mosfet biasing

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ed
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#16 Re: mosfet biasing

Post by ed »

pre65 wrote: Sat Aug 12, 2017 11:15 am
Nick wrote: Sat Aug 12, 2017 11:06 am

Phil, the problem isn;t the bias voltage changing, its the supply voltage.

Sorry, I thought the supply voltage change was affecting the bias.

My bad.
it is but it's not as simple as a SE bias circuit...if you look at the circuit you will see how crucial the balance is between the top and bottom of the out put pair.
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Ali Tait
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#17 Re: mosfet biasing

Post by Ali Tait »

ed wrote: Sat Aug 12, 2017 11:12 am
Nick wrote: Sat Aug 12, 2017 11:06 am The output devices will never be perfectly matched, so a change in Vds for each will lead to a non equal change in Ids so the DC offset will move.

exactly so!...I think some sort of servo mechanism could be rigged, I even thought of an avr tracker but at the end of the day I want to have a play with a regenerator anyway...it may give up secrets with the other amps I have.

It was very interesting comparing notes with other builders of this circuit. I have deduced that the US mains is more stable thatn ours and it was interesting to find that one other UK builder had exactly the same bias problems as me...He came from lincolnshire which is not to far away, which may or may not be relevent. I naively think that the variations in mains are localised to sub stations or even street cabling and connectivity. Maybe Ali has an angle on this.

I do know that since the underground was relaid in our street(The leccy man said the previous line was getting too hot) We've had higher voltages.
The grid transformer at your local supply point will tap up and down automatically as demand varies, so yes the voltage will wander. Depending on what industry is around you, some places will obviously vary more than others. There is not a lot close to me, so I'm fortunate to have pretty stable mains around 240.

Coincidentally I'm currently installing two new 90MVA transformers at a substation which are replacing two 60 odd year old 45MVA. Lot of this going on due to end of life cycle and so many wind-farms coming on line.
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#18 Re: mosfet biasing

Post by Nick »

On a similar subject, I saw this the other day, which is a good thing IMHO,

http://www.linearsystems.com/lsnews/lsj689.html
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#19 Re: mosfet biasing

Post by jack »

Nick wrote: Sat Aug 12, 2017 10:37 pm On a similar subject, I saw this the other day, which is a good thing IMHO,

http://www.linearsystems.com/lsnews/lsj689.html
I noticed that some of their JFETs appeared in the native libraries of LTspice a while ago... nice devices...
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#20 Re: mosfet biasing

Post by IslandPink »

Low capacitance - nice !
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#21 Re: mosfet biasing

Post by ed »

this may alter the playing field if we're lucky:

https://www.powersystemsdesign.com/arti ... on/8/11932
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#22 Re: mosfet biasing

Post by ed »

Nick wrote: Sat Aug 12, 2017 10:37 pm On a similar subject, I saw this the other day, which is a good thing IMHO,

http://www.linearsystems.com/lsnews/lsj689.html
or this:
http://www.exicon.info/PDFs/ecx10n20.pdf
and its compliment
http://www.exicon.info/PDFs/ecx10p20.pdf

these are very tempting to try at the current price...
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