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JFET current source

We already saw that the JFET is a voltage-controlled current source. If VGS remains constant, so will ID. The circuit above makes use of that behaviour. The transistor is a BF245B. VGS = 0V. According to the datasheet ID will be 10mA. In this simple circuit we use it to supply an LED with a current flow that is independant of the source voltage.

Note that we cannot use this current source in the JFET amplifier in the previous section. The JFET in the current source has the same parameter variety as the one in the amplifier. That's no problem for driving LEDs, but it is in an amplifier.

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