INDIANA UNIVERSITY, DEPT. OF PHYSICS, P400/540 LABORATORY FALL 2008 Laboratory #5: More Transistor Amplifier Circuits Goal: Use and measure the behavior of transistor circuits used to implement different amplifier configurations: common emitter, DC feedback, push-pull, and a differential amplifier. 1. Ebers-Moll Equation Wire up the circuit below that you used in a previous lab to measure β. Again, use the substitution box for R, to generate collector currents going from a few microamps to a few milliamps. Plot the logarithmic increase of V BE with I C, and confirm the "60 mv/decade" law (plot the log of V BE versus I C on a linear scale [check out the Ebers-Moll equation to determine why this should be a straight line...] using a computer or semi-log paper). A 0! 10 ma (or less) 1k +5 V R 4.7k DVM, 0! 1 V range V
2. Biasing with DC Feedback We generally use an emitter resistor to stabilize common-emitter amplifiers against temperature effects. Another way to do it is with DC feedback as shown below. This application of feedback will be used over and over again when we begin work with operational amplifiers. Wire up the circuit below. (= V ) CC Input R 1 22 µf! + R 2 68k 6.8k Note that this is dc feedback: that is, it stabilizes the quiescent point, but does not affect circuit gain. This arrangement provides some bias stability. The nominal collector quiescent point is 11 V BE or roughly 7 volts. Why? If the quiescent collector voltage were more than that, for instance, the base divider would drive the transistor into heavy conduction, restoring the proper operating point. Similarly, the proper operating point would be restored if the quiescent point were to drop. Check to see fi the quiescent collector voltage is approximately correct. Since V BE depends on temperature, you should be able to shift the collector voltage a small amount by warming the transistor between your fingers: which way should does it move?
3. Push-Pull Amplifier (NPN) Input 2N3906 (PNP) 6.8k Using a NPN and a PNP transistor, build the push-pull amplifier stage shown above. Use it to explore "cross-over distortion". Drive the circuit with sine waves of at least a few volts amplitude with a frequency of about 1 khz. Examine the output closely and sketch it. If things behave strangely, you may have a "parasitic oscillation". It can be fixed by placing a 470 Ω resistor in series with the common base lead. If necessary, also add a 100 pf capacitor from circuit output to ground. Vary the amplitude up and down and describe what is observed. 4. Differential Amplifier Predict the differential and common-mode gains for this amplifier (don't neglect r e.) and then build the circuit. 1.0 µf 1.0 µf! + Input 1 Input 2 11k (two 22k resistors in parallel) 100 100 11k
As input to the circuit, we will use two frequency generators, where one generator will drive the other. The external frequency generators supplied will be "floated" and disconnected from absolute or "world" ground by plugging it in the line voltage using a "cheater" plug that bypasses the grounding prong of the plug. When you connect the two function generators to your amplifier, take care to avoid "defeating" the "floating" of the external function generator. Remember that BNC cables and connectors can make implicit connections to absolute ground. Avoid tying the external generator to absolute ground through such inadvertent use of a cable and connector. Using the BNC to banana converter connector and then alligator clips will be safest. Now let the breadboard's function generator (that cannot be "floated") drive the external function generator's local ground. Use the output of the external function generator to feed your differential amplifier as shown on page 135 of the Horowitz & Hill lab manual. 1. Measure common-mode gain: Shut off the differential signal (external function generator) while driving the amplitude. Does the common-mode gain match your prediction? 2. Measure differential gain: Turn on the external function generator while cutting the common mode amplitude to a minimum (there is no Off switch on the breadboard function generator). Apply a small differential signal. Does the differential gain match your predictions? 3. Now turn on both generators and compare the amplifier's output with the composite input. To help distinguish the two signals, it helps to use two frequencies far apart; this is not required for proper operation, but it makes things easier. This experiment should give you a sense of what "common-mode rejection ratio" means: the small amplification of the common signal, and relatively large amplification of the difference signal. This circuit still lets a large common-mode signal produce noticeable effects at the output.
5. Improving the Differential Amplifier Replace the "tail" resistor with a 2 ma current source: 12k 2 ma 2.7k 1k This change should make the common-mode amplification negligible. See how this improved circuit treats a signal that combines common-mode and differential signals. Estimate the differential and common-mode gain, and the common-mode rejection ratio (CMMR).