Please enter valid values. Configuration parameters must be non-zero and pinch-off voltage cannot be zero.
RESULTS
Input Parameters Specification
Supply Voltage (VDD)The positive direct current supply rail applied to bias the JFET drain terminal.
Desired Voltage (VRS)The target voltage drop required across the source resistor RS to configure active gate-to-source bias.
Pinch-Off Voltage (Vp)The gate-source cut-off threshold at which JFET channel conduction completely ceases.
Zero-Gate Current (IDSS)The maximum saturated channel current that flows through the JFET when gate-source potential is zero.
Practical Operational Examples
Low-Noise Audio Buffer
JFET Type: 2N5457
IDSS = 3.0 mA, Vp = -1.5 V
Desired VRS = 0.5 V, VDD = 12 V
High Impedance Active Probe
JFET Type: J113
IDSS = 10.0 mA, Vp = -3.0 V
Desired VRS = 1.0 V, VDD = 15 V
Guitar Effects Pedal
A standard 9V battery supply biasing an MPF102 JFET (IDSS = 6.0mA, Vp = -2.5V). Setting VRS = 0.8V stabilizes the analog signal headroom.
Oscilloscope Input Stage
Sizing high-impedance front-end probes utilizing low pinch-off JFETs to reduce loading down input voltage nodes completely.
Diagrams & Theory
N-channel Junction Field-Effect Transistors (JFETs) function as voltage-controlled semiconductors. When configured as a source follower, the stage provides near-unity voltage gain, low output impedance, and extremely high input impedance, functioning as a high-fidelity analog buffer.
By inserting a source resistor (RS) between the source terminal and ground, the channel current (IDS) forces the source terminal positive. Since the gate terminal is referenced to ground, the gate-to-source bias voltage automatically becomes negative (VGS = -VRS). This negative feedback stabilizes the operating point of the transistor against manufacturing variances.
Formulas & Mathematical Logic
Gate-Source Bias Voltage: VGS = -VRS
Shockley's Drain Current Equation: IDS = IDSS * (1 - VGS / Vp)²
Source Resistor Sizing: RS = -VGS / IDS * 1000
The mathematical model uses Shockley's square-law equation to calculate drain current based on physical channel dimensions, translating parameters seamlessly to predict required resistances.
Step-by-Step Example
Example: Configuring a 2N5458 JFET (IDSS = 6.0mA, Vp = -3.0V) to deliver a source drop VRS of 1.0V.
Step 1: Calculate the gate-to-source bias voltage: VGS = -VRS = -1.0V.
Step 2: Apply Shockley's equation to solve for drain current: IDS = 6.0 * (1 - (-1.0 / -3.0))^2.
Step 3: Execute step calculations: IDS = 6.0 * (1 - 0.3333)^2 = 6.0 * 0.4444 = 2.667 mA.
Step 4: Calculate the source resistance RS: RS = -(-1.0V) / 0.002667A = 375 ohms.
How to Use This Calculator
Enter the DC supply voltage VDD in the dedicated input field.
Enter the target voltage drop VRS desired across the source resistor.
Specify the negative pinch-off voltage Vp according to your JFET's datasheet parameters.
Enter the zero-gate drain current IDSS value in milliamperes (mA).
Click Calculate to instantly view the calculated drain current (IDS), source resistor value (RS), and gate-source bias voltage (VGS).
About This Calculator
Model, analyze, and scale self-biasing networks for N-Channel Junction Field-Effect Transistors with CalcBoy's professional suite.
This design tool estimates active channel current, required source resistance values, and matching bias voltage bounds for low-noise JFET buffers and source-followers.
Designing discrete analog preamplifiers, instrument buffers, and high-impedance probes requires meticulous selection of biasing networks. Because of their physical structure, JFETs draw virtually zero gate leakage current. This results in incredibly high input impedance (often reaching the giga-ohm range), making JFET buffers indispensable in guitar effects pedals, condenser microphones, and oscilloscope front-end inputs.
A self-biasing scheme uses the voltage drop across the source resistor (RS) to generate the negative gate-source potential necessary for linear operation. As channel current flows through RS, the source terminal rises above ground potential, while the gate remains referenced to ground. This naturally creates a negative feedback loop: any increase in drain current raises the source voltage, making the gate more negative and stabilizing the channel current against thermal and manufacturing variances.
Calculating this balanced operating point is highly sensitive to individual device parameters. IDSS and Vp can vary widely across identical part batches. This CalcBoy utility uses Shockley's square-law model to calculate accurate source resistor values based on your target voltage parameters, reducing prototype assembly and bench tuning efforts.
Typical ApplicationsCondenser microphone preamplifiers, guitar effects buffers, high-impedance analog probes, and sensor matching stages.
Calculated DeliverablesDrain-to-source current (IDS), source bias resistance (RS), and operating bias voltage (VGS).
Target AudienceElectronic circuit designers, audio electronics developers, hobbyists, and engineering students.
Datasheet TipJFET properties vary significantly. Measuring the actual IDSS of your physical components using a simple test jig ensures highly accurate biasing calculations.
Tip: Always choose a source resistor with a wattage rating higher than the calculated power dissipation (Pd = IDS * VRS) to prevent thermal drift.
Frequently Asked Questions
1. What is a JFET source-follower buffer circuit?
A source follower is a JFET circuit configuration where the input is applied to the gate and the output is taken from the source. It offers high input impedance, low output impedance, and near-unity voltage gain, serving as an isolation buffer.
2. Why does the input pinch-off voltage (Vp) need to be negative for N-Channel JFETs?
N-channel JFETs require a negative gate-source voltage (VGS) to narrow the internal channel and restrict current. A positive VGS would forward-bias the gate junction, causing current to flow into the gate and defeating JFET operation.
3. What happens if the desired VRS exceeds the magnitude of the pinch-off voltage (Vp)?
If VRS is greater than the magnitude of Vp, the gate-to-source voltage (VGS) will drop below the pinch-off threshold. This completely cuts off the JFET channel, reducing the drain current to zero.
4. Why is IDSS a critical parameter in JFET biasing?
IDSS represents the maximum possible current that can flow through the channel under zero-bias conditions. It defines the absolute upper boundary of the JFET's load capabilities and operating curves.
5. Can I use this calculator for P-channel JFET configurations?
The core mathematical equations are identical, but P-channel JFETs require opposite voltage polarities. For P-channel calculations, the applied voltages and current polarities must be reversed.
6. How does the source resistor RS affect overall gain and output impedance?
A larger RS value increases negative feedback, stabilizing the operating point and bringing the voltage gain closer to unity (1.0). It also helps minimize output impedance, making the buffer more efficient at driving low-impedance loads.