Junior Physics Lab: Building & Analyzing Transistor & Op-Amp Circuits, Study notes of Physics

Download Junior Physics Lab: Building & Analyzing Transistor & Op-Amp Circuits and more Study notes Physics in PDF only on Docsity! 1 Junior Physics Laboratory Exercise on Analog Circuits In this exercise you will assemble and operate some simple transistor and op-amp circuits. The examples chosen are typical of those used elsewhere in our labs and in research. Your write-up should explain clearly what you did in each part, following the general organization of this guide. Include a schematic of each circuit as you built it, using the schematics in the notes as a guide to standard practice. Provide complete answers to any questions posed in the text. A neatly hand-written report with clear free-hand figures is entirely acceptable. Before starting the lab work you should review the topical notes on Analog Circuits, available on the course web site. Sections 2.01-2.09, 2.15, 4.01-4.10, and 15.02 of The Art of Electronics by Horowitz and Hill will also be helpful. A. Circuit construction The most convenient way to set up test circuits is on a breadboard, a large plastic block with sockets to mount transistors, integrated circuits, resistors, capacitors, etc. Pre-cut jumper wires are used to make connections by plugging in to the interconnected sockets. An auxiliary circuit board plugs into the main board to provide connections for positive and negative supply voltages and for external signals. The connections to the outboard can be understood by careful examination. Pin identifications for semiconductor devices are posted near the work areas. Connections to other components should be evident. Note that the larger-value capacitors are polarized, as indicated (usually) by a negative sign near one lead. Be sure to observe polarity in your circuit to avoid malfunction. B. Transistor circuits Two transistor types are provided: 2N3904 (NPN) and 2N3906 (PNP). The manufacturer specifies 100 < hFE <300, maximum collector current of 200 mA, and maximum collector - emitter voltage 40 V. The ±12 V supply will be within ratings for the collector, but you will destroy the transistor if you exceed IC = 200 mA or apply supply voltage directly to the base, even briefly. docsity.com 2 1. Common emitter amplifier Set up the NPN common emitter circuit of Fig. 1(a) and drive it with sine-wave inputs of various amplitudes. Using the scope, compare the input and output wave forms. You should be able to qualitatively explain the severe distortion you will see, and the additional clipping at large amplitudes, in terms of the general characteristics of transistors. Also try driving the input with a fairly large amplitude square wave to demonstrate that the circuit can be a logic inverter. Now construct the biased common emitter circuit of Fig. 1(b). Again, drive the input with sine waves and qualitatively explain the output wave form, including the DC component. Be sure to determine the phase and amplitude of the output signal relative to the input so you can calculate the AC voltage gain and demonstrate that the amplifier inverts. What happens when the input amplitude becomes large? 2. Push-pull follower Construct the push-pull output stage shown in Fig. 2(a), driving the input with a sine wave. Sketch and compare the input and output wave forms, and explain the distortion near the zero crossings. Find the gain for large amplitude signals, and determine the maximum amplitude before noticeable clipping. Can you demonstrate a larger output amplitude from this circuit than from the biased common emitter? Is the DC component essentially absent, as claimed in the notes? Next add the bias network shown in Fig. 2(b). Biasing should remove or minimize the cross-over distortion. Does it? Sketch the wave forms, noting any remaining irregularities. What is the maximum undistorted amplitude from this arrangement? in V Vout 100K 10K +12V in V out V +12V 100K 10K 10K 1K 2N3904 0.047µF 2N3904 (b)(a) Fig. 1 Practical common emitter amplifier circuits, with and without biasing. docsity.com 5 Construct the circuit as shown, with R1 a 1 K! variable resistor and R2 = 1 K!, and observe the effect on the output of changing the variable resistor setting. You should be able to find resistor settings for which the output voltage is essentially zero, a stable-amplitude sine wave, or a badly clipped waveform. Determine the variable resistance value needed to produce a stable sine wave, and calculate the expected gain. Is it approximately 3? Does the circuit oscillate at the expected frequency? This circuit is not very useful because the amplitude of the oscillations is quite sensitive to the amplifier gain, 1 + R2/R1, which may drift with time or temperature. It can be stabilized by replacing R1 with a small incandescent light bulb. As the amplifier output increases more current is drawn through the lamp, causing the resistance of the metal filament to increase. This negative feedback decreases the amplifier gain and the output is reduced until a steady state is reached. Demonstrate this effect by using a light bulb for R1 and the variable resistor for R2. Adjust R2 to get stable oscillations, and then note the effect of R2 values somewhat larger or smaller than the stable setting. It is also interesting to watch the oscillations build up after the power is turned on when R2 is at the stable setting. You should be able to see the effects of the thermal response time of the lamp quite clearly. (Historical note: The use of a light bulb to stabilize an oscillator was invented by William R. Hewlett and patented by him in 1942. A variable-frequency audio oscillator based on this circuit was the first product from the Hewlett-Packard company.) 4. Power booster Construct a unity-gain inverter, using 10K! input and feedback resistors. Find the maximum output voltage with an open circuit, and again when the output is driving a 100! resistor. What is the output current capability of the op amp? Is this consistent with the - + V i R load out V +12V -12V 2N3904 2N3906 10K 10K Fig. 5 An op-amp inverter with power output stage. docsity.com 6 specification for the 741? Now construct the circuit shown in Fig. 5, and again check the output voltage for Rload = ! and for Rload = 100". Do you see a substantial increase in output current capacity, as predicted? This circuit can also be used to demonstrate the use of feedback to suppress distortion. Sketch the output waveform of the circuit as built. Then reconnect the feedback resistor to the normal inverter configuration, from the op amp output to the inverting input. Sketch the new output waveform, and comment on the differences you see. For a more dramatic demonstration, connect a small loudspeaker to the output of the unity-gain inverter. What is the maximum output voltage before the signal distorts? Now drive the speaker with the power booster circuit, and again check the maximum undistorted amplitude. You should also hear a large increase in acoustic output. Note: Do these tests very quickly, to minimize overheating of the transistors. Real amplifiers need heat sinks to keep the transistors from melting. docsity.com