of your Amplifier (Equation 12) and compare it with the theoretical maximum of 25%. Record your result in Table 5.2. If you calculate a value greater than 25%, then repeat Step 4.9, 4.10 and Try to determine the source of your error. Page 5 of 8

6 Name : Date : Matrix No : 5. RESULTS: Table 5.1: Class A Amplifier Bias Parameter Parameter Measured Value Expected Value % Error V B V E V CEQ I CQ Table 5.2: Class A Amplifier Efficiency Parameter Measured Value V o(rms) Parameter Expected Value P o(rms) P DC %η 6. CALCULATION: Instructor Approval : Date : Page 6 of 8

7 Name : Date : Matrix No : 7. QUESTIONS: 1. For the circuit in Figure 5.1, the value of R L that centers the Q point is approximately 2. For the best performance, the Q point should be centered on 3. If the negative peaks of the output waveform are clipped, this condition represents 4. Making R L much larger than 1kΩ would result in 5. For the Class A Amplifier, the maximum peak-to-peak output voltage that can be obtained without clipping a 500 Ω load is approximately Instructor Approval : Date : Page 7 of 8

8 Name : Date : Matrix No : 8. DISCUSSION: 9. CONCLUSION: Page 8 of 8