JFET Calculator

A Junction Field-Effect Transistor (JFET) is a voltage-controlled semiconductor device used in amplifiers and switches — this calculator analyzes its DC operating point and AC performance for a given circuit. Enter your Supply Voltage (VDD), JFET parameters (IDSS and Pinch-off Voltage VP), Gate-Source Voltage (VGS), and resistor values (RD, RS, RG), then select a circuit configuration (Common Source, Source Follower, or Common Gate) to get the Drain Current (ID), Drain and Source voltages, Transconductance (gm), Voltage Gain (Av), and Power Dissipation.

V

Power supply voltage

mA

Maximum drain current when VGS = 0

V

Gate voltage that reduces drain current to zero

V

Applied gate-to-source bias voltage

Ω

Drain load resistor value

Ω

Source resistor for bias stabilization

Ω

Gate bias resistor (typically 1MΩ)

Select circuit configuration

Results

Drain Current (ID)

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Drain Voltage (VD)

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Source Voltage (VS)

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Transconductance (gm)

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Voltage Gain (Av)

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Power Dissipation

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Frequently Asked Questions

What is a JFET and how does it work?

A JFET (Junction Field Effect Transistor) is a voltage-controlled device where the gate voltage controls the drain current. Unlike BJTs, JFETs have extremely high input impedance and are normally-on devices that require negative gate voltage (for N-channel) to reduce current flow.

What is the significance of IDSS and VP parameters?

IDSS is the maximum drain current when VGS = 0V, representing the fully-on state. VP (pinch-off voltage) is the gate voltage that completely cuts off drain current. These parameters define the JFET's operating characteristics and are found in datasheets.

How do I choose the right bias voltage (VGS)?

VGS should typically be set between VP/4 and VP/2 for linear operation. This ensures the JFET operates in the active region with good linearity and reasonable gain while maintaining thermal stability.

What's the difference between common source and source follower configurations?

Common source provides voltage amplification with phase inversion, ideal for amplifier stages. Source follower (buffer) has unity gain but very high input impedance, making it perfect for impedance matching and buffering applications.

Why is the gate resistor (RG) typically so large?

Large gate resistors (1MΩ or more) take advantage of the JFET's extremely high input impedance. This maximizes input impedance, minimizes loading on the previous stage, and reduces bias current flow through the gate circuit.

How does source resistor (RS) affect JFET operation?

RS provides negative feedback for bias stabilization and affects the operating point. Larger RS values improve thermal stability but reduce gain. The voltage drop across RS (VS) helps establish the proper VGS bias point.

What causes JFET parameters to vary and how to compensate?

JFET parameters vary significantly between devices and with temperature. Use source degeneration (RS), voltage divider biasing, or current source biasing to improve stability and reduce dependence on device parameters.

How do I calculate transconductance (gm) for a JFET?

Transconductance gm = (2 × IDSS / |VP|) × (1 - VGS/VP), which represents the change in drain current for a change in gate voltage. Higher gm values provide better voltage gain in amplifier applications.