What Is a JFET?
A JFET (Junction Field-Effect Transistor) is a transistor that, like a MOSFET, is voltage-controlled but simpler in structure. It stands out for its very high input impedance.
How Does It Work?
A JFET is conductive by default; the reverse voltage applied to the gate narrows the drain-source channel and reduces the current. So as the gate voltage rises, the current is pinched off (depletion type).
Difference From a MOSFET
| Property | JFET | MOSFET |
|---|---|---|
| Structure | Simple, robust | Insulated gate |
| Default state | Conductive (normally-on) | Usually off |
| ESD sensitivity | Lower | High |
The High Input Impedance Advantage
Since the gate draws almost no current, a JFET doesn't load the source. This lets it read sensitive sensor and measurement signals without disturbing them.
Where Is It Used?
Low-noise amplifiers, high-impedance sensor interfaces, analog switches and instrument inputs.
N-Channel and P-Channel
The JFET also comes in two polarities: N-channel (most common, pinched off by a negative gate voltage) and P-channel (reverse polarity). Since N-channel offers higher performance, it's preferred at sensitive amplifier inputs.
Common Mistakes
- Driving the gate into forward bias: a JFET works with reverse gate voltage; in the forward direction current leaks.
- Expecting it to act like a MOSFET: a JFET is usually "normally-on", so the logic is different.
- Using it at high power: a JFET is a low-to-medium-power signal component.
Frequently Asked Questions
What's the difference between a JFET and a MOSFET?+
Why is a JFET low-noise?+
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