The pinch-off voltage of a JFET is about _______

Question

Examoogle · Accepted Answer

5 V

Explanation: The pinch-off voltage ($V_P$) of a JFET is the gate-source voltage ($V_{GS}$) at which the depletion region completely closes the conductive channel, reducing the drain current ($I_D$) to nearly zero. Typical commercial JFETs are designed with pinch-off values commonly in the range of 1 V to 6 V, making 5 V a standard representative value in technical examinations.

Working Principle: As the reverse bias voltage at the gate-source ($V_{GS}$) increases, the depletion layers surrounding the gate extend into the n-type (or p-type) channel. When $V_{GS}$ reaches the pinch-off voltage $V_P$, the depletion regions from both sides meet, 'pinching off' the channel. Beyond this point, any further increase in $|V_{GS}|$ ensures the channel remains closed, keeping the device in the cut-off state.

Key Formulas: $I_D = I_{DSS} \left(1 - \frac{V_{GS}}{V_P}\right)^2$ — Shockley equation for JFET drain current; $V_{GS(off)} = V_P$ — Relation between gate-source cut-off and pinch-off voltage

Key Points: Pinch-off voltage ($V_P$) is a negative value for n-channel JFETs.; The magnitude $|V_P|$ is often referred to as the threshold or cut-off voltage.; It is a fabrication-dependent parameter determined by channel doping and geometry.

Advantages: High input impedance; Low noise performance

Disadvantages: Temperature sensitivity; Lower gain compared to BJTs

Applications: Voltage controlled resistors; High impedance pre-amplifiers

Additional Info: Option B (0.6 V) corresponds to the barrier potential of a standard silicon PN junction diode.. Options C and D (15 V, 25 V) are generally too high for standard signal-level JFET pinch-off voltages.

Answer: A is correct — The pinch-off voltage of a JFET is typically around 5 V for most general-purpose devices used in practical applications.

Exam Tip: Remember that $V_{GS(off)}$ and $V_P$ are identical in magnitude for a JFET; if an exam asks for the gate voltage required to cut off the current, that is your $V_P$.

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