3401-Lab-BJT-Amp-Behavior-r3c_SH (2)

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Kennesaw State University *

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3401

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Feb 20, 2024

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BJT Amplifiers EE 3401 Laboratory Exercise Electrical and Computer Engineering Department Kennesaw State University 1 Rev. 3c_SH Objectives: You will analyze, build and test NPN transistor amplifiers in order see how they perform. Three common-emitter (CE) amplifiers and one common collector (CC) amplifier are examined. The effect of an emitter resistor on the quality of the bias circuit and the amplifier gain are observed. Introduction: The CE amplifier in Fig. 1 has no emitter degeneration resistor. The resulting dc bias circuit quiescent point (Q-point) that is sensitive to β variation. However, it has high ac voltage gain. The function generator’s signal is connected to the base through a dc blocking capacitor. The second CE amplifier in Fig. 2 has an emitter resistor, which makes the Q-point much less sensitive to β variation. But, the gain is much lower than that of the first circuit. The third circuit uses a capacitor to “bypass” the emitter resistor for ac signals, increasing the gain. The capacitor does not affect the dc bias circuit, so the Q- point is the same as for the second circuit. The common-collector amplifier in Fig. 4 is a modification of the Fig. 2 circuit with the collector resistor removed. Equipment: Function/Arbitrary Waveform Generator Oscilloscope Digital Multimeter (DMM) DC power supply with +20 V output Cables and oscilloscope probes as needed Solderless breadboard Components: Resistors (Qty): 100 Ω (3), 1 k Ω (1), *56 k Ω (1), *82 k Ω (1), 180 k Ω (1) Capacitors (Qty): 4.7 µ F (2), 47 µ F (1) NPN transistor (Qty): 2N2222 (1) 2W 8 Ω speaker with alligator leads (provided by lab instructor) *The 56 k Ω and 82 k Ω resistors will be combined in parallel for R 2 (33 k Ω ) Prelab: 1. For the three amplifier circuits in Figs. 1, 2, 3, and 5: a. Calculate the Q-point values of collector current ( I C ) and collector-emitter voltage ( V CE ). Assume β DC =220, V BE = 0.65 V, V CE(SAT) =0.2 V, and V A = ∞ . Put your results in the prelab columns of Table 1. b. Calculate the small-signal voltage gain V out / V in . Assume β AC =220 and the capacitors are short circuits. Put your results in the prelab columns of Table 2. Record the gain magnitude in column 2 and the phase shift in column 3. A non-inverting gain has a phase shift of 0 ° and an inverting gain has a phase shift 180 ° .

2 Rev. 3c_SH Procedure: General Information 1. Consider the following when building and testing the circuit. a. Electrolytic capacitors are used in the circuits. These are polarized and must be installed in the correct direction as indicated by the “+” sign on the schematics. b. Measure sine wave input and output waveforms in peak-peak values. The oscilloscope can make the measurements automatically. Note: Use Hi Res or Average acquisition mode for best oscilloscope measurements. Note: Always turn off the power supply before making circuit changes. Common-Emitter Amplifier Circuits 2. Consider the circuit in Fig. 1. a. Measure the value of R C . R C value: b. Construct the circuit. Minimize the number of wires. c. Use the DMM to measure V CE and I C . For I C , measure the voltage across R C and divide by its value. Put the results in Table 1. d. Connect the oscilloscope to measure V in on channel 1 and V out on channel 2. Use a × 10 probe to measure V out . Set channel 2 for ac coupling so the output signal’s dc component does not appear on the oscilloscope. Note: Some probes can be switched between × 1 and × 10. Be sure it is set to × 10. If the probe scale factor is not automatically sensed by the oscilloscope, use the channel 2 menu to set the probe scale factor to × 10. Fig. 1. Common-emitter amplifier without emitter resistor.

3 Rev. 3c_SH e. Set the function generator to create a 10 mV peak , (20mV peak-peak ) waveform at 5 kHz: V in = 0.01sin[2 π (5000) t ] V. Connect it to the circuit and verify the peak-peak amplitude on the oscilloscope. Note: The oscilloscope may have trouble triggering on the channel 1 waveform. Connect a BNC-BNC cable from the generator’s sync output to channel 3 and trigger on that square wave instead. Once you set up the triggering, turn off channel 3 so the square wave does not appear on the display. f. Verify that the V out waveform is a sine wave with very little distortion. Note: If the waveform flattens at the top or bottom, you need to reduce the amplitude of V in . Do this by adding two 100 Ω resistors in parallel from the function generator’s connection point at the left side of C 1 to ground. This creates a voltage divider with scale factor 0.5 between the generator’s internal 50 Ω resistance and the 50 Ω resistance you created. g. Measure the peak-peak amplitudes of V in and V out and the phase shift of V out relative to V in . Record these in Table 2. Calculate the measured gain magnitude | V out / V in | and place the result in the table. Save an image of the oscilloscope screen showing the measured amplitudes and phase shift. Note: Be sure to use Hi Res or Average acquisition mode so that the waveforms are clean with minimum noise h. If you added 100 Ω resistors to reduce the input signal amplitude, remove them. Change the input signal to 20 mV peak (40 mV peak-peak ): V in = 0.02sin[2 π (5000) t ] V. The waveform should now flatten at the top or bottom or both. Save an image of the oscilloscope screen. 3. Consider the circuit in Fig. 2. a. Construct the circuit. b. Disable the function generator output or disconnect it from the circuit. Use the DMM to measure V CE and I C . Put the results in Table 1. Fig. 2. Common-emitter amplifier with emitter resistor.

