University of Utah Electrical & Computer Engineering Department ECE 2100 Experiment No. 8 Common-Collector Amplifier

Transcription

1 Uniersity of Utah Electrical & Computer Engeerg Department ECE 2100 Experiment No. 8 Common-Collector Amplifier Mimum required pots 48 Grade base, 100% 67 pots ecommend parts 67 pots (100%, ALL parts are recommended this time) A. Stolp, 3/7/00 re, 3/4/03 Objecties 1.) Obsere the problem of a high output impedance source connected to a low impedance load. 2.) Build a common-collector amplifier to sert between source and load and measure its characteristics. 3.) Look at similarities and differences between the common-collector and commonemitter amplifiers. Check out from stockroom: Wire kit & Two 10x scope probes Possibly one speaker Parts: 10, 100, 10 k, & two 20 k6 resistors one 1-10 µf & one µf capacitor 2N3904 transistor microphone and speaker (you may hae to check the speaker out) Experiment 1, Common-Collector ( pts, ecommended) Last week you measured the of your 2N3904 transistor with a circuit that was essentially a common-collector amplifier (the collector is connected to a DC oltage source, which looks like a signal ground, or common). At that time you simply measured the DC (bias) oltages and currents. We used this circuit because it s ery easy to bias and ery stable. Today you ll use this circuit with AC signals and see that although this circuit only has a signal oltage ga of about 1 (which explas its stability), it is still a ery useful circuit. First let s take a look at the problem that this circuit will sole. Impedance mismatch of a microphone and a speaker: Hook up your microphone as shown. The two 20 k6 resistors simply proide the microphone with some DC oltage that it needs to operate. Most electret microphones need some external DC oltage. (These mics are also called condenser mics because the actual microphone is a capacitor and the old-fashioned name for a capacitor is a condenser.) Hook the scope up to o and confirm that the microphone is workg. p1 ECE 2100 CC Amp Lab

2 Hook up your speaker to o. (The other speaker wire goes to ground, of course. In future, I won t mention the ground connection specifically. I ll assume that you can figure that part out on your own.) Does the scope still show a signal at o? If yes, is it anywhere near as large as it was before the speaker was connected? Can you hear anythg at the speaker? (Possibly the best way to test this is to hold the speaker right to your ear and blow on the microphone, but frankly, I don t thk a hurricane would make enough noise that you d hear it though this setup.) The mic has a large output resistance (called output impedance more general terms) and the speaker is a low impedance load (has a low put impedance). Een if the output oltage from the mic were sufficient to operate the speaker, it can t supply enough current. Comment your lab notebook concerng how the mismatched impedances effects operation of this circuit. Artificial Impedance mismatch: Because the output impedance of the microphone is difficult to measure, we re gog to switch to an artificial signal source and artificially make it s output impedance about as large as that of the mic. Set the bench function generator output to about 4 V pp (2 V pp on the HP) at 1 khz. Add a 10 k6 resistor series as shown. The function generator and the 10 k6 resistor together model a source with a 10 k6 output resistance. (Actually 10,050 6, but we ll ignore the 50 6 output resistance of the generator itself.) I ll call this V S & S from now on and you ll use it throughout this section of the lab. Measure the output oltage of V S & S with no load resistor. Now add the 10 6 load resistor ( L ) as shown and measure the output oltage aga. See how this mimics the microphone-speaker problem? Why is the output of V S & S so much lower than before? ecord the measurements so you can see the improement later. (If the signal across L is too small to measure accurately, state that your notebook lieu of a measurement.) Try the speaker place of the 10 6 resistor. Can you hear anythg from the speaker? In the next section you ll build an amplifier circuit to sert between the source, V S & S and the load, L. Common-Collector: Build the common-collector transistor circuit shown. Leae off the source ( V S & S ) and the load resistor ( L ) for now. Notice that stead of usg a separate oltage supply for the base bias (like the circuit from last lab), this circuit uses a oltage diider between Vcc (15V) and ground. p2 ECE 2100 CC Amp Lab

