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I want to make the laser (Iop < 20mA and Vop < 2.5V) blink with help of MicroController. I made the following circuit and tried to test it using a 10k resistor instead of the laser in order to check the current flowing through the drain of the transistor. enter image description here So when I was measuring the voltage at the source of the transisor it was showing the 1.4V and wasn't change when a signal at the non-inverting pin changed.

When I disconnected the output of the OpAmp from the gate the voltage at the drain became 0.4V and the voltage at the output of the OpAmp was 2.7V in spite of voltage quantity of the signal pulse (V_pulse = 3.3 or 0).

What was wrong? Is there any mosfet transistor (dip type) that will fit better for my goal? Or do they only work with high current? Is there a circuit that is better than what I want to make?

UPD: Is the schematics from this answer apropriate how do i create a current source? There are no transistor at all

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  • \$\begingroup\$ Are you using a JFET or a MOSFET there? The symbol and results from your sim make me think you've got a JFET instead of a MOSFET. \$\endgroup\$ – brhans Sep 28 '17 at 21:38
  • \$\begingroup\$ Yeah, I used JFET because it seems to me that all MOSFET are high current devices. \$\endgroup\$ – Jacob. St Sep 28 '17 at 21:41
  • \$\begingroup\$ Your 10k 'test' resistor and current sensing 200R are also far, far, far too high. 20mA through 200R needs 4V across the resistor. 20mA through 10k needs 200V ... \$\endgroup\$ – brhans Sep 28 '17 at 21:44
  • \$\begingroup\$ And there is another problem. The operational you use has a minimum operating voltage of 4 V. And it is not even rail-to-rail input/output (the common mode voltage must be 3V below V+ and 3V above V-). \$\endgroup\$ – next-hack Sep 28 '17 at 21:49
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MC4558 opamp isn't specified to run on 3.3V, minimum is 4V.

Its input and output are not rail to rail, so it won't work either. The opamp is used outside of its acceptable input and output ranges so it just won't work at all.

A RRIO opamp like this one would be a better choice. There are lots of cheap and nice CMOS rail to rail opamps from other manufacturers, pick your favorite!

If you have some 5V supply on hand, use this instead, you'll have a lot more headroom.

Now, your choice of 2SK212 JFET is... interesting. First, you have to pull the gate lower than 0V to turn it off, which the opamp can't do. And it won't do 8mA either.

Suggestion: drop the FET, use a proper opamp, and the schematic you linked will work well. The current-sensing resistor should be of a low enough value to ensure that with the max current you want, the drop on the resistor plus the Vf of the laser still leaves headroom for the opamp's output. With a 3V3 supply and a 2V5 laser you should put like 0.2V on the current sense resistor.

Why not just drive the laser from the 3V3 IO with a resistor? Unless you absolutely need a very precise current?...

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  • \$\begingroup\$ "Why not just drive the laser from the 3V3 IO with a resistor? Unless you absolutely need a very precise current?..." I don't know. Why do people use laser or led drivers? Laser that I have seems expensive. And it is used for laser spot detection system that is sensitive to change of spot's brightness. Your answer is kind of super. Thanks \$\endgroup\$ – Jacob. St Sep 28 '17 at 21:49
  • \$\begingroup\$ Ah OK if you need precise brightness then that explains the current source. You just need a rail to rail opamp. But opamps have offset voltage, so depending on its polarity it could be impossible to turn the laser completely off. The one I linked has 1mV typical, so on a 25R current sense resistor you'd get 40µA when off if the offset is of the wrong polarity. \$\endgroup\$ – peufeu Sep 28 '17 at 22:02
  • \$\begingroup\$ Сurrent-voltage characteristic shows that my laser needs current > 2mA to radiate... So I think 40µA is not big problem) Thank you very much \$\endgroup\$ – Jacob. St Sep 28 '17 at 22:08
  • \$\begingroup\$ Alright! Good luck then ;) \$\endgroup\$ – peufeu Sep 28 '17 at 22:10

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