BF981 mosfet lead identification - UK Vintage Radio Repair and Restoration Discussion Forum

Jolly 7 · 1st Feb 2019, 10:26 pm

I am trying to identify the leads of a BF981 correctly. The longest lead is the drain and easy to identify. But all of the legs seem to have notches, making it difficult to identify the source. Therefore I've taken a picture and marked out the resistance readings between adjacent leads. Can anyone please help me identify source, g1 and g2 ?

Nymrod121 · 1st Feb 2019, 11:14 pm

https://www.retro.co.za/zs1ke/TenTec/BF981.pdf

Note the package diagram comment re. 'type number marking' which should help you orientate the leadouts OK.
Best wishes
Guy

Philips210 · 2nd Feb 2019, 12:17 am

Hi.

As far as I recall, there are two possible combinations of leadouts for the BF981 dual gate MOSFET. I've seen three possible package outlines with different lead notches. It would seem G1 is always opposite the drain. The source and G2 leadouts can be opposite positions. The source always appears to have a distinctive notch in its leadout. So, I would say the correct leadout pattern for your device is in the attached pic.
The other possible arrangement would be for G2 and S to be reversed.

Regards,
Symon.

Jolly 7 · 2nd Feb 2019, 12:50 am

Thanks Symon and Guy. What got me confused was the package variation, which you have now confirmed. I am trying to construct something from a circuit diagram and the pinout showed there did not appear to match the datasheet diagrams. Funny that the datasheet does not seem to mention these variations. Very confusing for a first timer.

Diabolical Artificer · 3rd Feb 2019, 8:04 am

"Very confusing for a first timer." Know what you mean, I used to get confused by resistor and capacitor markings and still frequently get bamboozled by tranny pinouts but the more you use stuff the less intimadating it becomes.

Will the BF981 not have a body diode to help differentiale between S & D?

Andy.

Jon_G4MDC · 3rd Feb 2019, 1:07 pm

Agree with that.

On all I have seen when the ID mark is upper most and the drain has been found then S, G1 and G2 run around in a clockwise direction.

The extra tab on source lead makes it easy.

1st Feb 2019, 10:26 pm	#1
Jolly 7 Octode Join Date: Feb 2018 Location: Southampton, Hampshire, UK. Posts: 1,063	BF981 mosfet lead identification I am trying to identify the leads of a BF981 correctly. The longest lead is the drain and easy to identify. But all of the legs seem to have notches, making it difficult to identify the source. Therefore I've taken a picture and marked out the resistance readings between adjacent leads. Can anyone please help me identify source, g1 and g2 ? Attached Thumbnails

1st Feb 2019, 11:14 pm	#2
Nymrod121 Nonode Join Date: Apr 2012 Location: Worcestershire, UK. Posts: 2,052	Re: BF981 mosfet lead identification https://www.retro.co.za/zs1ke/TenTec/BF981.pdf Note the package diagram comment re. 'type number marking' which should help you orientate the leadouts OK. Best wishes Guy

2nd Feb 2019, 12:17 am	#3
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,579	Re: BF981 mosfet lead identification Hi. As far as I recall, there are two possible combinations of leadouts for the BF981 dual gate MOSFET. I've seen three possible package outlines with different lead notches. It would seem G1 is always opposite the drain. The source and G2 leadouts can be opposite positions. The source always appears to have a distinctive notch in its leadout. So, I would say the correct leadout pattern for your device is in the attached pic. The other possible arrangement would be for G2 and S to be reversed. Regards, Symon. Attached Thumbnails

2nd Feb 2019, 12:50 am	#4
Jolly 7 Octode Join Date: Feb 2018 Location: Southampton, Hampshire, UK. Posts: 1,063	Re: BF981 mosfet lead identification Thanks Symon and Guy. What got me confused was the package variation, which you have now confirmed. I am trying to construct something from a circuit diagram and the pinout showed there did not appear to match the datasheet diagrams. Funny that the datasheet does not seem to mention these variations. Very confusing for a first timer.

3rd Feb 2019, 8:04 am	#5
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,669	Re: BF981 mosfet lead identification "Very confusing for a first timer." Know what you mean, I used to get confused by resistor and capacitor markings and still frequently get bamboozled by tranny pinouts but the more you use stuff the less intimadating it becomes. Will the BF981 not have a body diode to help differentiale between S & D? Andy. __________________ Curiosity hasn't killed this cat...so far.

3rd Feb 2019, 1:07 pm	#6
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: BF981 mosfet lead identification Agree with that. On all I have seen when the ID mark is upper most and the drain has been found then S, G1 and G2 run around in a clockwise direction. The extra tab on source lead makes it easy. Attached Thumbnails