HomeworkAssignment02Solution

# HomeworkAssignment02Solution - 55:041 Electronic Circuits...

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55:041 Electronic Circuits. The University of Iowa. Fall 2011. 1 Homework Assignment 02 Question 1 a) Define the CMRR for a differential amplifier. What is the ideal value? (2 points) Answer: | | . Ideally, b) What is the purpose of in the circuit below, and what should the value be to be effective? (2 points) Answer: This compensates for the op- amp’s input offset current. The value should be || . c) What are the units of slew-rate? (1 point) Answer: Typically, slew rate is expressed in s . d) An engineer measures finds that with an input voltage , the output voltage is , so that the gain of the amplifier is 6. However, op-amp theory suggests the gain is . Give one phrase that could explain the difference. (2 points) Answer: Slew-Rate

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55:041 Electronic Circuits. The University of Iowa. Fall 2011. 2 e) Briefly (2 3 sentences) describe the purpose of the 100K resistor in the op-amp integrator circuit below. (2 points) Answer: This provides a DC current path to bias the noninverting input transistors. Without this resistor, the input current flows through the capacitor which will charge up. Even without any input, the output will then eventually saturate at the supply rail.
55:041 Electronic Circuits. The University of Iowa. Fall 2011. 3 Question 2 (a) Determine the voltage gain for the ideal op-amp circuit shown. (b) Determine for (c) Let For determine magnitude of the current in the resistor in the T-network. (d) Determine and indicate on the figure the direction of the current in the resistor in the T-network. (15 points) Hint: apply KCL at the junction of the T-network and the inverting input. Solution Part (a) There is a virtual short between and so that . Call the voltage at the junction of the three resistors that form the T-network, KCL at the T-network junction gives KCL at the inverting input gives Solving for in the second equation gives . Substituting this in the first KCL equation and some algebraic manipulation yields Part (b) ( ( . Part (c) ( ( The current in the resistor in the T-network is ( ( . The magnitude of the current is Part (d) The negative sign of associated with the current determined in Part (c) indicate that the current flows though R from the ground into the junction of the T-network. i

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55:041 Electronic Circuits. The University of Iowa. Fall 2011. 4 Question 3 For the amplifier shown, determine (a) the ideal closed-loop voltage gain. (b) What is the actual closed-loop voltage gain if the open-loop gain is and the op-amp is otherwise ideal? (c) What is the minimum open-loop gain such that the closed-loop gain is within 1% of the ideal?
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