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MOSFETs don't operate the same way as BJTs. MOSFETS react to voltage same way as BJTs do to current. When you can't draw a lot of current, you can select a high input impedance mosfet and control it with voltage.
In Edge-Connectors, the flat layer of 99.99% gold is covered to have an accurate impendance with the circuit connected on. For instance, 0.2Ohms is required for Blue-Tooth equippments. And the flawness of such devices could not pass up by 0.1%, in such cases!! And as I know, Aerospace productions in China requires it don't have errors.
MOSFET input is applied at gate. Gate of MOSFET is separated by oxide (insulator) that why input impedance is very high. Input impedance requirement depends on the application. For amplifier we need high input impedance to avoid loading of input signal.
That is like asking "What is the purpose of velocity? Why is it so high in meteors?" Input impedance is a characteristic of a circuit that can be changed, but not eliminated. It is high in MOSFETS because a lot of input voltage results in very little input current. You have to study MOSFETS to understand why that is so.
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