JFET-transistors

N-JFET, basic
N-JFET, basic

JFETscan be described as 3-electrodes, depletion-mode, voltage-controlled current sources. The electrodes are the control-input ´Gate´, the high-ohmic ´Drain´ and the low-ohmic ´Source´.


a small change in Gate voltage controls a large change of Drain/Source current. The ratio of Ugs to Id is called the ´Forward Transfer Admittance´ Yfs in mA/V or as mS (milli-Siemens).

Two polarities are known, the N-channel and the P-channel.

The N-channel outnumber the P-channel by far.

Beeing depletion-mode devices the maximum Drain current Idss flows when Ugs=0, hence Gate and Source are shorted together.

For N-channel JFETs applies, that for Ugs becoming more negative the Drain current sinks. The Source hence remains more positive than the Gate. If a resistor is now connected from Source to the negative supply a current lower than Idss flows, because the voltage drop over the resistor ´lifts´ the source potential, thereby making Ugs more negative.

A certain current Id is now fixed in value. A simple, yet effective constant current source is formed.

If asignal modulates the Ugs the Drain current swings around that working or Bias point and can be used for amplification purposes. The opposite polarities of Ugs and voltage potentials apply to the P-channel types.

Similar to the also depletion-mode vacuum tubes JFETs alow for simple circuits with a low parts number count.

Unfortunately is the number of available JFETs becoming smaller. HighVoltage types (>60V) and P-channel are manufactured in ever reducing numbers and complementary types are virtually all obsolete or exotic.

Due to the inherently higher tolerances in the production processes JFETs reqire almost always screening and matching in tolerance-sensitive circuits.

Almost all JFETs are lowpower types and prices are considerably higher than for ´equivalent´ bipolar parts.

The Id over Ugs graphs are more rounded and less steep as for bipolars.

Still though JFETs are quite poular among Audio afficionados.

The partly simple, low-parts number count circuits sound quite charming,  similar and similarly unique like tube circuits.