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What are the conditions for a FET to act as a variable resistance?

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can anyone explain me the ohmic region in BJT, JFET and MOSFET. I wanna do an active filter with a FET that change the cut-off frequency. I want to know what are the conditions for a FET to act as a variable resistance ?

Added after 30 minutes:

should I use UNJ to fix this probleme ?
 

Re: semiconductor

Ohmic region in case of FET: The transistor is on, but behaves as a voltage controlled resistor. When VDS is less than in the active region, the drain current is roughly proportional to the source-drain voltage and is controlled by the gate voltage.

In case of jfet,
cutoff region :
vgs ≤ vp
id=0

triode region
vp ≤ vgs ≤ o
vds ≤ vgs-vp
id=idss(2(1-vgs/vp)(-vds/vp)-(vds/vp)²)

saturation region
vp ≤ vgs ≤ o
vds ≥ vgs-vp
id=idss(1-vgs/vp)² (i+λvds)

where, λ is inverse of early voltage.
 

Re: semiconductor

sec,

read sedra and smith, microelectronic circuits for more in-depth understanding of FET. Pay special attention to the biasing and small-signal analysis of FET (chap 4). It will also explain the various configuration that you can place the FET amplifier (which you can think of as a variable resistor) and the various assumption/constraints that you must satisfy b4 you can apply the small signal model.
 

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