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The JFET transistor

The JFET transistor

Also called a field-effect junction transistor, this transistor is controllable in voltage (and not in current like the bipolar transistor).

    JFET = Junction Field Effect Transistor

Symbol :

3 branches: the gate (G), the drain (D) and the source (S).

Caractéristique :

 Junction Field Effect Transistor

Operation :

  • if Vgs> 0 or Vgs switch open
  • if Vgs_off <Vgs <0 then the JFET is on. Two cases can then arise:



        – if Vds < Vp then the JFET leads in ohmic zone
        – if Vds > Vp then the JFET leads in linear zone

Notes:

– Vgs_off  = -4 V (approximately, in general)
– Vp = -Vgs_off
– the JFET leads to the maximum when Vgs = 0 => Id = Idss
– the current is the same in the drain and in the source

Equivalent diagram in AC BF:

 Junction Field Effect Transistor

Between the grid and the source, the impedance is very high. We can therefore represent it by an open wire => the current on the grid is almost zero.

gm = ΔId / ΔVgs for Vds = constant. gm is a conductance (inverse of a resistance) and is expressed in Siemens: S (in general it will even be mS).

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