Why the MOFETs have a very small gate terminal current b Des

Why the MOFETs have a very small gate terminal current? (b) Describe the difference between the simple RC circuit based low pass filter and the OP-Amp based low pass filter.

Solution

a)

The gate is seperate from rest of the mosfet by a dielectric material and metal plate, hence gate is in effect connected to the rest of the mosfet by a low capacitance capacitor resulting in high impedance.

This results in small gate current.

b)

An RC filter is a passive filter. So output amplitude is always less than input amplitude due to loss in the circuit.

An opamp filter is an active filter. So an external source can be used to amplify the signal to cope with attenuation in the circuit.

Filter amplification can also be used to either shape or alter the frequency response of the filter circuit by producing a more selective output response, making the output bandwidth of the filter more narrower or even wider in active filters. Then the main difference between a “passive filter” and an “active filter” is amplification.