Mosfet bias.

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pre65
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#1 Mosfet bias.

Post by pre65 »

I need a 4.7v bias for the mosfet amp.

Is that + 4.7v ?

I was thinking of using a 4.7v zener diode, but I see there are several through hole types. What level of current (ma) is involved ?

OR, an LM329 fixed voltage ref (6.9v) and a trim pot ?
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#2 Re: Mosfet bias.

Post by Nick »

Is that + 4.7v ?
Hint: Have you noticed they don’t make -9v and +9v battery’s, just 9v ones?
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#3 Re: Mosfet bias.

Post by pre65 »

OK. :)

I was meaning is the bias positive, as opposed to grid bias on a valve which is negative.

I'll go for the 1.3W Zener unless otherwise advised. :wink:
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#4 Re: Mosfet bias.

Post by pre65 »

Would something like this (see link) be suitable for mosfet bias duties, with (for example) a 6v PSU (wall wart ?) instead of the solar panel.

http://www.theorycircuit.com/solar-powe ... r-circuit/

Or am I worrying too much about this ? :)
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#5 Re: Mosfet bias.

Post by Nick »

You want as low noise as possible, and I don't know if it need to be adjustable. I haven’t read the info on the amp design. I would (personally) avoid wall warts.
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#6 Re: Mosfet bias.

Post by pre65 »

Thanks Nick. I think there are a couple of 6v transformers in a box somewhere.

Does the bias supply (fixed @ 4.7v) have much current ?

I've tried searching on the web, and checking the IXFN140N20P data sheet but I can't seem to see any figures.

http://docs-europe.electrocomponents.co ... a3b4bf.pdf
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#7 Re: Mosfet bias.

Post by Nick »

Does the bias supply (fixed @ 4.7v) have much current
No.
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#8 Re: Mosfet bias.

Post by Mike H »

Also the actual bias might not be exactly 4.7V. Most obvious method is set it with a preset? There won't be any DC flow thru the gate.
 
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#9 Re: Mosfet bias.

Post by pre65 »

Mike H wrote:Also the actual bias might not be exactly 4.7V. Most obvious method is set it with a preset? There won't be any DC flow thru the gate.
I'm reading what you have written, but not sure what you mean. :oops:
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#10 Re: Mosfet bias.

Post by SimonC »

A nice lead in to the basics of mosfet amps http://www.electronics-tutorials.ws/amp ... ifier.html
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#11 Re: Mosfet bias.

Post by Mike H »

pre65 wrote:
Mike H wrote:Also the actual bias might not be exactly 4.7V. Most obvious method is set it with a preset? There won't be any DC flow thru the gate.
I'm reading what you have written, but not sure what you mean. :oops:
Sorry, I meant preset is simple and easy to do as there is no current flowing through the gate, so the preset only has to provide a Voltage.

However the final circuit configuration may make this method completely irrelevant; what is this MOSFET being used for?
 
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#12 Re: Mosfet bias.

Post by pre65 »

This. There is a thread in projects. It's a single ended mosfet, the mosfet is the size of an ice hockey puck.

Image
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#13 Re: Mosfet bias.

Post by Mike H »

A-HA!! Now I see. :D

OK so where it says '4.7V' at the bottom, could still be from a preset, with say 12VDC across it. Would provide a means of adjusting the ouptut DC Volts.
 
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#14 Re: Mosfet bias.

Post by pre65 »

Yes.

I'm not au fait with mosfets (or transistors) so this is a steep learning curve.

The 4.7v zener idea appealed to me as a fit and forget option. :)

Any alternative suggestions welcome. :wink:
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#15 Re: Mosfet bias.

Post by Mike H »

pre65 wrote: The 4.7v zener idea appealed to me as a fit and forget option. :)
Understandaby, and would be great IF the actual MOSFET gate bias Voltage matches exactly.
 
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