Previous blogs:
Experimenting with MOSFETS: Total Gate Charge Experimenting with MOSFETs: Transfer Characteristic
Introduction
More experimenting with MOSFETs. In the last blog I looked at the transfer characteristics but it wasn't very easy to see from the graph I plotted where the threshold was. The threshold is measured at a drain current of 250uA and that is down in the noise for the current probe I was using. So this is a different approach to finding the threshold voltage.
Circuit
Here's the circuit I ended up with.
The current through the MOSFET is turned into a voltage by the source resistor. That gets compared to a voltage derived from a voltage reference and the op amp drives the gate in such a way as to make them match. Once I've adjusted it for 250uA flowing in the drain, the gate voltage (relative to the source) is the Vgs threshold voltage. [In case it's not obvious, I'm copying the constant-current load circuit that has been the subject of several blogs on the site here.]
Here's the circuit built on a small piece of pcb material.
The op amp I used was an OP177. It's a precision op amp and maybe a bit 'over the top' for a simple experiment like this but it was close to hand. Although all the specs are for +-15V operation, it works fine for this on a single supply of 12V. The 47nF decoupling capacitor is underneath the package in case you're wondering why you can't see it. The gate resistor was simply being used to span the gap between the op amp output and the gate's pad on the board - it's not necessary for the operation. I used a 15 ohm resistor, but only because I had plenty (it's an odd value that I don't seem to design with much). The voltage reference I used was an LT1009. It's far too good for this but it was lying around on the bench so it got thrown into the pot. I didn't bother with any additional compensation for the op amp.
First Test
I set the current by adjusting the preset potentiometer whilst looking at the drain current with a bench multimeter. Once I'd set it to 250uA, I got these readings for an IRL730 part - a threshold voltage of a fraction over 3V.
One thing I noticed a bit too late on in this is that their spec is measured for the case where the drain voltage is the same as the gate voltage. How much difference that makes I don't know. It might be simply to keep the dissipation down as the threshold voltage is temperature dependent.
More Tests
Having got this working, I thought I'd have a go at measuring a group of parts and see what the spread on them was. I don't have many IRL730 devices, but I did find an old tube with 17 STP55NF06LSTP55NF06L parts in it, so I tried those. The datasheet I've got gives a minimum of 1V and a typical of 1.7V (they are billed as being 'logic level' parts); it doesn't give a maximum figure.
This chart shows the Vgs for each of the 17 parts sorted by ascending voltage. Ten of the parts cluster around 1.8, the rest have values up towards 2V. Not quite as good as their typical value suggests but the spread is only 0.2V. Doesn't look like there's a very high chance of getting a 1V part.
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