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What is the pinch off voltage for a JFET?

JFET Pinch off voltage

“Pinch-off” is not locked. During pinch-off operation, the discharge current does not drop to zero. Instead, the Id current becomes constant, remaining relatively independent of the Vds voltage. The “pinch-off” mode in FET transistors is similar to the linear operating region of the BJT transistors.

Pinch off voltage: Pinch off voltage is the drain to source voltage after which the drain to source current becomes almost constant and JFET enters into saturation region and is defined only when gate to source voltage is zero.

Pinch off voltage region

Pinch off voltage in jfet

With the common-source common-source hook of a JFET to N channel, since gate voltage Vgs becomes more and more negative, the channel becomes narrower as the Mobile Depletion Zones invade the channel from the side.

Finally, these Depletion Areas meet but the channel does not close! Instead, the channel becomes a long passage of constant width. Within this channel, avalanche breakdowns occur while small portions of the channel try to close. But every time this happens, a larger voltage appears through the closed portion, which brings the DZ back and opens the channel again.

pinch off voltage

During pinch-off the channel behaves very strangely: it is no longer a resistance. Instead, when the drain-source Vds voltage increases, the conductive channel grows physically for longer! It is a magic resistor, a resistor that tries to maintain a constant current even against the variable voltages that lie on it.

What is pinch off voltage

A pinch-off FET is a constant-voltage controlled voltage source, while a pinch-off external FET is a controlled voltage resistor. For example, we can use FETs as variable resistors to act as audio volume controls or analog potentiometers. We do so by keeping the value Vgs low, so the channel remains completely open and does not enter Pinch-off mode.

Pinch off voltage triode

If we look at the set of operating curves for a FET, the Id / Vds chart, pinch-off mode is in the region where the Id curves have a flat tip, or tilt slightly upwards with strong voltage increases Vds.

So, if this is pinch-off, then what’s “pinch closed?” This is at the bottom of the curve family at large Vgs values, where the channel is actually closed, and Id is zero to all Vds values.

Pinch Closed vs Pinchoff Mode

Finally, where does the misunderstanding come to “pinch closed” against “Pinchoff mode”? Maybe it’s because when the Pinch-off starts, the AC resistance of the drain-source channel will rise to the stars.

It becomes very large, ideally infinite (which causes the V / I curves to become flat.) But this is not DC resistance. There may be an amplitude flowing into the exhaust circuit of your power mosfet, even though the AC discharge resistance is immense.

This is the main feature of any constant current source. It has a significant current, while for AC and dynamic changes it behaves like an open circuit.

But, the infinite ohm and “open circuit” does not mean zero current in which current sources are concerned. And so, when Vgs becomes big and the channel becomes a current source, it does not close. But it’s all weird! Enter the pinch-off operating mode.

How is the pinch off voltage calculated?

The pinch off voltage Vp is the value of Vds at which the drain current becomes constant and equal to Idss and is always measured at Vgs = 0 V. Switching off takes place for VDS values ​​below VP, if VGS is not equal to zero. Although VP is a constant, the minimum value of VDS at which ID becomes constant varies with VGS.

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