Exercise on Analog Circuits

Transcription

1 PHYS 331: Junior Physics Laboratory I 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 our labs and research. Your write-up should expla clearly what you did each part, followg the general organization of this guide. Include a schematic of each circuit as you built it, usg the schematics the notes as a guide to standard practice. Provide complete answers to any questions posed the text. A neatly hand-written report with clear free-hand figures is entirely acceptable. Before startg the lab work you should review the topical notes on Analog Circuits, available on the course web site. Sections , 2.15, , and 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, tegrated circuits, resistors, capacitors, etc. Pre-cut jumper wires are used to make connections by pluggg to the terconnected sockets. An auxiliary circuit board plugs to the ma board to provide connections for positive and negative supply voltages and for external signals. The connections to the board can be understood by careful examation. P 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 dicated (usually) by a negative sign near one lead. Be sure to observe polarity your circuit to avoid malfunction. B. Transistor circuits Two transistor types are provided: (NPN) and 2N3906 (PNP). The manufacturer specifies 100 < h FE <300, maximum collector current of 200 ma, and maximum collector - emitter voltage 40. The ±12 supply will be with ratgs for the collector, but you will destroy the transistor if you exceed I C = 200 ma or apply supply voltage directly to the base, even briefly. 1

2 K 0.047µF 100K (a) Fig. 1 Practical common emitter amplifier circuits, with and with biasg. (b) 1. Common emitter amplifier Set up the NPN common emitter circuit of Fig. 1(a) and drive it with se-wave puts of various amplitudes. Usg the scope, compare the put and put wave forms. You should be able to qualitatively expla the severe distortion you will see, and the additional clippg at large amplitudes, terms of the general characteristics of transistors. Also try drivg the put with a fairly large amplitude square wave to demonstrate that the circuit can be a logic verter. Now construct the biased common emitter circuit of Fig. 1(b). Aga, drive the put with se waves and qualitatively expla the put wave form, cludg the DC component. Be sure to determe the phase and amplitude of the put signal relative to the put so you can calculate the AC voltage ga and demonstrate that the amplifier verts. What happens when the put amplitude becomes large? 2. Push-pull follower Construct the push-pull put stage shown Fig. 2(a), drivg the put with a se wave. Sketch and compare the put and put wave forms, and expla the distortion near the zero crossgs. Fd the ga for large amplitude signals, and determe the maximum amplitude before noticeable clippg. Can you demonstrate a larger put amplitude from this circuit than from the biased common emitter? Is the DC component essentially absent, as claimed the notes? Next add the bias network shown Fig. 2(b). Biasg should remove or mimize the cross-over distortion. Does it? Sketch the wave forms, notg any remag irregularities. What is the maximum undistorted amplitude from this arrangement? 2

3 N3906 2N (a) Fig. 2 Push-pull amplifier circuits, with and with biasg. (b) C. Basic op-amp circuits The type 741 op-amp is supplied an eight p tegrated-circuit package. It requires both positive and negative supplies the range ±10 to ±36, but no explicit ground connection. There is provision for exactly zerog the apparent put voltage, but we will not use that feature. The put can reach with a volt or so of either supply voltage and produce up to 20 ma of put current. 1. Inverters and followers Followg the schematic the notes, design and build an verter with a ga of 10. The resistors should be chosen so that most of the current capacity of the op amp is available for drivg the external load. erify that your circuit is an verter with the specified ga at low frequencies, and fd the frequency at which the ga is reduced by a factor of 1/ 2 (This is called the 3 db pot because it is the frequency at which the power put is half the maximum.). Keep the put below a volt or two, to avoid large-amplitude effects. It is also terestg to explore some of the limitations of the verter. How large an put amplitude will it produce before clippg or severely distortg the signal? Try drivg the put with a small-amplitude square wave, ab 1-2 at the put, comparg put and put signals. What is the maximum rate of change of voltage, volts/µs, that the amplifier can produce? This is called the slew rate, and is important some applications. Next, set up a unity-ga non-vertg follower and aga check the frequency response and slew rate. Do you notice any difference from the verter? These characteristics will differ 3

4 0.047µF i - + o for various types of op amp various configurations, and must be considered when designg circuits for specific applications. Fig. 3 An tegrator/filter circuit. 2. Integrator filter Set up an averagg filter usg the component values Fig. 3. For an put signal of a volt or two, plot the put amplitude vs frequency on logarithmic scales. Is the 3dB pot where you would expect from the component values? Don't forget that! = 2"f. It is also terestg to look at the phase shift as a function of frequency. You can do this easily by displayg the put signal on channel 1 and the put on channel 2 of the scope. At low frequencies the put is verted, as expected, but you will notice an additional shift at higher frequency, which you should describe. Dependg on the tended use, the phase shift or equivalent time delay may be an important filter characteristic. 3. Oscillator In the Wien-bridge oscillator circuit shown Fig. 4 the RC network acts a frequencydependent voltage divider, feedg 1/3 of the put voltage to the non-vertg put when f = 1/(2"RC). Sce the RC network has no phase shift at this frequency, this is positive feedback and the circuit will oscillate if the ga, 1 + R 2 /R 1, is greater than 3. R NC 2 R o.01 μf.01 μf Fig. 4 Wien-bridge oscillator circuit. Note that there is no connection to the top of R 1, and that no signal source is needed to start the oscillation. Avoid connectg the resistor slider to either the ±12 supply or ground, as damage will result for some settgs. 4

5 Construct the circuit as shown, with R 1 a 1 K! variable resistor and R 2 = 1 K!, and observe the effect on the put of changg the variable resistor settg. You should be able to fd resistor settgs for which the put voltage is essentially zero, a stable-amplitude se wave, or a badly clipped waveform. Determe the variable resistance value needed to produce a stable se wave, and calculate the expected ga. 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 ga, 1 + R 2 /R 1, which may drift with time or temperature. It can be stabilized by replacg R 1 with a small candescent light bulb. As the amplifier put creases more current is drawn through the lamp, causg the resistance of the metal filament to crease. This negative feedback decreases the amplifier ga and the put is reduced until a steady state is reached. Demonstrate this effect by usg a light bulb for R 1 and the variable resistor for R 2. Adjust R 2 to get stable oscillations, and then note the effect of R 2 values somewhat larger or smaller than the stable settg. It is also terestg to watch the oscillations build up after the power is turned on when R 2 is at the stable settg. 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 vented by William R. Hewlett and patented by him A variable-frequency audio oscillator based on this circuit was the first product from the Hewlett-Packard company.) 4. Power booster Construct a unity-ga verter, usg! put and feedback resistors. Fd the maximum put voltage with an open circuit, and aga when the put is drivg a 100! resistor. What is the put current capability of the op amp? Is this consistent with the +12 i - + 2N3906 R load 5-12 Fig. 5 An op-amp verter with power put stage.

6 specification for the 741? Now construct the circuit shown Fig. 5, and aga check the put voltage for R load =! and for R load = 100". Do you see a substantial crease put current capacity, as predicted? This circuit can also be used to demonstrate the use of feedback to suppress distortion. Sketch the put waveform of the circuit as built. Then reconnect the feedback resistor to the normal verter configuration, from the op amp put to the vertg put. Sketch the new put waveform, and comment on the differences you see. For a more dramatic demonstration, connect a small loudspeaker to the put of the unity-ga verter. What is the maximum put voltage before the signal distorts? Now drive the speaker with the power booster circuit, and aga check the maximum undistorted amplitude. You should also hear a large crease acoustic put. Note: Do these tests very quickly, to mimize overheatg of the transistors. Real amplifiers need heat sks to keep the transistors from meltg. 6