I dont work much with mosfets so I dont really know how to go about this.


simulate this circuit – Schematic created using CircuitLab

So pretty much I have the circuit above, What's happening is that the Source of the mosfet is outputing around 3.3V and not close to 5V. I understand there will be resistance as RDS(on) however its 0.200ohms. Shouldn't make it 3.3V.



simulate this circuit

So I tried this configuration, what happens now is that:

  1. I am still not getting ~5V at the Drain

  2. The mosfet gets real hot

  3. Now the gate voltage is not even 5V, but once I take out the mosfet, 5V returns back at the output of the comparator.

  • \$\begingroup\$ If you edit the circuit to use less horizontal space, it will probably be easier to read in the preview image in your post. \$\endgroup\$ – The Photon Oct 26 '18 at 22:45
  • \$\begingroup\$ I edited the schematic because I forgot the pull up resistor. \$\endgroup\$ – Pllsz Oct 26 '18 at 22:46
  • \$\begingroup\$ Your second circuit is making the MOSFET try to short +5V to ground. \$\endgroup\$ – TimWescott Oct 27 '18 at 3:16

In order to turn on fully, the gate to source voltage as to be above some threshold. When you have an enhancement-mode MOSFET wired as a source follower (like that one), the source will have to be below the gate.

You could get the effect that I think you want by using a P-channel FET and reversing the sense of the MCP6548.

| improve this answer | |
  • \$\begingroup\$ I dont understand what is that though. Did I do something wrong or its the limitations of my mosfet \$\endgroup\$ – Pllsz Oct 26 '18 at 22:45
  • 1
    \$\begingroup\$ It's a limitation of nearly all MOSFET's. That's just how they work, other than depletion mode devices, which are only slightly more common than hen's teeth. The gate voltage simply has to be higher than the source voltage for the FET to conduct -- it's how the device works. \$\endgroup\$ – TimWescott Oct 26 '18 at 22:53
  • \$\begingroup\$ Is there a better way to essentially turn on the op amp \$\endgroup\$ – Pllsz Oct 26 '18 at 22:59
  • \$\begingroup\$ Well, you could take my suggestion about a P-channel MOSFET! Or use a PNP junction transistor and a suitable base resistor. Why the resistance to a P-channel FET? \$\endgroup\$ – TimWescott Oct 27 '18 at 3:13
  • \$\begingroup\$ I am just curious how does a PNP work better than a N \$\endgroup\$ – Pllsz Oct 27 '18 at 6:13

MCP6548 has an open drain output. So it can only pull the gate of M low, it can't drive a high voltage onto the gate.

In addtion, RFP12N10 has a Vgs(th) of 1 to 2 V, so even if you do pull the gate to 5 V, in order for current to flow through the channel the source will be at 4 V or lower.

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  • \$\begingroup\$ That makes one wonder why the circuit works at all! \$\endgroup\$ – TimWescott Oct 26 '18 at 22:43
  • \$\begingroup\$ Interesting, I wouldnt expect it to go that low. How do people go around this problem then? \$\endgroup\$ – Pllsz Oct 26 '18 at 22:45
  • \$\begingroup\$ @Pllsz. you could use a PMOS as in Tim's answer. Or use a dedicated power switch device. Or boost the gate drive voltage above 5 V. Or use the NMOS as a low-side switch instead of high side. Or... \$\endgroup\$ – The Photon Oct 26 '18 at 22:49
  • \$\begingroup\$ Oh no, isn't complete I am just stuck at this stage The purpose right now, is to essentially "sense" specific voltage peaks of an audio signal and amplify when the threshold is to low, and if its low amplify it and if its not low I was going to have a buffer just to pass on the signal. The N channel was to be a gate if the signal is low and needs amplification and P channel for when the signal doesnt need amplification After this stage was going to be a pre-amp stage with a series of filters to signal condition. \$\endgroup\$ – Pllsz Oct 26 '18 at 22:49
  • \$\begingroup\$ I forgot to mention the MOSFET gets hot over quite some time, and I dont understand why this MOSFEt isn't doing what its suppose to do? In the datasheet it says RDS(on) 0.200 when VG = 5V? Thought this was a logic mosfet and what if I change it from source to the drain? and how can a PNP help out? \$\endgroup\$ – Pllsz Oct 26 '18 at 22:51

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