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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Feb 2019, 7:44 pm   #1
Jolly 7
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Default Testing a dual gate mosfet

I recently received a batch of BF981 mosfets and need to establish that they are working.
I found the following instructions on the net,; can anyone please advise if they are valid tests for dual gate mosfets ? Thanks.

Firstly, measure the resistance between both gates (connect them together) and drain and source, the resistance should be near infinite, >100M.

Secondly connect both of the gates to the drain. Measure the resistance between the drain (meter positive) and the source (meter negative), it should be under 1k.

Finally connect the 5V supply's positive to the source and negative to the gates. Measure the source - drain resistance, it should be much higher than the previous measurement >1M.
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Old 13th Feb 2019, 10:44 pm   #2
Biggles
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Default Re: Testing a dual gate mosfet

Be wary of static discharge damage when testing. I would use an anti static mat to reduce the chance of damaging the gate while connecting up.
Alan.
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Old 13th Feb 2019, 11:58 pm   #3
Jolly 7
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Default Re: Testing a dual gate mosfet

Both sets of the mosfets arrived in ordinary plastic packets inside jiffy bags with no antistatic packaging whatsoever. Maybe they're already damaged on arrival.
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Old 14th Feb 2019, 12:11 am   #4
mickm3for
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Default Re: Testing a dual gate mosfet

Hi used in vhf amps has proct diodes on both g1 and g2 data sheet if any help
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Old 14th Feb 2019, 12:58 am   #5
Jolly 7
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Default Re: Testing a dual gate mosfet

I think the protection diodes for the BF981 make the tests I mentioned in post 1 invalid as none of my results match, apart from the 5V test which I haven't done yet.

I tested three different BF981 mosfet samples and all have very similar resistance readings as shown in the attached photo. Makes me think that they are fine after all.
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Old 14th Feb 2019, 2:21 am   #6
Terry_VK5TM
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Default Re: Testing a dual gate mosfet

Hi Joydip.

Perhaps time to think about building yourself some simple test equipment?

This one will do Dual Gate Mosfets, Fets and bipolar transistors.

http://makearadio.com/visitors/nick-fet-tester.php
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Old 14th Feb 2019, 5:56 pm   #7
Biggles
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Default Re: Testing a dual gate mosfet

The best way of checking them would of course be in an RF circuit. This would prove gain, noise etc but I appreciate not everybody has suitable radio equipment to hand to do that. I did hear once that some dual gate mosfets require a source resistor and some don't, with the source connected straight to negative or ground. I can't remember the type numbers but it was probably a comparison between the older ones (40673/3N201), and the "newcomers" (BF981). Hope that helps.
Alan.
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Old 14th Feb 2019, 6:47 pm   #8
Jolly 7
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Default Re: Testing a dual gate mosfet

@Terry: nice idea. Hope I can get around to building it someday soon.

@Biggles: I have a DIY 455 kHz signal generator with audio tone.
Would that be of any use ?
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Old 14th Feb 2019, 9:47 pm   #9
Biggles
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Default Re: Testing a dual gate mosfet

I think the 455KHz oscillator could be pressed into use if you have a scope to compare the input and output waveform levels to check gain. A lot of dual gate mosfets are designed to operate at VHF/UHF but it would be very tricky to test them dynamically at those sorts of frequencies without an appropriate test circuit and some good test gear. Substitution in the front end of a VHF receiver would be one way but of course that will mean soldering and possible cropping of leads to make them fit so the DC testing is going to be the quick and easy option.
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Old 15th Feb 2019, 1:27 am   #10
G0HZU_JMR
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Default Re: Testing a dual gate mosfet

For bit of nostalgia I had a rummage and managed to find my old FET tester. It looks an ugly mess but this thing is old... I made this in my student days as a quick and dirty tester for J310, 2N3819, 40673 and BF981 devices.

The tester contains a 7.68MHz crystal oscillator and the idea is that the FET under test is a buffer amplifier after the oscillator and the tester measure the Gm at 7.68MHz using a diode detector.

I can't remember how I calibrated it but it was probably with my old Tek 585 scope. With a 4 pin socket it could also test the classic 40673 MOSFET and a BF981 could be tested by placing/pressing it on top of the 4 test pins that are around the test socket. Not the best arrangement but it was quick and effective.

It was at its best when selecting J310 or 2N3819 JFETs for similar (or highest) Gm and it could also spot any duds really easily. I haven't used it for many years but it still has the little piece of paper inside with the FET pinouts.

It looks a mess but this was made in a hurry on a shoestring using salvaged parts from an old CB radio and an old tobacco tin
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Old 15th Feb 2019, 8:01 am   #11
MrBungle
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Default Re: Testing a dual gate mosfet

Interesting bit of kit. Similar to what I’m doing to establish fT on BJTs - made an amp fixture and check the small signal gain at 15MHz.

Also a flash back to my youth there as my father used to get through a metric ton of that tobacco.
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Old 16th Feb 2019, 12:55 pm   #12
G8HQP Dave
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Default Re: Testing a dual gate mosfet

Quote:
Originally Posted by Biggles
I did hear once that some dual gate mosfets require a source resistor and some don't, with the source connected straight to negative or ground. I can't remember the type numbers but it was probably a comparison between the older ones (40673/3N201), and the "newcomers" (BF981).
I read somewhere that 'old' and 'new' dual-gate MOSFETs needed different biasing, but in this respect I think BF981 are 'old' types like 40673 i.e. the details of optimum voltage might be slightly different, but they don't require the completely different bias of a 'new' type MOSFET. I am not sure if there was a change from depletion to enhancement types just before dual-gates disappeared altogether or if it was something else.
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Old 16th Feb 2019, 9:50 pm   #13
Biggles
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Default Re: Testing a dual gate mosfet

Most of the VHF front end circuits I have seen in the past (they may be different now) have the source strapped directly to 0 volts and the input tuned circuit directly between the g1 connection and 0 volts, indicating zero bias on g1. However the g2 connection always seems to have some bias on it provided by a resistive potential divider between +ve and 0 volts. Presumably the device is using g2 to provide bias in enhancement mode. To be honest I have never experimented much with these devices. It was always change them if suspect, when I used to repair receivers as a job. They actually rarely gave trouble.
Alan.
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