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CALVIN ENGR 311 - BJT Amplifiers ­ Complete Model

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ENGR 311BJT Amplifiers - Complete ModelThe common emitter amplifier is one of the most widely used amplifier configurations due to itshigh gain. Other configurations are the common collector and common base amplifiers whichrespectively have the collector and base of the transistor grounded, or common to the input andoutput AC signals. This exercise will look only at the common emitter amplifier.Amplifier performance is measured by a number of parameters: gain, bandwidth, distortion,input impedance, output impedance, noise figure/signal to noise ratio, powerdissipation/efficiency, and phase shift. Many of these parameters are difficult to measure withoutspecialized equipment, so we will concentrate on only a few of these in this exercise.Experiment1. Study the data sheet for the 2N3904.2. Calculate the DC operating point of the amplifier shown in Figure 1 (use MathCAD).3. Calculate the AC small signal gain (vo/vs instead of transducer gain) of the amplifier (setupequations and use MathCAD). Make the simplifying assumption that all capacitors are shortcircuits at the frequency of interest. Keep in mind that, although these are the usual assumptionsmade for hand calculations, the value calculated is commonly referred to as the mid-band gain.4. Use PSpice to obtain a plot of the gain magnitude of the circuit as a function of frequency.Repeat the simulation with the emitter capacitor set at 1μF and again with the emitter capacitorremoved.5. Set vin at 0.5v and take gain magnitude measurements at three frequencies: a) somewhere inthe middle of the band, b) at the lower 3 dB point (-3dB with respect to your first measurement),and c) at the upper 3dB point. You will have to use a function generator, a frequency counterand an oscilloscope. Be sure to use scope probes and verify the compensation of the probesbefore taking measurements. The cutoff frequencies should occur at approximately 1KHz and1MHz. 6. Remove the emitter capacitor and qualitatively observe the effects on midband gain andbandwidth. Can you explain the observed changes?7. Replace the emitter capacitor with a 1μF capacitor. Measure the new lower cutoff frequency.Can you explain this result?8. Use a frequency somewhere in the midband. Measure the AC voltage at the base of thetransistor and the voltage drop across the 10kΩ resistor connected to the input (the AC current).From this data, calculate the small signal input resistance of the amplifier for inclusion in yourreport.9. Remove the 1kΩ load resistor and measure the output voltage. Now attach a load resistor ofabout 300Ω (any low value of resistance is fine as long as its exact value is known). Measure thenew output voltage. From the change in voltage and the known value of RL, calculate the smallsignal output resistance.10. Report all your finding and observations. Report due in two


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