1. An engineer measures the (step response) rise time of an amplifier as. Estimate the 3-dB bandwidth of the amplifier. (2 points)

Transcription

1 Exam 1 Name: Score /60 Question 1 Short Takes 1 point each unless noted otherwise. 1. An engineer measures the (step response) rise time of an amplifier as. Estimate the 3-dB bandwidth of the amplifier. (2 points) 2. True or false: one can use an ordinary glass Si diode as a photodetector. 3. The abbreviation/term ESR is often encountered in data sheets of capacitors. What does ESR stand for? 4. The output voltage of a three-terminal voltage regulator is 5 5 ma load, and A load. What is the regulator s load regulation? (2 points) ( ) ( ) ( ) 5. True or false: the diffusion capacitance of a pn junction is negligible when the junction is reverse-biased. 6. True or false: a diode, forward biased at I D = 1 ma, has a small-signal or incremental resistance of about. (2 points) Answer: 1

2 7. True or false: the turn-on voltages of Schottky diodes are less than that of Si diodes. However, their reverse leakage/saturation currents are also higher. 8. True or false: The turn-on voltage of red LEDs is larger than the turn-on voltage of blue LEDs. 9. Which of the following depicts the correct current direction? Circle one. 10. In the context of diodes, the term PIV means: 11. True or false: the diffusion capacitance of a pn junction is generally much larger than the junction capacitance. 12. True or false: a silicon diode is biased so that at 25 o C. V D changes with 2 mv/ o C, so that at 125 o C, will be = 0.9 V 13. True or false: in the circuit below, even though the diode equation is nonlinear, the photocurrent is essentially linear with photon flux density. 2

3 Question 2 What is the voltage in the circuit below? Assume an ideal op-amp. (6 points) Call the voltage at the output of the op-amp, and note that the voltage at node is 5 V. Then two KCL equations for node A and the output node are: Solving yields and. for the differential amplifier circuit below. Assume ideal op- Question 3 Find the expression for amps. (6 points) Recognizing that op-amp is wired as an inverting amplifier with voltage gain,it immediately follows that the voltage at the output terminal of is. Note that is wired as a noninverting summer, so that its output voltage is ( ) ( ) 3

4 Question 4 For each of the circuits below, determine the output voltage. (2 points each) This is a current-to-voltage converter with ( )( ) This is a follower where. Thus This is a noninverting summing amplifier where Thus ( ) 4

5 Question 5 The circuit below has input, output, and capacitors voltages that are zero at. It is driven by the input signal shown below. The resistance and capacitance in the circuit are, and. Sketch and label the resulting output waveform. Assume an ideal op-amp. (8 points) This circuit is an op-amp integrator. The input, output, and capacitor voltages are initially 0, so that ( ) ( )( ) ( ) ( ) For, we can write ( ), so the output is That is, the voltage decreases linearly with time. At, the value is. For, we can write ( ), so the output is. That is, the voltage increases linearly with time. At, the value is. This pattern repeats. The output voltage is shown below. 5

6 Problem 6 An engineer designs a power supply that consists of a transformer, a full-wave, 4-diode bridge rectifier and a smoothing capacitor. The nominal load current is 1.2 A. By what percentage will the ripple voltage increase/decrease if the supply is used with an actual load of A? Assume the transformer and rectifier diodes are capable of handling the increase in load current. (4 points) The ripple voltage for a full-wave rectifier is: ( ) The term represents the peak current though the load resistance. Increasing the load from A to A (a 25 % increase) will increase the ripple voltage by the same percentage, or 25 %. Problem 7 Draw the schematic of a power supply that consists of a step-down transformer, full-wave rectifier bridge, smoothing capacitor, and 7805 linear regulator. (5 points) 6

7 Problem 8 An engineer designs a power supply that consists of a transformer, a full-wave, 4-diode bridge rectifier and a smoothing capacitor. The transformer secondary voltage is 7.4 VAC at, and the load resistance is. The bridge rectifier diodes are silicon. Determine the value of the smoothing capacitor if the output ripple voltage is to be less than 500 mv. (5 points) ( )( ) ( )( )( ) Problem 9 An engineer measures the bandwidth of the simple low-pass filter below by driving it with a sinusoidal signal and measuring the attenuation at various frequencies. She uses an oscilloscope with an input impedance of along with a probe. What is the true bandwidth of the circuit? What will the engineer measure? (6 points) ( )( ) ( ) ( ) ( )( ) 7

8 Question 10 Consider the current-to-voltage converter circuit shown. The current source has a finite output resistance, and the op-amp is ideal except for a finite open-loop gain. Show that the input resistance indicated in the figure is given by ( ). (6 points) Turn off independent sources add a test voltage and determine the resulting current. Then. To turn off a current source, we remove it from the circuit. The circuit below indicates the setup to determine. The output voltage is yields, no current flows into the inverting input and applying Ohm s law ( ) Since, it follows that ( ). 8

9 Question 11 An amplifier is designed to provide a 12 V peak-to-peak swing across a load. Assume sinusoidal signals. Assuming the amplifier has output resistance how much power will the load dissipate? (4 points) A 12 V peak-to-peak sinusoidal signal has an amplitude of 6 V and with, the signal amplitude across the load is, using voltage division, ( ( )). The rms value is, and the power is ( ) Problem 12 In the circuit shown, is a dc current and is a sinusoidal voltage signal. Capacitors and are very large; their function is to couple the signal to and from the diode but block dc current from flowing into the signal source or the load (not shown). Ignore junction- and diffusion capacitances. Use the diode small-signal model to show that the signal component of the output is (5 points) The small-signal equivalent circuit is shown right. The small-signal incremental resistance of the diode is given by. Applying voltage division gives 9

10 Question 13 We would like to measure the voltage in the circuit below with a voltmeter. What is the value of, and what is the common-mode voltage associated with? What CMMR (in db) is required of the voltmeter if we are to measure to within 0.01%? (6 points) The current through the resistance is ( ). The voltage across the resistor is therefore 3.75 V. Further, ( ), and The common-mode voltage is then ( ) The error must be less than or or 3.75 V, which is mv. Thus, the multimeter must suppress the 7.5 V common-mode voltage to less than mv. In other words, the CMMR must be at least This is equivalent to 86 db. Question 14 Determine a value for in the circuit below such that ma. Assume. The LED s I-V plot is shown below. (4 points) 10