as low as possible for proper operation this keeps the device in the linear region of operation high signal levels on the drain cause drain to source resistance modulation which can result in distortion and instability of the negative inductor circuit the voltage controlled resistor technique uses the principle of applying a fraction of the voltage appearing on one side of a resistor to its other side to increase its effective resistance this is resistor r4 in fig1 how far the effective resistance of r4 is increased is determined by the gain of the non inverting amplifier consisting of the op amp z2 resistors r5 through r8 and the jfet transistor q1 varying the drain to source resistance of the jfet transistor q1 varies the gain of the non inverting amplifier and thus the effective resistance or resistor r4 the overall voltage controlled resistor provides a 180 degree phase shift because as the resistance of the jfet transistor goes up the resistance of the voltage controlled resistor goes down this is a distinct advantage because if the resistance of the voltage controlled resistor ever becomes greater for ac than it is for dc the negative inductor op amp breaks into oscillations because of the characteristics of the jfet transistor its drain to source resistance can only become greater for ac than dc.