high input resistance of JFET transistor..

[sohail qureshi]

why the input resistance of JFET is very high............................?? what is its concept...? tnx in advance.......

 

[srizbf]

simply said , the output current is dependent on input voltage only.

not on input current , like bjt.

so input voltage alone is responsible for current variation.
assuming zero input current(ideal) , you can say input impedance is infinity.

sidenote:are you asking for JFET or MOSFET ?

 

[sohail qureshi]

simply said , the output current is dependent on input voltage only.

not on input current , like bjt.

so input voltage alone is responsible for current variation.
assuming zero input current(ideal) , you can say input impedance is infinity.



sidenote:are you asking for JFET or MOSFET ?

sorry to say, but i think this is not a proper answer....! my question is how the input resistance is high than the output resistance.......plz explain... i m asking for JFET thanks

 

[kak111]

The FET is a three terminal device like the BJT, but operates by a different principle.
The three terminals are called the source, drain, and gate.
The voltage applied to the gate controls the current flowing in the source-drain channel.
No current flows through the gate electrode, thus the gate is essentially insulated from the source-drain channel.
Because no current flows through the gate, the input impedance of the FET is extremely large (in the range of 10^10...10^15 Ω).

The two common families of FETs, the junction FET (JFET) and the metal oxide semiconductor FET (MOSFET) differ in the way the gate contact is made on the source-drain channel.

In the JFET the gate-channel contact is a reverse biased pn junction.
The gate-channel junction of the JFET must always be reverse biased otherwise it may behave as a diode.
All JFETs are depletion mode devices they are on when the gate bias is zero (VGS = 0).

Look all parts here...........
Field Effect Transistors
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