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COLORADO STATE UNIVERSITY ECE 332: ELECTRONIC PRINCIPLES II HOMEWORK 11 1. A negative-feedback amplifier has a close loop gain π΄π = 100 and an open-loop gain π΄ = 104 . What is the feed-back factor π? If a manufacturing error results in a reduction of π΄ to 103 , what closed-loop gain results? What is the percentage change in π΄π corresponding to this factor of 10 reduction in π΄? 2. Consider the op amp circuit shown in the figure, where op amp has infinite input resistance and zero output resistance but finite open-loop gain π΄. a) Verify the feedback factor π = π 1 /(π 1 + π 2 ) b) If π 1 = 10πΞ©, find π 2 that results in the closedloop gain π΄π = 10π/π for the following three cases: (i) π΄ = 1000π/π; (ii) π΄ = 100π/π; (iii) π΄ = 12π/π. c) For each of the three cases in (b), find the percentage change in π΄π that results when π΄ decreases by 20%. Comment on the result. 3. The noninverting buffer op amp configuration shown in the figure below (the bottom diagram) is a direct implementation of the feed-back loop of the feedback diagram (the top diagram). Assuming that the op amp has infinite input resistance and zero output resistance, what is the feedback factor π? If π΄ = 103, what is the closed-loop voltage gain? What is the amount of feedback (in dB) (the amount of feedback is defined as 1+T)? For ππ = 1π, find ππ and ππ . If π΄ decreases by 10%, what is the corresponding percentage decrease in π΄π ? Page 1 of 2 4. The feedback current amplifier in the figure utilizes an op amp with an input differential resistance π ππ , an open-loop gain π, and an output resistance ππ . The output current πΌπ that is delivered to the load resistance π πΏ is sensed by the feedback network composed of the two resistances π π and π πΉ , and a fraction πΌπ is fed back to the amplifier input node. Assume the loop gain is large, find expressions for, π β‘ πΌπ / πΌπ , and π΄π β‘ πΌπ / πΌπ , assuming that the feedback causes the voltage at the input node to be near ground (i.e. it is a virtual ground). For π π = 100Ξ©, and π πΉ = 10πΞ©, find π and π΄π . 5. Figure below shows a feedback transconductance amplifier utilizing an op amp with openloop gain π, very large input resistance, and a very small output resistance, and an NMOS transistor π. The amplifier delivers its output current to π πΏ . The feedback network, composed of resistor π , senses the equal current in the source terminal of π and delivers a proportional voltage ππ to the negative input terminal of the op amp. Assume R is much smaller than the output impedance of Q. a) Show that the feedback is negative. b) Open the feedback loop by breaking the connection of π to the negative input terminal. Find an expression for π΄ β‘ πΌπ / ππ . c) Find an expression for π β‘ ππ / πΌπ . d) Find an expression for π΄π β‘ πΌπ / ππ . e) What is the condition to obtain πΌπ β ππ /π Page 2 of 2