THE UNIVERSITY OF HONG KONG Department of Electrical and Electrical Engineering Experiment EC1 The Common-Emitter Amplifier Location: Part I Laboratory CYC 102 Objective: To study the basic operation and analyze the characteristics of the common-emitter amplifier. Apparatus: HP E3611 DC Power Supply 1 HP 6116 A Pulse/Function Generator 1 HP 34401 A Multimeter 1 HP 54600 An Oscilloscope 1 Components: Resistors: 150 Ω 1 1 kω, 1 2 kω 2 4.7 kω 1 10 kω 1 Capacitor: 0.033 μf 2 1 μf, 1 2.2 μf 2 10 μf 1 Transistor: 2N3904 NPN transistor 1 Others: A breadboard 1 Reference: A. S. Sedra and K. C. Smith, Microelectronic Circuits, 5 th Edition, Oxford Press, 2004 Useful Formulas Voltage gain from base to collector vout (1) Av v is (2) A v RC RL (normal circuit) r e (3) Av R r C (no load) e (4) A v R R C L E R + r e (no bypass capacitor)
Transistor AC-emitter resistance (at room temperature) 26mV (5) re I E Quiescent DC-base voltage RV 2 CC (6) VB R + R 1 2 Quiescent DC-emitter voltage V V V (7) E B BE Quiescent DC-emitter current VE (8) IEQ ICQ R E Quiescent DC-collector voltage V V I R (9) C CC CQ C Quiescent DC-collector-emitter voltage V V I ( R + R + R ) (10) CEQ CC CQ C E1 E2 Amplifier-input impedance R R R β ( r ) (11) in 1 2 e Voltage gain in db (12) Av in db 20log Av Frequency response due to the input coupling capacitor C 1 1 (13) f1 where R G signal source impedance 2 πc ( R + R ) 1 in G Frequency response due to the bypass capacitor C 2 (14) f2 1 R1 R2 RG 2 πc2[( + re) RE] β Frequency response due to the output coupling capacitor C 3 1 (15) f3 2 πc ( R + R ) 3 C L Preparation: 1. Before coming to the laboratory, run the PSPICE simulator to analyze the circuit in this experiment. Obtain the simulation results for Tables 1 7 and complete sketches 1-5. 2. Derive Equations 2, 3, 4, 11, 13, 14 and 15.
3. Suppose a resistor R is inserted between the signal generator and C 1. Derive a relation between V in and the AC voltage at point B. Procedures: 1. Wire the circuit shown in Fig. 1 on the breadboard provided. Do not connect the signal generator and the power supply to the circuit yet! 2. Check all connections. Apply the 15-V supply voltage. Use the DMM to individually measure the transistor DC-base voltage V B, emitter voltage V E and collector voltage V C with respect to the ground. Record the results into Table 1. Based on the resistor values in Fig. 1, calculate the expect values of these three voltages using Eqns. 6, 7 and 9, assuming a base-emitter voltage drop of 0.7 V. Complete Table 1. 3. Using the measured value for the DC-emitter voltage V E obtained in Step 2, calculate the DCemitter current I EQ using Eqn. 9 and the transistor AC-emitter resistance r e using Eqn. 5. Complete Table 2. 4. Connect Channels 1 and 2 of the oscilloscope to points I (v in ) and point O (v out ), respectively, of the circuit in Fig. 1. Then connect the signal generator to the circuit and adjust the sine wave output level of the generator to 0.02 V p-p (peak-to-peak) at a frequency of 5 khz. Measure the actual p-p output voltage out v and the p-p input voltage v in, calculate the voltage gain by dividing v out by v in, and the expected value using Eqn. 2. Record them in Table 3. Sketch the output voltage waveform in Sketch 1. 5. Slowly increase the amplitude of v in to obtain the maximum symmetrical v out without getting any clipping on the output waveform. Measure the p-p values of v in and v out, and then calculate the gain. Also, calculate the expected values using Eqn. 2. Record them in Table 3. Sketch the output waveform in Sketch 2. 6. Increase the amplitude of v in in Step 5 by about 20%. Measure the p-p values of v in and v out, and then calculate the gain. Also calculate the expected value using Eqn. 2. Record them in Table 3. Sketch the output waveform in Sketch 3. 7. Remove R L. As in Step 4, experimentally determine the voltage gain by measuring the p-p voltages of v in and v out, and calculate the expected value using Eqn. 3. Record them in Table 3. 8. Reconnect the 2-kΩ resistor as in the original circuit of Fig. 1. Remove the 10-μF emitter bypass capacitor from the circuit. As in Step 7, experimentally determine the voltage gain and calculate the expected value using Eqn. 4. Complete Table 3. 9. Reconnect the 10-μF emitter bypass capacitor as in the original circuit of Fig. 1. Keep v in at 0.02V (p-p). Vary the frequency of the input signal as indicated in Table 4 and measure v out at each frequency. Plot the frequency response in Sketch 4. (Alternately, you can use PSPICE to generate
