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Experiment 10 β Bipolar Junction Transistor: Common-Emitter Amplifier Physics 242 - Electronics Introduction Transistors are particularly useful in applications involving the amplification of signal power. In this lab, you will investigate the common emitter amplifier, one of the most commonly used transistor circuits. Procedure and Questions 1. Measure the component values first, then build the common emitter amplifier shown above. With the input voltage disconnected, measure the DC voltages VCC, VC, VE, and VB characterizing the quiescent operating point. VC, VE, and VB refer to the voltages at the collector, emitter, and base terminals of the transistor, measured with respect to ground. Q1: Assuming ο’ο = 150 and π£π΅πΈ = 0.6 V, calculate the predicted values of VCE and πΌπΆ . Show your calculations and use measured component values. Find the percent difference between the measured and predicted values of collector current IC and collector-emitter voltage ππΆπΈ . Note that you can find your measured VCE from your experimentally measured values as ππΆπΈ = ππΆ β ππΈ , and you can find your measured πΌπΆ from your π βπ measured values as πΌπΆ = πΆπΆπ πΆ. πΆ Q2: Solve for the predicted values IC and ππΆπΈ , assuming ο’ = 50. How much (by what percent) are these predicted values moved from the predicted values you found in Q1 where you assumed that ο’ο = 150. 2. Use the sine-wave generator to provide a 1 kHz sinusoidal input signal, and monitor output waveform on the βscope. Reduce the amplitude of the input signal so that the output waveform is not clipped. Measure the amplitudes of the input and output waveforms (from which the voltage gain can be found). Then increase the amplitude of the input signal until the wave begins to clip, and sketch the waveform, indicating maximum and minimum voltage levels with respect to ground. Q3: Calculate the expected value of the voltage gain π£π π£π for your circuit (using measured component values). Find the percent difference between measured and predicted gains. Q4: Show your sketch. Indicate where clipping occurs. Explain why the clipping occurs at the voltage you observed and show a supporting calculation. Don't expect exact agreement, since it is difficult to detect the exact onset of clipping. 3. The combination of the capacitor and the parallel combination of π 1 , π 2 , and the effective resistance looking into the emitter π πΈ,πππ form a high-pass filter for input signals. Vary the frequency of the input signal to find the cut-off frequency ππ , the 1 frequency at which the gain has dropped by a factor of or 0.707. β2 Q5: Show your calculation of the predicted cut-off frequency, and calculate the percent difference between the predicted and measured values. 4. In this part, you will measure the input impedance π π of your common-emitter amplifier. Set the signal generator to provide a 10 kHz sine wave that is not clipped. First, measure the peak-to-peak voltage of the input signal, with the signal generator connected directly to the 'scope, and disconnected from the circuit. Then, insert a resistor π π₯ as shown in the figure above and measure the voltage πππ with the 'scope. Make this measurement for the following resistance values of π π₯ : 2 kο, 5 kο, 10 kο, and 20 kο. Q6: a. Make a plot from which you can find the input resistance of your common-emitter amplifier circuit. In finding the input resistance, the circuit is modeled as a resistor π π as shown in the picture above. To decide what to plot, write the voltage divider equation describing the simplified circuit shown in the picture. Find the input resistance using the slope or y-intercept values from your plot. b. Calculate the predicted input resistance of the circuit. Use measured component values. You may assume the capacitor is effectively a short circuit at 10 kHz. Find the percent difference between the predicted and measured input resistance values. 5. In this part, you will measure the output impedance of the amplifier. Set the signal generator to provide a 10 kHz sine wave that is not clipped. Then connect a 0.5 οF capacitor πΆπ and a load resistor π πΏ as shown in the picture above. The function of the coupling capacitor is to pass the AC part of the output signal while blocking the DC level. Measure the output voltage ππΏ across the load resistor using the 'scope. Make this measurement for the following resistance values for π πΏ : 2 kο, 5 kο, 10 kο, 20 kο, and open circuit (omit π πΏ and connect the 'scope between the circuit output and ground, where π πΏ would be). Q7: a. Make a plot from which you can find the output resistance of your commonemitter amplifier circuit. In finding the output resistance, you should model the circuit as a Thevenin-equivalent source ππ in series with a Thevenin-equivalent resistor π π as shown in the picture above. Find the output resistance π π from your plot. b. Calculate the predicted output resistance of the circuit. The resistance looking into the collector is very large (it is a reverse-biased diode); you may assume it is infinite. Use measured component values. You may assume the capacitor is effectively a short circuit at 10 kHz. Find the percent difference between the predicted and measured input resistance values.ο