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why is the input impedance of JFET and MOSFET very high ?

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desperado1

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jfet input impedance

In comparision to BJT why is the input impedance of JFET and MOSFET very high ?
 

fet high input impedance

The gate lies on an insulated thin oxide layer (dielectric) above the substrate between the source and drain. The gate is effectively one side of a capacitor. This is the input impedance being referred to - the fact that the gate has no DC path to either source, drain or substrate.
 

mosfet input impedance

Awarapunshee said:
The gate lies on an insulated thin oxide layer (dielectric) above the substrate between the source and drain. The gate is effectively one side of a capacitor. This is the input impedance being referred to - the fact that the gate has no DC path to either source, drain or substrate.
Click to expand...
thanks for the reply but i am still a bit confused. can u please show it pictorically ?
 

why is the input impedence to a jfet so high?

What about in case of JFET...?
There are no insulations in this case...
Is the High input impedance due to the reverse biased junctions at the gate...?
 

fet input impedance

yes. it is. In case of BJT the input impedence is between emmiter and base and the junction is forward biased. Had the base been highly doped, the input impedence wiuld have been very low. With light doping in the base this impedance is increased.
 

why is the input impedance to a jfet so high?

in jfet the gate-channel junction is always reverse biased so the input impedance is high.if it was forward biased the input impedance bocomes very low.
 

why emitter configuration is always and mostly used in BJT.....?
 

Re: why is the input impedance of JFET and MOSFET very high

..emitter configuration..
you mean 'common emitter configuration?

srizbf
13thmay2010
 

Re: why is the input impedance of JFET and MOSFET very high

Used in BJT...what? amplifiers?.... pls specify..
 

Re: why is the input impedance to a jfet so high?

sasaberali said:
in jfet the gate-channel junction is always reverse biased so the input impedance is high.if it was forward biased the input impedance bocomes very low.
Click to expand...
gate to channel junction ....?? do u mean gate to drain junction by this...??
 

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