LABORATORY MODULE ENT 162 Analog Electronics Semester 2 (2006/2007) EXPERIMENT 6 : Amplifier Low-Frequency Response Name Matrix No. : : Name Matrix No. : : PUSAT PENGAJIAN KEJURUTERAAN MEKATRONIK KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA (KUKUM) Page 1 of 8
EXPERIMENT 6 Amplifier Low-Frequency Response 1. OBJECTIVE: 1.1 To demonstrate the factors that contributes to the low-frequency response of a Common Emitter Transistor Amplifier. 2. PARTS AND EQUIPMENT: 2.1 Resistor 1/4W 150 Ω - 1 Pc 2.2 Resistor 1/4W 2.7 kω - 2 Pcs 2.3 Resistor 1/4W 3.9 kω - 1 Pc 2.4 Resistor 1/4W 47 kω - 1 Pc 2.5 Resistor 1/4W 100 kω - 1 Pc 2.6 Capacitor 25V 2.2 µf - 2 Pcs 2.7 Capacitor 25V 10 µf - 1 Pc 2.8 Capacitor 0.033 µf - 2 Pcs 2.9 Capacitor 1 µf - 1 Pc 2.10 2N3904 NPN Silicon Transistor - 1 Pc 2.11 0 15V 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 Page 2 of 8
3. INTRODUCTION: The low-frequency response of a typical Common-Emitter Amplifier is determined in part by the input and output Coupling Capacitors and the Emitter Bypass Capacitor. The result is essentially a combination of three high-pass filter networks that allow signals having frequencies greater than the cutoff frequency of the dominant network to pass through while attenuating all others. Although the cutoff frequencies associated with these three paths can be made equal such is rarely the case. The values of the Capacitors are intentionally made abnormally small in order to allow the frequency response to be easily measured. Useful formulas: 1. V B B = [ R2 R + R 1 2 ] V CC - Dc Base voltage 2. V E = V B B - VBE - Dc Emitter voltage 3. I E = R VE + R E1 E 2 - Dc Emitter current 4. r e = 25mV I E - Dc Emitter resistance 5. R in = R 1 ll R 2 ll β ac (r e + R E1 ) - Ac input impedance 6. A V = 7. AV = V V out in RCllRL RE1+ r' e - Voltage gain - Voltage gain 8. db = 20 log (A V ) - Decibel voltage gain 9. f 1 = 10. f 2 = 1 2πC ( R in + R 1 G ) 1 R1llR 2llRG 2πC [( + RE1 + r' e) llr β 2 E 2 ac ] - Frequency response due to C 1 - Frequency response due to C 2 11. f 3 = 1 2πC ( R C + R 3 L ) - Frequency response due to C 3 Page 3 of 8
4. PROCEDURE: 4.1 Wire the circuit shown in Figure 6.1. Do not connect the Signal Generator and the Power Supply yet. Figure 6.1 : Experiment Setup 4.2 Check all connection. Apply only the 15 V supply voltage to breadboard. With a multimeter, individually measure the transistor s quiescent dc Emitter voltage with respect to Ground. From this value determine the transistor s ac Emitter resistance, r e (Equation 4). Calculate the expected midband voltage gain of the Amplifier in decibels (Equations 7 and 8). Record your results in Table 6.1. 4.3 Connect Channel 1 of your Oscilloscope at point I, V in and Channel 2 to point O, V out. Then connect the Signal Generator to the circuit. Adjust the sine wave output level of the Generator at a frequency of 50 khz so that the peak-to-peak output voltage of the Amplifier spans 7.1 vertical divisions when Channel 2 is set at a sensitivity of 0.5V/division. Measure the peak-to-peak input voltage level and determine the Amplifier s db voltage gain (Equation 8). Record this value in Table 6.1. Page 4 of 8
4.4 In order to determine the Amplifier s low-frequency 3 db point due solely to the effects of the input Coupling Capacitor, C 1, replace C 1 (2.2μF) with a 0.033μF Capacitor. Adjust the sine wave output level of the Generator at a frequency of 50 khz so that the peak-to-peak output voltage of the Amplifier spans 7.1 vertical divisions when Channel 2 is set at a sensitivity of 0.5V/division. Then slowly reduce the input frequency until the peak-to-peak output voltage drops to 5 vertical divisions. This reduction in output voltage is 5/7.1 or 0.707, which is equivalent to -3 db. Using your Oscilloscope, measure the frequency at which this value occurs, f 1. Record this frequency, f 1 in Table 6.2 along with the expected value (Equation 9) for comparison, assuming a typical ac Beta, β ac of 150. Replace C 1 with a 2.2μF Capacitor. 4.5 In order to determine the Amplifier s low-frequency 3 db point due solely to the effects of the Emitter Bypass Capacitor C 2, replace C 2 (10μF) with a 1μF Capacitor. Again adjust the sine wave output level of the Generator at a frequency of 50 khz so that the peak-to-peak output voltage of the Amplifier spans 7.1 vertical divisions when Channel 2 is set at a sensitivity of 0.5V/division. Reduce the input frequency until the peak-to-peak output voltage drops to 5 vertical divisions. From your Oscilloscope, measure the frequency at which this value occurs, f 2. Record this frequency, f 2 in Table 6.2 along with the expected value for comparison (Equation 10), assuming a typical ac Beta, β ac of 150. Replace C 2 with a 10μF Capacitor. 4.6 In order to determine the Amplifier s low-frequency 3 db point due solely to the effects of the output Coupling Capacitor C 3, replace C 3 (2.2μF) with a 0.033μF Capacitor. Again adjust the sine wave output level of the Generator at a frequency of 50 khz so that the peak-to-peak output voltage of the Amplifier spans 7.1 vertical divisions when Channel 2 is set at a sensitivity of 0.5V/division. Reduce the input frequency until the peak-to-peak output voltage drops to 5 vertical divisions. From your Oscilloscope, measure the frequency at which this value occurs, f 3. Record this frequency, f 3 in Table 6.2 along with the expected value for comparison (Equation 11). Page 5 of 8
Name : Date : Matrix No : 5. RESULTS FOR EXPERIMENT 6: AMPLIFIER LOW-FREQUENCY RESPONSE Table 6.1 : Amplifier Midband Response Parameter Value V E measured r e measured V in V out A V (db) measured A V (db) expected % error Table 6.2 : Amplifier Low-Frequency Response (Critical Frequencies) Frequency Measured Expected % Error f 1 f 2 f 3 Instructor Approval : Date : Page 6 of 8
Name : Date : Matrix No : 6. CALCULATIONS: Instructor Approval : Date : Page 7 of 8
Name : Date : Matrix No : 7. DISCUSSION: 8. CONCLUSION: Page 8 of 8