3 Bias: Calculate the Théen equialent of the base bias circuit (Vcc, B1, and B2 ) to fd V BB and BB and show that this is ery close to a separate 7.5 V supply with a series 10 k6 resistor. Measure and record the DC bias oltages of the common-collector circuit (measure V B and V E with a DC oltmeter). Make a reasonable assumption for (look back at the last lab) and calculate these oltages. (If we re not yet that far class, look at the circuit I e drawn with a diode and a resistor that s times bigger than E. Use that circuit model to calculate V B and V E.) Compare your measurements and calculations. Signals: Connect V S & S to the put of this circuit, as shown before. Set both channels of the scope to 1 V/di, DC couplg and adjust both ground leels to the bottom of the screen. This way you ll be able to obsere both the AC an DC characteristics of both B and E at the same time. Don t press the auto button or you ll mess thgs up. Connect the scope probes to the base and to the emitter. Fd V AVE (DC) for both signals. These should be the same as the DC bias oltages that you measured earlier. Notice that the signals swg aboe and below these bias leels. This is classic AC + DC superposition and you ll see this many more times this class. Make a sketch of these oltages your notebook, showg AC signals ridg on the DC aerage oltages. Note: Leae the scope set this way so that you can see both signals on the same scale along with the DC biases. 0.7 V difference: What is the difference between the two oltages? Is the difference constant? Notice that E simple tracks or follows B, just 0.7 V under. The AC signal oltages are the same for both (signal oltage ga 1), just the DC is different. The common-collector circuit is also known as an emitter follower because the emitter oltage follows the base oltage. Loaded: Add the load resistor ( L 10 6). Measure the signal oltage across the load. It s still not ery big, but that s because the load is so small. (emember, we were tryg to mimic the mismatch between a microphone and a speaker, which is an outrageously bad mismatch.) Contrast the load oltage you measure now to what you measured when this load was connected directly to V S & S. Try the speaker place of the 10 6 resistor. Can you hear somethg from the speaker now? You see that the current ga of the emitter follower circuit does help, een though it has no oltage ga. It begs to fix the mismatch between the source and the load resistances (or source and the load impedances). Input resistance: From the standpot of the signal source ( V S & S ) the rest of this circuit can be modeled as a sgle resistor p3 ECE 2100 CC Amp Lab

4 hooked to ground ( ). emember the general models of amplifiers from chapter 1 of your textbook. Deise a way to measure. (No, you can t just use an ohmmeter to fd this.) You may hae to add another part to your circuit. emember that you cannot moe the ground of your scope to some pot other than the ground from the signal generator. (They are both connected to bench-ground and if you hook then to different pots the circuit, those two pots will be shorted together.) Make whateer measurements you need order to fd the signal put resistance of this amplifier. Consider this a little design problem. If you can t figure out a way to fd, then look the appendix for the answer. Howeer, if you do hae to look the appendix then you ought to ask yourself why you can t engeer a solution to this simple problem after nearly two years electrical engeerg. emoe L from the common-collector circuit and fd the circuit s put resistance ( ) aga. Assume a reasonable and calculate the theoretical put resistance of the commoncollector circuit each case (with and without L ). Compare these to what you found from measurements, aboe. (If we re not yet that far class you may leae these calculations and comparisons until later.) Output resistance: Deise a method to fd the signal output resistance of this amplifier. Make whateer measurements and changes as necessary so you can calculate o from your measurements. Aga, if you can t figure this out, look the appendix, but do ask yourself why you can t figure this out. Assumg a reasonable, calculate the expected output resistance and compare. Back to the microphone and speaker: Change your circuit to the one shown at right. Notice that put couplg capacitor is gone. That s because I m dog somethg a little tricky-slimy this circuit. I m borrowg the DC oltage necessary to operate the microphone from the DC bias circuit for the transistor. Can you measure an audio signal at o now? Can you hear anythg at the speaker if you hold the speaker right to your ear and blow on the microphone? Admittedly this isn t a P.A. system yet, but is a phenomenal improement for just one lousy little transistor. Ma pots of the common-collector: (hts for a conclusion) 1) Emitter oltage tracks or follows the base oltage, just 0.7 V DC less. Signal oltage ga 1 not countg losses due to put and output resistances. 2) Emitter follower >> o (which also means that it has current ga). p4 ECE 2100 CC Amp Lab

5 3) is [( E L ) ] B1 B2 which is usually much greater than o and hopefully L too. 4) o is E [( S B1 B2 )/] which is usually much less than and hopefully less than S. Next lab: Leae this circuit tact for the next lab. Next time you ll look at the CE amplifier. Appendix To Measure Input resistance: Add a resistor ( test ) between the source and the circuit put. Measure to ac signal oltage on both sides of test usg the scope (peak-to-peak is alright). I test test I Alternatiely, if you know the alue of s, then you don t hae to add an test. Measure the oltage V with the circuit connected and then aga without the circuit connected (with and then without ). From those measurements and the alue of S, you can calculate I & then. Be sure to measure AC signal oltages (peak-to-peak is alright). To Measure Output resistance: Measure the signal oltage output without L. This is the open-circuit output ( nl ). emember that the open circuit oltage relates directly to the Théen oltage of a Théen equialent circuit. econnect L and measure the loaded output ( L ). Use these two measurements to calculate the output resistance ( o ) of this amplifier. Be sure to measure AC signal oltages (peak-to-peak is alright). O nl L L L p5 ECE 2100 CC Amp Lab