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What is the total output impedance at node x?(a) R1 || ro1 || 2 * ro2 || ro2 || 2*ro4… || ron(b) R1 || 2 * ( ro1 || ro2 … || ron)(c) R1 || ro1 || ro2 … || ron(d) R1 + (ro1 || ro2 … || ron)This question was posed to me in quiz.My question is based upon MOSFET Amplifier with CG Configuration topic in portion FET Amplifiers of Analog Circuits

Answer» RIGHT answer is (c) R1 || ro1 || ro2 … || ron

The best EXPLANATION: We note that all the MOSFET’s are in parallel. The output impedance of the i^th MOSFET is given by ROI. Hence, the total impedance at node X is simply R1 || ro1 || ro2…|| ron. Note that R1 is parallel to each MOSFET. This is because, while dining the output impedance, we SET Vin and Vcc to zero. Note that R1 is connected from node X to ground, all the MOSFET’s are connected form node x to ground.


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