(b) 25% (b) increases

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1 Homework Assignment 07 Question 1 (2 points each unless noted otherwise) 1. In the circuit 10 V, 10, and 5K. What current flows through? Answer: By op-amp action the voltage across is and the current through and is 10 10K 1 ma. 2. Consider a first-order RC low-pass filter with 3-dB frequency 60Hz. By how much does it delay a 50 Hz sine wave? Express you answer in ms. Answer: The phase shift at 60 Hz is 45 and increasess at 45 / decade. 50 Hz is log decades higher than 60 Hz. (Thee negative sign implies 50 Hz is 0.08 decades before 60 Hz.) Thus, the phase shift is The period of a 50 Hz sine wave is 20 ms, so the delay is ms An alternate and more accurate calculation for the phasee is tan and delay of 2.2 ms. 3. Consider a linear power supply consisting of a transformer, a full-wave, 4-diode bridge rectifier, smoothing capacitor, and a load current 1.2 A. By what percentage will the ripple voltage increase if the load current increases to 1.5 A? a) 100 % Answer: 25% (b) 25% (c)) Stay the same (d) 50% 4. A current source supplies a nominal current 1mA. When connected to a 5K load, only 0.95 ma flows through the load. What is the internal resistance of the current source? Answer: The voltage across the load is V. A current 0.05 ma flows through the current source s internal resistance, which has value K 5. For a forward-biased diode, the barrier potential as temperature increases. (a) decreases (b) increases (c) stays the same Answer: Option (a). 1

2 6. What is the magnitude of the current phase angle for a 5.6 F capacitor and a 50-ΩΩ resistor in series with a 1.1 khz, 5 VAC source? (a) 72.9 (b) 62.7 (c) 27.3 (d) 17.1 Answer: The impedance of the RC circuit is Ω. the phase angle is tan Thus,, (c) is the answer. The magnitude of 7. Pick the word/ phrase that best completes the followingg sentence. To obtain a frequency response plot of a circuit in SPICE, one must perform analysis (a) an AC (b) Transfer function (b) Harmonic Distortion (c) Transient Answer: (a) 8. Which of the circuit is a voltage-to-currentt converter? What is the output in terms terms of the component values and the input? Answer: Circuit (b) By op-amp action. The current flowing through and the load is, which is independent from. 9. Consider a power supply with V 20 V, and V 19.8 V. The load regulation is (a) 0% (b) 0.1% (c) 1% (d) 10% Answer: Load regulation is %, so the answer is (c). 2

3 10. Consider the current mirror below, and neglect base currents. What is? Answer: 0.25 ma 83 A In the current mirrors below, neglect base currents and take 30 A, What is? (a) 30 A (b) 30 A 3 10 A (c) 30 A A Answer: (a) 12. Estimate assuming 100 and 1 ma. (a) 82K 22K 17K (b) 82K 22K 100Ω 100Ω (c) 82K 22K 1K 1K (d) 82K 22K 12.5K 7.3K (e) Need additional information Answer: Using BJT scaling the resistance looking into the transistors base is about assuming 100 and 1 ma this about 15K. This is in parallel with the 82K and 22K resistors, so (d) is the correct answer. and 13. Many BJT datasheets do not list explicitly, but list ann equivalent h-parameter instead. What is this parameter? Answer: 14. Typically, the C-E saturation voltage for a BJT, namelyy, is in the range of (circle one) (a) V (b) 0.7 V (c) 2 3 V (d) Need additional information Answer: (a) 3

4 15. In the circuit below 1 ma and all the capacitors aree large enough to be considered shorts. Estimate the midband gain. (a) 6.8 (b) 3.4 (c) (d) Answer: (d) 16. In the circuit below 1 ma and all the capacitors aree large enough to be considered shorts. Estimate the midband gain (3 points) (a) 12.1 (b) 6.1 (c) (d) Answer: Estimate the voltage gain of the amplifier below if 6.3 ma, 200, and. (a) 10 (b) 10 (c) 252 (d) 252 Answer: A R R 10 4

