- JFET Biasing Definition: JFET biasing involves setting the correct voltage levels at the gate and drain to control the transistor’s operation.
- Pinch-Off Voltage: The pinch-off voltage is the drain voltage at which the depletion layers in a JFET meet, stopping the increase in drain current.
- Gate Cut-Off Voltage: The gate cut-off voltage is the negative voltage at which the JFET drain current drops to zero, crucial for determining the operating range.
- Self-Biasing Method: Self-biasing uses a resistor at the source terminal to create a voltage drop, establishing the proper gate-source voltage.
- Voltage Divider Biasing: This method uses resistors to divide the voltage and set the gate voltage, ensuring the JFET operates correctly within its parameters.
JFET biasing establishes the no-signal drain current, drain-source voltage and gate-source voltage. Pinch-off and cutoff are different limits, so the voltage symbol and test condition must be stated before using either value in a bias calculation.
Pinch Off Voltage
For an n-channel JFET with source and gate grounded, a positive drain voltage causes conventional drain current. Channel potential rises toward the drain, so the reverse-biased gate-channel junction has a wider depletion region near the drain.
At low VDS, drain current rises approximately linearly. As VDS increases, the channel narrows near the drain. Drain-end pinch-off marks the transition to the constant-current, or saturation, region; it does not mean that the whole channel closes. Current continues through the short high-field region and rises slightly in a real device because of channel-length modulation. At excessive VDS the junction can enter avalanche breakdown. Amplifier bias therefore needs the intended signal swing to remain in the saturation region and within every voltage, current and power rating.
The shown circuit equation relates supply voltage, drain resistor, drain current and drain-source voltage.
Here IDSS is the saturation-region drain current specified at VGS = 0 under the datasheet’s test conditions, not a universal current at one exact drain voltage.
Making an n-channel JFET gate negative relative to its source increases junction reverse bias, widens the depletion region and lowers drain current.
At the gate-source cutoff voltage, drain current falls to a small specified off-state value. Datasheets usually call this VGS(off). Some texts use the symbol VP for a related pinch-off parameter, with opposite sign under one convention, but the equality depends on the device model and definitions. 
An amplifier input is a small signal around its gate bias, not a universal swing from 0 to −VGS(off). The positive limit must avoid forward-biasing the gate junction, while the negative limit, drain load line and required linearity set the other boundary. VGS(off) and IDSS vary widely between individual JFETs, so robust biasing uses datasheet ranges and feedback. Fixed gate bias, self-bias and divider bias are common methods, but current-source and active-feedback circuits also exist.
Biasing of JFET by a Battery at Gate Circuit
A separate gate supply can establish negative VGS for an n-channel JFET. Because the gate-channel junction is reverse biased, only leakage current normally flows and the voltage drop across a practical gate resistor is often small but nonzero. Leakage rises with temperature. The drain circuit then sets the corresponding VDS through its supply and load resistance.
Note: The diagrams include an AC input, but DC bias analysis sets its average value to zero and finds the quiescent point first.
With negligible gate leakage, the gate voltage follows the bias source closely.
The approximate transfer equation can estimate ID from device parameters IDSS and VGS(off) (= – VP) when the JFET is in its saturation region. These parameters have wide production tolerances and the square-law model is approximate.
KVL in the drain circuit then gives VDS for the chosen load and supply.
The Q point is the coordinate (VDS, ID) where the circuit load line and selected device characteristic intersect.
Self Biasing of a JFET
Self-bias places source resistor RS between the source and ground.
Drain current develops voltage across RS as shown. 
The gate is returned to ground through resistor RG. If gate leakage is neglected, VG is approximately zero.
The resulting gate-source voltage VGS is negative for an n-channel device.
If drain current rises, source voltage rises and VGS becomes more negative, providing DC negative feedback. 
Solve the transfer relation and circuit equation together to find ID and VDS. The operating point is then (VDS, ID). This feedback improves stability but does not remove device spread or temperature effects.
Voltage Divider Biasing of a JFET
A resistor divider establishes a gate voltage from the drain supply. With negligible gate current, the unloaded divider rule is a useful first estimate. A source resistor develops VS = IDRS, so VGS = VG − VS. For an n-channel JFET the bias is negative when source voltage exceeds gate voltage. Gate leakage, resistor tolerances, supply variation and the wide JFET parameter range affect the actual Q point. The source resistor supplies negative feedback, while a bypass capacitor can change AC gain without changing the DC bias.
