12/26/2023 0 Comments Fet transistor for multiple ledsWe remember that the JFET is a depletion mode device which is always “ON”, so requires a negative gate voltage for N-channel JFET’s, and a positive gate voltage for P-channel JFET’s to turn them “OFF”. Thus the channels drain current, I D will always flow as long as the JFET device is operated within its active region as shown. When V GS is at its V GS(off) cut-off voltage level there will be zero drain current, (I D = 0) as the channel is closed. So a JFET can be biased to operate as an FET current source device at any current value below its saturation current, I DSS when V GS equals zero volts. Thus a high R DS(ON) value results in a low I DSS and vice versa. When an FET is conducting, this channel resistance is commonly known as R DS(ON) and is at its minimum resistive value when V GS = 0. However, the channel resistance is not completely zero but at some low ohmic value defined by the manufacturing geometry of the FET and which can be as high as 50 or so Ohms. As we have seen, when V GS = 0, maximum drain-to-source current flows, therefore the JFET’s channel resistance, R DS must be at its minimum, and this is true. Also note that I DSS will vary alot between devices of the same part number as manufacturers quote on their data sheets, minimum and maximum values of this zero gate voltage drain current, I DSS.Īnother point to note is that an FET is basically a voltage-controlled resistor whose conductive channel has a resistive value in series with the drain and source terminals. However, the key to creating an accurate and reliable FET based constant current source depends on using low transconductance FET’s as well as precision resistor values to convert the current into a precise and stable voltage.įor example, the 2N36xx or the 2N43xx n-channel JFET series is only a few mill-amperes (mA), whereas the larger n-channel J1xx or PN4xxx series can be several ten’s of milli-amperes. 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. But why would you want a constant current? Constant current sources and current sinks, (a current sink is the reverse of a current source) are a very simple way of forming biasing circuits or voltage references with a constant value of current, for example, 100uA, 1mA or 20mA using just 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 LED’s at a constant brightness. An FET Current Source is a type of active circuit which uses a Field Effect Transistor to supply a constant amount of current to a circuit.
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