5 18. True or false: the of a transistor is a function of temperature, but essentially independent of collector current. Answer: False 19. A single-pole op-amp has an open-loop low-frequency gain of 10 and an open loop, 3-dB frequency of 4 Hz. If an inverting amplifier with closed-loop low-frequency gain of 50 uses this op-amp, determine the closed-loop bandwidth. Answer. The gain-bandwidth product is 4 10 Hz. The bandwidth of the closed-loop amplifier is then is 4 10 /50 8 khz. 20. Assume that your SPICE simulation software (such as Micro-Cap SPICE) do not have a photodiode part. Explain in 1 2 sentences how you can nevertheless simulate a photodiode. Answer One can model a photodiode with a current source. 21. Consider a first-order RC low-pass filter with 3-dB frequency 25Hz. What is the phase shift in degrees at 75 Hz? (3 points) Answer: The phase shift at 25 Hz is 45 and the phase plot has a slope 45 / decade. 75 Hz is log decades higher than 25 Hz. Thus, the phase shift is A more accurate calculation gives the phase shift as tan

6 Question 2 For the circuit shown, determine the quiescent values for the base, emitter, and collector currents for both transistors. (8 points) Assume 0. 7 V and 80. Solution The emitter of is at a potential 0.7 V lower thann its base so that 0.7 V, and the voltage across is 4.7 V. Consequently ma. 20K The emitter of is at a potential 0..7 lower than its base so that 1.4 V, and the voltage across is 3.6 V. Consequently 3.6 1K 3.6 ma, ma ma, 3.56 ma ma ma ma ma A, ma 6

7 Question 3 Consider the circuit shown. Determine the input resistance. Hint: it is known that the voltage gain is 3.3. (5 points) 10M 10K 10K 47K ma V Solution Determine the input resistance using the standard procedure: turn off independent sources, drive the circuit with a test source and determine the current that flows. Then,. Referring to the figure, we can simply replace with and then Using the hint, we know that 3.3 so we can write 3.3 so that M 7

8 Question 4 Assume that op-amp in the non-inverting buffer configurationn below has nfinite input resistance, zero output resistance, and an open-loop gain of 1,000. Determine the closed-loop gain. For the closed-loop gain, provide your answer to four decimal places. (6 points) Solution KVL around the loop shown below gives 0 Where is the voltage across. KVL equations becomes However, no current flows into the op-amp, so 0, and the 0 Now 1,000 1,000, which means 1,000, 1,000 0 Solving for yields 1, , so the KVL equation becomes 8

9 Question 5. An op-amp has a voltage gain of 100 db at dc and a unity-gain frequency of 5 MHz. (a) What is, the low frequency 3-dB cutoff frequency? (2 points) (b) Write an expression for the transfer function for the open loop gain of the op amp. (3 points) (c) The op-amp is used in a non-inverting configuration with a gain of 40 db. What is the bandwidth of the feedback amplifier? (3 points) (d) Write an expression for the transfer function for the feedback gain of the op amp. (3 points) (e) By how much does the amplifier delay a 10 khz sine wave? (4 points) Assume that the op-amp has a single-pole frequency response. Solution (a) 100 db of voltage gain is equivalent to a voltage gain of 10 and the GBW is 5 MHz. Thus, the low frequency 3-dB point is Hz. (b) The transfer function is (c) The GBP is 5 MHz, so that an amplifier with gain 100 ( 40 db) will have a bandwidth of khz. (d) The transfer function is (e) The phase at 10-kHz is tan The period of a 10-kHz sine wave is 100 s so that corresponds to a delay of Δ s 3.14 s 360 9

10 Question 6 For the circuit shown, determine,,. Assume the Si transistor is operating in the forward active mode. (8 points) 5 V 75 20K 10K 2K Solution K0.7 20K Further (note that both and flow through and contribute to the voltage drop across K Solving these two equations yield 4.61 A. Then and ma K 1.49 V 10