ECE 145a /218A problem set 6a Power amplifiers Problem 1:...

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1 ECE 145a /218A problem set 6a Power amplifiers Problem 1: The FET has a small-signal parameters as below, and large-signal parameters as given to the right. g m W m mS g / 0 . 1 m i g R / 0 . 1 g gd W m fF C / 0 . 0 g gs W m fF C / 0 . 1 g ds W m mS G / 1 . 0 The maximum safe drain-source voltage is 1.1 Volts, and the maximum drain current is 0.5 mA per micron of gate width Wg. The maximum gate width for an individual FET finger is 2 microns, if we wish to maintain the low input resistance Ri given above. a) If we construct a multi-finger FET with *100* such fingers, each of 2 microns width, in parallel, find the overall gm, Ri, Cgs, Rds, of the device. What is the maximum drain current ? What are f and max b) For this 100-finger device, what is the optimum load impedance for maximum saturated output power ? Caution: you should *not* include the parallel loading of RDS in this calculation; this is a subtle point which arises because, in real FETs, RDS is bias- f ?

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