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[SOLVED] Current source with using Mosfet

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alikemal

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Hi. How can i construct a current source with using mosfets? And can you explain the working principle of it ? Thank you...
 

KlausST

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Hi,


a single FET won´t do. You need extra parts like resistors, and voltage source.
Maybe you need capacitors, zener diodes, opamps or even more parts.


it depends on your needs:
max voltage
current (if adjustable: min ... max)
precision
frequency range
...

***
usually one has a resistor in the source path of the fet. the higher the current the more the voltage drop at the resistor.
Then a opamp regulates gate voltage in a way that the desired curent (voltage drop) is across the resistor.


Klaus
 

alikemal

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I need 10 uA current sıurce with a voltage source is 1.8V.
 

Audioguru

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Current regulator diodes were inexpensive 30 years ago. They were made with only a Jfet and a resistor. Today they are VERY expensive.
 

alikemal

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but i will construct it with using ltspice
 

KlausST

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Hi,

either you are satisfied with a 180k resistor, or you have to provide more information.


Klaus
 

alikemal

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i made a current mirror with using a current source. But my ibstructor told me that you should construct current without using a current source.

 

asdf44

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Ok 1.8V but what's the rest of the context? Is this for an IC or a PCB or what?

Here is an option for a simple topology:
Image4.gif


The left is a depletion mode mosfet which isn't common, but works pretty well.

The right puts a constant voltage on a "normal" (enhancement mode) mosfet gate, and as the current increases, the voltage on the R increases which makes the voltage of the mosfet source go up. That decreases Vgs which shuts the mosfet off. So it ends up self regulating at a constant current depending on the voltage and the R.

The voltage source could be created with a simple zener diode pulled up to your Vcc. Though neither of the above is a particularly accurate current source (and low voltage zeners are terrible), and would require manual trimming on each circuit to get the current exactly on target. Though in simulation it will work great.
 
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