Q. Why FET is called a voltage-controlled device? Why its input resistance is high?

In the case of a FET the output current ID is a function of the voltage VGS applied to the input circuit. Hence FET is known as a voltage controlled device. The equation that governs the working of a FET is called Shockley's equation given by,

I_D = I_DSS[1 - (V_GS/V_P) ^₂

where, ID is the drain current

I_DSS is the maximum saturation current

V_GS is the voltage applied between gate and source

V_P is the pinch-off voltage

A FET requires virtually no input current and hence IG is always made zero. This gives an extremely high input resistance to FET. The range of its input impedance is at a level of 1 to several 100MWs.

Posted Date: 6/11/2013 6:07:08 AM | Location : United States

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