LAB 6: BJT AMPLIFIER

Transcription

1 LAB 6: BJT AMPLIFIER LEARNING OUTCOMES: In this lab, students design and implement single-stage BJT amplifiers and observe amplitude and frequency response. Breadboard and the Analog Discovery Kit are to be used. The students will use various tools and functions from the Waveform software to perform measurement and plotting amplifier response. The students will compare the gain and frequency response of the BJT amplifier and the MOSFET amplifier in Lab 5. MATERIAL AND EQUIPMENT Material 2N2222 npn BJT transistor Capacitors Resistors 2N2907 pnp BJT transistor Equipment Breadboard Multimeter Analog Discovery Digilent Waveform software Curve tracer PRE-LAB Look at the handout and be familiar with the CE amplifier. You need to know how to calculate the DC voltage, current and small-signal gain. Design an amplifier with a voltage gain of -4, a collector current of 5 ma, bias the collector voltage to provide a maximum output voltage swing (i.e., ½ of Vcc). Use a Vcc of +5 V from the power supply of the Analog Discovery unit. PROCEDURES 1. NPN transistor characteristics Obtain 2N2222 Si npn from the lab instructor or from the technician in room 2C97. Measure their βdc at VCE =5V using the Tektronix curve tracers. Use a scale of 0-10 ma for IC and 0-10 V for VCE. The step current for the base is 10uA. You need to capture the transistor characteristics to your lab report. Label βdc on the curves. The normal βdc of 2N2222 is between Also use the information in Lab 5 BJT IV curve. 2. Common-Emitter amplifier Construct the circuit shown in Figure 1. You can use either 1 uf for capacitor C B and 4.7 uf for the capacitor C C. A. Measure the DC currents and voltages: IB, IC, IE, VC, VB, VE. Remember to measure one at a time and NO input AC signal. In your report, make a table and list the results from your experiment and hand calculations. In your hand calculation, use β determined from your IV curve. Assume VBE = 0.7V.

2 B. At this time, do NOT connect the load resistor RL yet. Generate an input signal from AWG1 (100 mvpp, 1 KHz) and send the signal to the input of the amplifier. Observe the input and output voltage waveforms using two probes of an oscilloscope (Channel 1 and Channel 2). First observe the waveforms of the input signals that are before and after C B. Note that how DC voltage at the base is preserved by using coupling capacitor. Then observe the waveforms of signals Vc and Vo that are before and after CC. The waveform at Vc has both AC and DC components and waveform at Vo is a pure AC signal because of the coupling capacitor CC. Capture the waveforms at Vi, V B, Vc and Vo copy them to your lab report. Find the voltage gain from the waveforms (Vo/Vi). +5V To Channel 1 of the oscilloscope R1 R C V C C C To Channel 2 of the oscilloscope AWG1 ~ V i C B R2 V B R E 2N2222 C E V O R L Figure 1: CE amplifier C. Perform hand calculations for the output voltage and voltage gain. List the results from your experiment, hand calculation as a table in your lab report. D. Connect the load resistor RL = 1K to the amplifier. Read the output voltage from the oscilloscope and find the voltage gain. E. Connect a polarized capacitor CE =22uF to the circuit as shown in Figure 1 so that it is in parallel with RE. At this time, the emitter is AC shorted to ground. Observe the output waveform at Vc and Vo and copy it to your lab report. Explain why the waveforms are distorted in your report. Once you have finished this step, recover the circuit connection according to Figure 1 by removing CE and RL. F. At the last step, you need to adjust the frequency of the input signal from the signal generator. First you need to reduce the frequency and observe the voltage gain. The gain will reduce after certain frequency. Find the frequency, fl, when the voltage gain decreases to 70%. Then you need to increase the frequency and also find the input signal frequency, fh, when voltage gain decreases to 70%. The amplifier bandwidth is defined as fh-fl. Document the frequencies in your report. G. Use Sweep function in the Arbitrary Waveform Generator to sweep the frequency of the input signal and observe the response of the amplifier. Describe the response of the amplifier. H. Compare the results with the n-channel MOSFET amplifier in Lab 4. I. Optional: Repeat the steps using 2N2907 (pnp) transistor. Comment on the differences between npn and pnp amplifiers. 2

3 Scope sample npn BJT (2N2222) amplifier: R1=10K, R2=3.9K, Rc=560, Re=150, 1KHz, 100mV, Vb=1.35V. Ve=0.709V, Vc=2.463V 3

4 Scope sample pnp BJT (2N2907) amplifier: (R1=10K, R2=27K, RE=100, RC=560) (VB=3.85V, VC=2.6V, VE=4.54V) +5V To Channel 1 of the oscilloscope R1 R E C E AWG1 ~ V i C B R2 V B R C 2N2907 V C C C V O To Channel 2 of the oscilloscope R L 4

5 REFERENCES 1. Sedra/Smith, Microelectronic Circuits, 6 th Edition, Oxford University Press, Mark N. Horenstein, Instructor s Resource Manual, Microelectronic Circuit and Devices, 2 nd Edition, Prentice Hall, A Complete Analog Design Kit for the Price of a Textbook, Diligent, -DISCOVERY 4. n-channel MOSFET specification: 5. p-channel MOSFET specification: 6. n-channel MOSFET N2222 npn BJT specification: D.PDF 8. 2N2907 pnp BJT specification: URE/DATASHEET/CD pdf 5