Discussion9

# Discussion9 - b The amplifier is to be compensated so that...

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Discussion #9: Review Problems EE3115 Fall 2010 1. Consider the oscillator circuit shown below. The circuit is supposed to produce sinusoidal oscillations at 27 kHz but the circuit does not function properly. Enumerate what is wrong with the circuit and briefly describe what corrective action to take. Be quantitative where possible. - 40 k Ω 4 k Ω 4 k Ω 4 k Ω 0.1 μ F 0.1 μ F V o + - Amplifier characteristics: Rin = Rout = 0 Open loop gain = 200 (1+ jf 50 ) 2. Two moderate gain amplifiers, A1 and A2, are connected in cascade to emulate an op amp. Negative feedback is used around the combination to achieve a specified gain. The characteristics of the amplifiers A1 and A2 are listed below. Blank graphs for doing amplitude and phase Bode plots are provided. + - V i o V + - + - 1 k R f A1 A2 A1 Characteristics A2 Characteristics R in = 100 k Ω R in = 1 M Ω R out = 100 k Ω R out = 100 Ω A(s) = 100 (1 + jf/10 4 )(1 + jf/10 6 ) A(s) = 100 (1 + jf/10 5 )

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a. If R f = 9 k Ω , is the closed loop amplifier stable or unstable? Support your answer quantitatively. A simple one or two word answer is unacceptable.

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Unformatted text preview: b. The amplifier is to be compensated so that it is stable for any value of R f . Dominant pole compensation with 45 degrees of phase margin is implemented with the capacitor C c as shown below. Determine the proper value of C c . +-V i o V +-+-1 k Ω R f A1 A2 C c 3. Consider the feedback amplifier circuits shown below. a. Specify the type of feedback used in amplifier ckts A and B. b. The input impedance R in diagrammed on both circuits must be less than 5 k Ω . Which circuit (A or B) should be used. Qualitatively justify your answer. c. Find the closed loop gain V o I s for R f = 100 k Ω for amplifier circuit A. Solutions to Discussion #9 Review Problems 1. a. Op amp input connections wrong. Reverse them. b. Open loop amplifier gain too large. Reduce gain to approximately three by reducing 40 k Ω resistor to an 8 k Ω resistor. c. d. 2. a. b. 3....
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Discussion9 - b The amplifier is to be compensated so that...

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