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4 Rev. 3c_SH c. Set the function generator to create the waveform V in = 0.02sin[2 π (5000) t ] V. Connect it to the circuit and verify the peak-peak amplitude on the oscilloscope. d. Measure the peak-peak amplitudes of V in and V out and the phase shift of V out relative to V in . Record these in Table 2. Calculate the measured gain magnitude | V out / V in | and place the result in the table. Save an image of the oscilloscope screen showing the measured amplitudes and phase shift. 4. Consider the circuit in Fig. 3. a. Construct the circuit. b. Disable the function generator output or disconnect it from the circuit. Use the DMM to measure V CE and I C . Put the results in Table 1. c. Set the function generator to create the waveform V in = 0.02sin[2 π (5000) t ] V. Connect it to the circuit and verify the peak-peak amplitude on the oscilloscope. d. Measure the peak-peak amplitudes of V in and V out and the phase shift of V out relative to V in . Record these in Table 2. Calculate the measured gain magnitude | V out / V in | and place the result in the table. Save an image of the oscilloscope screen showing the measured amplitudes and phase shift. 5. Consider the circuit in Fig. 4. a. Disable the function generator output from the circuit in the previous step. Disconnect the oscilloscope from the circuit but leave the function generator connected. b. Add a 4.7 µ F capacitor and the 2W 8 Ω speaker to the circuit as shown in Fig 4. Fig. 3. Common-emitter amplifier with bypassed emitter resistor.

5 Rev. 3c_SH c. Enable the function generator output to supply V in = 40 mV peak-peak at 5 kHz (from the previous step). You should be able to hear a tone. Increase V in to 100 mV peak-peak . The tone should get louder. d. Decrease the frequency of the function generator until you can’t hear the tone. Record the lowest audible frequency on your datasheet. e. Increase the frequency of the function generator until you can’t hear the tone. Record the highest audible frequency on your datasheet. Common-Collector Amplifier Circuit 5. Consider the circuit in Fig. 5. Fig. 5. Common-collector amplifier. Fig. 4. Common-emitter amplifier with bypassed emitter resistor and resistive load

6 Rev. 3c_SH a. Remove R E from the circuit and measure its value. Do not measure the resistor while it is in the circuit. R E value: b. Construct the circuit. c. Disable the function generator output or disconnect it from the circuit. Use the DMM to measure V CE and I E . For I E , measure the voltage across R E and divide by its value. Assume I C ≈ I E since β is large. Put the V CE and I C results in Table 1. d. Set the function generator to create the 200 mV peak (0.4 V peak-peak ) waveform V in = 0.2sin[2 π (5000) t ] V. Connect it to the circuit and verify the peak-peak amplitude on the oscilloscope. e. Measure the peak-peak amplitudes of V in and V out and the phase shift of V out relative to V in . Record these in Table 2. Calculate the measured gain magnitude | V out / V in | and place the result in the table. Save an image of the oscilloscope screen showing the measured amplitudes and phase shift. Results Documentation Summary 6. You should have the following documentation from the procedure steps. a. You should have five oscilloscope screen shots, two from the circuit 1 and one each from circuits 2, 3, and 5. b. Be sure that you recorded data and calculated results in steps you were asked to. c. Check with your instructor for additional documentation requirements. Analysis: 1. Examine your oscilloscope image from step 2(h) and refer to the BJT common-emitter characteristics in Fig. 6. If the waveform flattens at the top, is the transistor in saturation or cutoff? If the waveform flattens at the bottom, is the transistor in saturation or cutoff? 2. Calculate the percent error between measured and calculated I C and V CE values in Table 1. Put the results in the appropriate columns in the table. Explain the cause(s) of the differences. 3. Calculate the percent error between measured and calculated gain magnitude values in Table 2. Put the results in the appropriate column in the table. Explain the cause(s) of the differences.

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7 Rev. 3c_SH Fig. 6. Typical BJT common-emitter characteristics with a CE amplifier load line.

8 Rev. 3c_SH Appendix 1: Data Tables Table 1. Calculated and measured amplifier Q-point data. Amplifier Calculated I C (mA) (Prelab) Calculated V CE (V) (Prelab) Measured I C (mA) Measured V CE (V) I C Percent Error V CE Percent Error CE, no R E , Fig. 1 CE, with R E , Fig. 2 CE, bypassed R E , Fig. 3 CC, Fig. 5 Table 2. Calculated and measured amplifier gain data. Amplifier Calculated Gain Magnitude | V out / V in | (Prelab) Calculated Gain Phase Shift (Prelab) Measured V in (V) Measured V out (V) Measured Gain Magnitude | V out / V in | Measured Gain Phase Shift Gain Magnitude Percent Error CE, no R E , Fig. 1 CE, with R E , Fig. 2 CE, bypassed R E , Fig. 3 CC, Fig. 5