How fet is used as vvr?

FET AS A VOLTAGE -VARIABLE RESISTOR (VVR):

FET is operated in the constant current portion of its output characteristics for the linear applications .In the region before pinch off , where Vds is small the drain to source resistance rd can be controlled by the bias voltage Vgs.The FET is useful as a voltage variable resistor (VVR) or Voltage Dependent resistor.

In JFET the drain source conductance gd = Id/Vds for small values of Vds which may be expressed as gd = gdo [ 1-( VgsVp)1/2 ] where gdo is the value of drain conductance when the bias voltage Vgs is zero.

The variation of the rd with vgs can be closely approximated by rd = ro / 1- KVgs ro - drain resistance at zero gate bias and K constant dependent upon FET type.

Small signal FET drain resistance rd varies with applied gate voltage Vgs and FET act like a VARIABLE PASSIVE RESISTOR.

Advantagesof JFET

Very high input impedance order of 100 ohm

Operation of JFET depends on the bulk material current carriers that do not cross junctions

Negative temperature coefficients

Very high power gain

Smaller size longer life and high efficiency

Ac drain resistance rd it is the ratio of change in drain - source voltage to the change in drain current at constant gate source voltage

Transconductance it is the ratio of change in drain current to the change in gate source voltageat constant drain source voltage

Amplification factor it is the ratio of change in drain source voltage to the change in gate source voltage at constant drain current.