Given a any mosfet circuit (common source,gate ,drain) how do you find the input and output resistance?
how do you find the the total resistance?
.......my lecture notes says that Rup parallel to rdown but i do not lnow how to get the values the equivalent of rup and rdown????
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It sound like your lecture was about a very simple circuit that is rarely used in real products. Rup and Rdown are probably the bias resistor values, so read them off the schematic. Any amplifier that uses feedback, including a simple common drain circuit, will have an artificially low output impedace because the amplifier reacts to loading. Feedback also effects the input impedance, depending on how the feedback is wired.
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To calculate those resistor value, it depending on the circuit requirement like gain,power,type of FET is chosen and impedance match those data.