Discrete mosfet (single mosfet) current source (sink) - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: May 21, 2021, 11:23:05 am
content: Hi,the question is simple: can one build a single-mosfet current source/sink?I mean: the bjt current sink is a very easy circuit, which exploit the VBEdrop when the tranny is on.Since, afaik, there's no way to "program" the source voltage in such a fashion, I wouldn't be able to create a similar circuit with a mosfet.tAoE is unclear (or perhaps I'm stupid) about it, and just discusses the JFET current source/sink, which is very easy (although it brings its own caveats to the table).Can you dissipate this part of my ignorance, please?Cheers

author: ledtester
author_rank: Super Contributor
timestamp: May 21, 2021, 02:06:47 pm
content: The JFET is a depletion mode device which is to say that it conducts when V_GS = 0. The MOSFETs most frequently used in circuits are enhancement mode devices which require V_GS > 0 in order to conduct.But there are depletion mode MOSFETs, so you could play the same game with them to create current sinks.

author: Zero999
author_rank: Super Contributor
timestamp: May 21, 2021, 02:15:38 pm
content: In theory, it's possible to use a single MOSFET to both source and sink current, but a low value pull-up resistor would be required to source the current, assuming it's an N-channel device. This means that the MOSFET would have to sink a lot more current, than the load, when in sinking mode.In reality, it's a bad idea. A push-pull circuit is much better in an application which needs to both source and sink current.

author: Moriambar
author_rank: Unknown
timestamp: May 21, 2021, 02:40:00 pm
content: Quote from: Zero999 on May 21, 2021, 02:15:38 pmYes I was thinking about either sourcing or sinking...

author: Moriambar
author_rank: Unknown
timestamp: May 21, 2021, 02:40:40 pm
content: Quote from: ledtester on May 21, 2021, 02:06:47 pmwhat about enhancement mosfets?

author: Zero999
author_rank: Super Contributor
timestamp: May 21, 2021, 03:00:25 pm
content: The power transistor in the classic two transistor constant current sink, can be an enhancement N-MOSFET.https://www.radiolocman.com/shem/schematics.html?di=113034

author: Moriambar
author_rank: Unknown
timestamp: May 21, 2021, 03:04:14 pm
content: Quote from: Zero999 on May 21, 2021, 03:00:25 pmso two transistors are needed, there's not a single-mosfet (as per the subject) solution?I really hope so, this means that I was correct!

author: Zero999
author_rank: Super Contributor
timestamp: May 21, 2021, 03:27:30 pm
content: Quote from: Moriambar on May 21, 2021, 03:04:14 pmA enhancement MOSFET normally a much higher threshold voltage, than a BJT's base-emitter junction. It's also has a much wider tolerance. In theory, you could replace the BJT with a MOSFET and the two silicon diodes with a zener diode, but it wouldn't be as stable, as the classic BJT circuit.

author: Moriambar
author_rank: Unknown
timestamp: May 21, 2021, 06:51:35 pm
content: Quote from: Zero999 on May 21, 2021, 03:00:25 pmWell here's what escapes me.In bjt you bias the base and then obtain the emitter voltage one diode drop below that. so now you have a voltage to work with and can select a resistor to obtain a specific current (as long as you keep the collector voltage within the compliance).But to set the source voltage in an enhancement mosfet you have to bias the source as well as the gate, since there's no definite drop you have: VGSis determined by the biasing, not by the transistor intrinsic characteristics, as per the BJT.Where am I wrong?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2021, 07:49:09 pm
content: There are depletion mosfets that can be use as a 1 transistor current source, much like the JFET solution with a resistor at spource and the gate at ground.Depl. Mosfets are also available for quite high votlages. This type of current source/sink is used in some LED lamps.For higher output resistance a 2nd FET could be used as a kind of kaskode to get a stable drain voltage.

author: Zero999
author_rank: Super Contributor
timestamp: May 21, 2021, 10:10:27 pm
content: Quote from: Moriambar on May 21, 2021, 06:51:35 pmJust set the gate voltage. The source will sit at the threshold voltage and thus the current through it will remain constant, even if the drain resistance is changed.

author: julian1
author_rank: Frequent Contributor
timestamp: May 21, 2021, 10:42:46 pm
content: Presumably the advantage of a bjt (2n3904), rather than just a diode (1n750) to sense the voltage drop across the resistor would be better frequency response to load variation. Since the common-emitter amplifier can drive the gate/ gate capacitance better? Edit.

author: Moriambar
author_rank: Unknown
timestamp: May 22, 2021, 09:06:40 am
content: Quote from: Zero999 on May 21, 2021, 10:10:27 pmAh. I did not know this. So unless source is hard-tied to gnd when VG>Vth, VSwill sit at the threshold voltage.Nice indeed, thanks!Not that I have a practical use for the circuit, but that is exactly what I wanted to learn!

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2021, 05:43:44 pm
content: Quote from: Moriambar on May 22, 2021, 09:06:40 amSorry, I was sloppy with my wording.The source will sit a at threshold below the gate, thus fixing the voltage across the source resistor and therefore the current through it. In the case of the 2N7002, the threshold voltage is around 2.2V, so if the gate is held at 4.6V, the source will sit at 2.4V.

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2021, 07:41:36 pm
content: I have used successfully the IXYS IXCP10M45S as a floating two-terminal constant-current device (see Fig 1 of the data sheet cited below). It is based on a depletion-mode MOSFET, using a resistor in the source lead to set the current with the gate-source voltage. (I used them as 6 mA plate loads with Nuvistor triodes in audio amplifiers, and as a current regulator into a string of Zener diodes at 85 mA with an appropriate heat sink.) I assembled them with a gate-stopper resistor and a source-feedback resistor to a standoff screwed to the drain tab for low power, or mounted on a heat sink for higher power. The initial source resistor was chosen to be a bit large, and I determined a shunt resistor to trim the current to the correct value by connecting a decade box in parallel with it to find the appropriate final resistor. The data sheet is athttps://ixapps.ixys.com/DataSheet/98704.pdfI like them because of their high voltage rating and ease of use. They are in stock at Mouser.The first attached file is my measurement of the dynamic resistance of the circuit (as a function of DC current), measured at DC between 10 and 20 V. At higher frequency, the capacitance shunts the resistive component. The second file shows the frequency-dependent impedance at 65 V and 7.5 mA, where the dashed line on the right is the capacitive reactance of 18.2 pF (important at higher audio frequency).