Homework Assignment 13

Transcription

1 Question 1 Short Takes 2 points each. Homework Assignment Classify the type of feedback uses in the circuit below (i.e., shunt-shunt, series-shunt, ) 2. True or false: an engineer uses series-shunt negative feedback to extend the bandwidth of a voltage amplifier this will also increase the input resistance. 3. What type of negative feedback (series-shunt, series-series, ) is used in the following amplifier? 4. True or false: voltage regulators use negative feedback to stabilize/regulate their output voltages: a side effect is that their output resistances are high. 5. An amplifier has gain of 800. After adding negative feedback, the gain is measured as 25. Find the loop gain. 1

2 6. An amplifier has gain of 800. After adding negative feedback, the gain is measured as 25. Find the feedback factor. 7. An amplifier with gain of 200 has a 10% variation in gain over a certain frequency range. Using negative feedback, what value of β should one use to reduce the gain variation to 1%? 8. An amplifier has gain of 100,000, and a 20% variation in gain over a certain temperature range. Negative feedback is used to reduce the gain to 10. What is the variation in gain with temperature of the feedback amplifier? 9. An op-amp has an open-loop gain of 120 db and an input resistance of 50 MΩ. An engineer wants to use negative feedback to obtain an amplifier with input resistance of 5 GΩ. What is the gain (in db) of the feedback amplifier? (2 points) 10. A single-pole op-amp has an open-loop low-frequency gain of A = 10 5 and an open loop, 3-dB frequency of 4 Hz. If an inverting amplifier with closed-loop low-frequency gain of A f = 50 uses this op-amp, determine the closed-loop bandwidth. 11. A single-pole op-amp has an open-loop gain of 100 db and a unity-gain bandwidth frequency 5 MHz. What is the open-loop bandwidth of the amplifier? The amplifier is used as a voltage follower. What is the bandwidth of the follower? 2

3 Question 2 A certain audio power amplifier with a signal gain of 10 V/V is found to produce a 2-V peak-to-peak 60-Hz hum. We wish to reduce the output hum to less than 1 mv peak-topeak without changing the signal gain. To this end, we precede the power stage with a preamplifier stage with gain a 1 and then apply negative feedback around the composite amplifier. What are the required values of a 1 and β? Provide β to four significant digits. (6 points) Original amplifier with 60-Hz hum problem. Preamplifier and negative feedback to fix hum problem. 3

4 Question 3 The amplifier below has an open-loop gain A OL = 80 db. What is β, the loop gain T, and the closed-loop gain A f (6 points) R 1 = 1K R 2 = 47K R L = 4.7K Question 4 For the non-inverting op-amp circuit below, the parameters are A = 10 5, A vf = 20, R i = 100K, and R o = 100 Ω. Determine R if and R of respectively (6 points) 4

5 Question 5 The parameters of the ideal shunt-series amplifier below are I i = 20 μa, I fb = 19 μa, R i = 500 Ω, R o = 20K, and β i = A/A.. The open-loop gain is A i = 2,000 A A. Determine the values and units for I ε, I o, A if, R if, and R of. (8 points) 5

6 Question 6 The open-loop gain and input resistance of the op-amp below is 10 6 and 1 MΩ respectively. Further, R 1 = 99K, R 2 = 1K. What is the closed-loop gain and input resistance? (5 points) Question 7 An op-amp having a single-pole at 100 Hz, and a low-frequency gain of 10 5 is operated is a feedback loop with β = (a) What is the factor which feedback shifts the pole? (2 points) (b) To what frequency? (2 points) (c) If β is changed to a value that results in a closed loop gain of +1, to what frequency does the pole shift? (2 points) 6

7 Question 8 Part (a) of the figure below shows a non-feedback amplifier with gain A that delivers 5 W into a 5 Ω speaker when the amplifier input is 50 mv rms. The nonlinear distortion in the amplifier output is 1% of the total signal. In part (b) negative feedback is employed to reduce the nonlinear distortion. A preamplifier is used to compensate for changes in gain the feedback introduces (a) Find the numerical value of the voltage gain A. (3 points). (b) Find the value of β required to reduce the distortion to 0.1% with the same output signal amplitude. Find the value of the preamplifier voltage gain A PR. You can assume the preamplifier nonlinear distortion is negligible. (5 points) 7

8 Question 9 Consider an op-amp having a single-pole open-loop response with A o = 10 5 and an open-loop 3-dB bandwidth of 10 Hz. The amplifier is ideal otherwise. The amplifier is connected in the non-inverting configuration with a nominal low-frequency closed-loop gain of 100. (a) Find the feedback factor β. (2 points) (b) Make neat Bode plots showing the open-loop gain and the phase of the open-loop amplifier. (6 points) (c) Add a plot of the loop gain T to you figure and find the frequency at which T = 1. (3 points) (e) Find the phase margin of the closed loop amplifier. (3 points) (f) Is the amplifier stable? (1 point) 8

9 Question 10 The open-loop voltage gain of an amplifier is given by A v = j f f j 10 5 An engineer used the amplifier to design a feedback amplifier with closed-loop gain A fv = 100. Will the amplifier be stable? If so, what is the phase margin? (15 points) 9

10 Question 11 Consider a feedback amplifier with loop gain transfer function β(100) T(s) = s Determine the stability with β = 0.2 (12 points) 10