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, 5th Edition, Oxford Press, 2004
Useful Formulas
Voltage gain from base to collector
(1)
(2) (normal circuit)
(3) (no load)
(4) (no bypass capacitor)
Transistor AC-emitter resistance (at room temperature)
(5)
Quiescent DC-base voltage
(6)
Quiescent DC-emitter voltage
(7)
Quiescent DC-emitter current
(8)
Quiescent DC-collector voltage
(9)
Quiescent DC-collector-emitter voltage
(10)
Amplifier-input impedance
(11)
Voltage gain in dB
(12)
Frequency response due to the input coupling capacitor C1
( 13) where RG = signal source impedance
Frequency response due to the bypass capacitor C2
(14)
Frequency response due to the output coupling capacitor C3
(15)
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 C1. Derive a relation between Vin 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 VB, emitter voltage VE and collector voltage VC 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 VE obtained in Step 2, calculate the DC- emitter current IEQ using Eqn. 9 and the transistor AC-emitter resistance re using Eqn. 5. Complete Table 2.
4. Connect Channels 1 and 2 of the oscilloscope to points I (vin ) and point O (vout ), 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 vin, calculate the voltage gain by dividing vout by vin, 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 vout without getting any clipping on the output waveform. Measure the p-p values of vin and vout, 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 vin in Step 5 by about 20%. Measure the p-p values of vin and vout, 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 RL. As in Step 4, experimentally determine the voltage gain by measuring the p-p voltages of vin and vout, 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 vin at 0.02V (p-p). Vary the frequency of the input signal as indicated in Table 4 and measure vout 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 vin = 0.02 (p-p) at 50kHz. Measure vout. 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 C1 only, replaced C1 with a
0.033 μF capacitor. Adjust the signal generator to give vin = 0.02 V (p-p) at 50 kHz. Measure vout. Slowly reduce the frequency of the input signal until vout drops to 0.707 (i.e. 1/ ·、 ) 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 C1 with the original 2.2 μF capacitor.
12. Replace C2 with a 1-μF capacitor and repeat the procedures outlined Step 11. Record the measured value off2 in Table 6. Calculate the expected value using Eqn. 14 and record it in Table 6. Replace C2 with the original 10-μF capacitor after the measurement.
13. Replace C3 with a 0.033-μF capacitor and repeat the procedures outlined in Step 11. Record the measured value off3 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