This current entering the JFET drain terminal is labelled I D. Here the gate-source voltage V GS is equal to the gate supply, or input voltage V G which sets the reverse bias between the gate and the source, while V DD provides the drain-to-source voltage and current flow from the supply from drain to source. The image shows the standard arrangement and connections for a common source configured N-channel JFET with normal biasing when used in its active region. ![]() ![]() We saw in our tutorial about JFETs that JFETs are depletion devices and that the N-channel JFET is a “normally on” device, until the gate-to-source voltage (VGS) becomes negative enough to turn it “off.” The P-channel JFET which is also a normally on depletion device requires the gate voltage to become positive enough to turn it “off.” In its simplest form, the JFET can be used as a voltage-controlled resistor where a small gate voltage controls the conduction of its channel. ![]() However, the key to creating an accurate and reliable constant current source depends on using low transconductance FETs as well as precision resistor values to convert the current into a precise and stable voltage.įield-effect transistors are commonly used to create a current source with junction FETs (JFETs) and Metal-oxide Semiconductor MOSFETs already being used in low current-source applications. Resistive voltage references can also be formed using constant current sources, because if you know the value of the resistance and the current flowing through it is constant and steady, then you can simply use Ohm’s law to find the voltage drop. ![]() Constant current sources and current sinks (the opposite of a current source) are a simple way to form biasing circuits or voltage references with a constant current value - for example, 100 uA, 1 mA, or 20 mA - using a single FET and resistor.Ĭonstant current sources are commonly used in capacitor charging circuits for accurate timing purposes or in rechargeable battery charging applications, as well as linear LED circuits for driving strings of LEDs at a constant brightness. FET constant current sources use JFETs and MOSFETs to deliver a load current, which remains constant despite changes in load resistance or supply voltage.Ī FET constant current source is a type of active circuit that uses a field-effect transistor to supply a constant amount of current to a circuit.
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