JFET
Transistor
A junction FET: a normally-ON transistor whose channel is pinched off by reverse-biasing the gate. Prized for very high input impedance and low noise in analog front-ends.
Schematic symbol
JFET (N-channel)
How jfet appears in a circuit diagram.
Types & variants
Key specs
Idss
mADrain current with gate shorted to source — the 'fully on' current.
Vgs(off) / pinch-off
VGate voltage that turns the channel off (negative for N-ch).
Transconductance (gm)
mSGain: how much Id changes per volt of Vgs.
Input capacitance
pFMatters at RF.
Markings
Standard part number on TO-92/SOT-23 body (J113, MMBFJ201, BF256...).
Standard values
There are no values — but Idss varies wildly between samples of the same part; audio builders buy batches and match them.
How to choose
Low-noise high-gm parts for audio preamps; check Idss range fits your bias plan. For a simple switch, a MOSFET is almost always easier.
Pinout & package
3 leads: gate, drain, source (D and S are often interchangeable). TO-92 pinouts vary by part — check the datasheet.
Example circuits
- High-impedance buffer for a piezo/guitar pickup
- Phaser/compressor voltage-controlled resistor
- RF amplifier front-end
Common failures
Gate junction blown by static or forward-biasing it hard; parameter drift makes circuits that relied on a specific Idss stop biasing correctly.
How to test
Diode mode: gate-to-channel behaves like a diode (conducts G→D and G→S on N-ch). Channel D–S reads a low resistance both ways that a negative gate bias can pinch off.
Substitutes
Another JFET with overlapping Idss/Vgs(off) range, or a MOSFET plus bias tweaks for switching duties. Classic audio JFETs are obsolete — look at Linear Systems reissues.