Method to set FET bias and supply voltage

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martin manning
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Re: Method to set FET bias and supply voltage

Post by martin manning »

Sorry guys, I was not thinking straight when I posted above about the revised value for the Rs bypass cap. It should be increased to 22uF for a 2k Rs. It's corrected in the post and the .pdf now.
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ChrisM
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Re: Method to set FET bias and supply voltage

Post by ChrisM »

Hey Martin,
So with a NTE452 I ended up using a 150K/10K divider.

8K2 source resistor and a 24K drain resistor. I'm getting lot's of "swing" across that drain resistor.

V+ is 16V and the Vd is 8V.
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martin manning
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Re: Method to set FET bias and supply voltage

Post by martin manning »

So it's working now? If it were me I'd go back to the standard 10k Rd load, and find the Rs to get V/Vd = 2 using 18VDC battery. THEN mess with the divider to get about 20V at the top of the drain resistor.
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ChrisM
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Re: Method to set FET bias and supply voltage

Post by ChrisM »

martin manning wrote:So it's working now? If it were me I'd go back to the standard 10k Rd load, and find the Rs to get V/Vd = 2 using 18VDC battery. THEN mess with the divider to get about 20V at the top of the drain resistor.
Yep, working great now.

The board I got was hand etched by a friend of mine. There was a a hairline trace between the input and ground.

Before getting into amps I spent many years building pedals. I've never heard of adjusting the source resistor to bias so it seems odd to me. I am used to seeing trimmers on the drain. Check out Run Off Groove for amp emulation with FETs, you'll see the biasing scheme commonly used. http://www.runoffgroove.com/fetzervalve.html This article is all about biasing a single stage common source FET.
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martin manning
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Re: Method to set FET bias and supply voltage

Post by martin manning »

Interesting article, thanks for the link. The Fetzer Valve described there has trimmers on both Rs and Rd. Not necessarily tryng to emulate a vacuum triode here, though... You can certainly find a DC op point (Id) that drops half of the supply voltage across Rd either way, but you will get different gain and distortion characteristics. Gain goes like (gm Rd)/(1+gm Rs), so you will get higher gain with the larger Rd. You'll get more headroom if you bring the voltage at the top of the drain resistor up a bit more, to say 20V.
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rogb
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Re: Method to set FET bias and supply voltage

Post by rogb »

Thanks Martin, I have just finished my 50w #102 and I used your method to set the bias, 9.5v on 18vDC. 20.5v on Rd from the B+.

It sounds really good, I have never used an FET input before so I really appreciate the set up notes.

Regards
Rog
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Structo
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Re: Method to set FET bias and supply voltage

Post by Structo »

I made this a Sticky. :D
Tom

Don't let that smoke out!
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glasman
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Re: Method to set FET bias and supply voltage

Post by glasman »

ChrisM wrote:I am using the NTE452. I have two of them, one is in another build with stock values and working fine but this other NTE452 isn't passing signal.

I'll have to try your biasing method tonight.
With some FET's the 3M3 resistor is too large, if your FET isn't passing gas, try dropping it to a 2M.

Old thread, but thought the information was relevant.
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martin manning
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Re: Method to set FET bias and supply voltage

Post by martin manning »

An updated and expanded procedure is now in the first post.
rootz
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Re: Method to set FET bias and supply voltage

Post by rootz »

Great update Martin. I'm going to experiment with the FET more and your guide gave me some new insights.
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Re: Method to set FET bias and supply voltage

Post by Stephen1966 »

Hi Martin,

Is there any reason we can't leave a trimpot permanently in place of the Rs after biasing to 50% using the battery method? I don't have a B+ power supply yet, but by the stage that's ready the FET board will be harder to access. In other words, is it usually necessary to bias the source resistor again after the supply is switched over to B+?

Stephen
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martin manning
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Re: Method to set FET bias and supply voltage

Post by martin manning »

Stephen1966 wrote: Fri Aug 20, 2021 9:27 amIs there any reason we can't leave a trimpot permanently in place of the Rs after biasing to 50% using the battery method?
You can do that and use it to experiment with different bias points.
Stephen1966 wrote: Fri Aug 20, 2021 9:27 amI don't have a B+ power supply yet, but by the stage that's ready the FET board will be harder to access. In other words, is it usually necessary to bias the source resistor again after the supply is switched over to B+?
If you arrive at more or less the same Vdd (supply voltage at the FET drain resistor) when powered by the HV supply then the bias setting will be good. If not, then either the Rs or the voltage divider could be adjusted. I had planned to provide a trimmer for both the Rs and the voltage divider at one point, but when the first PCB set became available I went with that.
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Re: Method to set FET bias and supply voltage

Post by Stephen1966 »

Super, thank you. The circuit as built, seems to be asking for a source resistor about 3.8k but the closest available is 3.9k which drops my bias voltage to just slightly less than 50% - pretty close but I like the idea of experimenting with the bias and I have a 5k trimmer on hand. I recall you saying that the Vdd is more or less independent of the bias but if I were to build this circuit again, I would use a trimmer there as well, or at least eyelets on the pcb I designed.

Stephen
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