the plot and then plot the measured values on the same figure.) Record the lower and upper 3-dB frequencies in Table 4. 10. Adjust the output level of the signal generator to v in 0.02 (p-p) at 50kHz. Measure v out. Determine the voltage gain of the amplifier in db using Eqn. 12 and the expected voltage gain in db using Eqns. 2 and 12. Complete Table 5. 11. In order to determine the amplifier s low frequency 3-dB point due to C 1 only, replaced C 1 with a 0.033 μf capacitor. Adjust the signal generator to give v in 0.02 V (p-p) at 50 khz. Measure v out. Slowly reduce the frequency of the input signal until v out drops to 0.707 (i.e. 1/ 2 ) of that measured at 50 khz input frequency. Record the frequency (f 1 ) at which this occurs in Table 6. Calculate the expected value using Eqn. 13, assuming β 150 for the 2N3094 transistor, and record it in Table 6. Replace C 1 with the original 2.2 μf capacitor. 12. Replace C 2 with a 1-μF capacitor and repeat the procedures outlined Step 11. Record the measured value of f 2 in Table 6. Calculate the expected value using Eqn. 14 and record it in Table 6. Replace C 2 with the original 10-μF capacitor after the measurement. 13. Replace C 3 with a 0.033-μF capacitor and repeat the procedures outlined in Step 11. Record the measured value of f 3 in Table 6. Calculate the expected value using Eqn. 14 and record it in Table 6. NB when you use PSPICE to get the values for steps 11-13, you can first plot the frequency response for each case and then get the 3-dB frequency from the plot. Figure 1
Report on: EC1 The Common-Emitter Amplifier Name: Student No.: Stream: Group No.: Date of experiment: Date of submission: Table 1 (Step 2) Parameter Measured Calculated %Error Simulated %Error Value Value Value Parameter I EQ (measured) r e (calculated) I EQ (simulated) %Error Table 2 (Step 3) Value Condition Normal circuit (step 4) Normal circuit with maximum symmetrical v out (step 5) Normal circuit with distorted v out (step 6) No load(step 7) No bypass capacitor (step 8) Table 3 Comparison of gains obtained from different methods (Steps 4-8) v in v out Measured Expected %Error Simulated %Error p-p p-p Gain Gain Gain
Table 4 Frequency Response for v in 0.02 peak-to-peak (Step 9) Freq (Hz) 50 100 500 5k 10k 20k Output (p-p) V A v Lower 3-dB frequency is 20 log A v Gain in db (simulated) Freq (Hz) 500 k 1 M 10 M 30 M 50 M 100 M Output NA (p-p) V A v Lower 3-dB frequency is 20 log A v Gain in db (simulated) NA Table 5 Amplifier mid-band response (Step 10) Parameter v in (p-p) measured v out (p-p) measured A v (db) measured A v (db) expected %Error A v (db) simulated %Error Value
Table 6 Amplifier low-frequency response (Steps 11-13) Frequency Measured Expected %Error Simulated %Error f 1 due to C 1 f 2 due to C 2 f 3 due to C 3 Scale: V/division Sketch 1: Waveform v out with v in 0.2 (p-p) (Step 4)
Scale: V/division Sketch 2: Maximum symmetrical waveform v out for the amplifier (Step 5) Scale: V/division Sketch 3: Distorted waveform v out with v in increased by 20% (Step 6)
Sketch 4: Amplifier Frequency Response (Step 9) Scale: V/division Sketch 5: DC and AC load lines
Postlab Questions: 1. Derive equations for the DC load line and the AC load line for the normal circuit shown in Fig. Using the measured I CQ and V CQ, draw both the DC and AC load lines in Sketch 5. Determine the maximum symmetrical p-p v out. Does this agree with that obtained in Step 5? Discuss. 2. In Steps 4 and 5, are the positive and negative peaks of v out symmetrical and equal in magnitude? Explain the discrepancies. 3. Comment on the advantages and disadvantages of this type of amplifier circuits. Version:20170831