- 1). Compute the potential drop needed in the circuit and compute the resistance necessary to achieve the voltage. A field-effect transistor (FET) used as a voltage-controlled resistor will operate within a specific range of resistance values. Review FET specifications to identify the component that will provide the specified resistance.
- 2). Design the circuit around the FET being used in place of a fixed- or variable-value resistor. The potential drop will occur across the source and drain leads. The gate junction must be connected to the source of your control voltage, as it is this voltage that controls the resistance characteristics of the transistor.
- 3). Apply negative (n-channel) or positive (p-channel) voltage to the gate to increase the resistance of the FET. Diminishing the voltage has the opposite effect until zero volts are present at the gate, at which point the resistance of the FET will be at its lowest specification.
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