Method to set FET bias and supply voltage

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martin manning
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Method to set FET bias and supply voltage

Post by martin manning »

This technique using batteries to set FET bias can be done before the FET board is installed in the amp.

Some info and discussion around measurements taken from a HAD amp has led to a revised and expanded FET bias procedure. Two options and two techniques are now described: One using two 9V batteries for the center-biased, clean boost configuration as originally described here, and an alternate procedure using B+ power that can be used to configure the FET for clean boost or a lower voltage, off-center biased version which mimics the HAD-built amp. Waveforms from simulations have been added to show the effect on headroom and distortion levels. The DIY eyelet board version is used for the illustrations since the PCB shown previously is no longer available.

More discussion regarding FET bias here: https://ampgarage.com/forum/viewtopic.php?f=4&t=34108
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Last edited by martin manning on Sun Aug 22, 2021 1:21 pm, edited 10 times in total.
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Re: Method to set FET bias and supply voltage

Post by amplifiednation »

Awesome Martin! I never would have thought to link two 9v batteries together!!
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Re: Method to set FET bias and supply voltage

Post by Drumslinger »

excellent, thanks Martin.
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Re: Method to set FET bias and supply voltage

Post by amplifiednation »

After rigging up some 9V battery terminals I was able to get this to work.

Where do you get the 22k value?

I changed the 22k to 8.2k, (keeping the 2K drain) and got 18.3V down to 9.7V.

How close to 50% should I be shooting for?
Last edited by amplifiednation on Tue Jun 19, 2012 1:38 am, edited 1 time in total.
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Re: Method to set FET bias and supply voltage

Post by martin manning »

The 22k is the lower leg of a voltage divider with the 150k. The drain is the 10k and the source is ~2k. The idea is to force 18V to appear at the top of the drain (22k/10k junction) using the batteries and set the source resistor to get 9V at the FET drain. Then tweak the 22k to get about 20V at the top of the drain R with the B+ hooked up and the bias will still be good. The thing is when you change the bias the idle current changes and the voltage drop across the 150k changes a lot.
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Re: Method to set FET bias and supply voltage

Post by amplifiednation »

Alright I had a misread with one of the leads, I didn't solder in the resistors so i lost continuity of Rs, sorry for my confused post.

So here we go...this is with an 8.2k in place of the 22k..

Rs 2.19k * top=18.2v bottom=9.6v
Rs 1.80k * top=18.19v bottom = 8.2v

then i found a 1% Dale 2K in my drawer

Rs 2.00k * top=18.16v bottom=8.96v

So I think we have a winner!
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Re: Method to set FET bias and supply voltage

Post by amplifiednation »

and Martin, this is true genius my friend!! i can't believe how easy this was to do with your help!! (assuming i got it right??). thank you!
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Re: Method to set FET bias and supply voltage

Post by martin manning »

OK cool. I think you will find that you will need about 22k in the divider to get the 20V you want to see at the top of Rdrain when you power it with the B+5. Check the voltage on both sides to see if you still drop about half.
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Re: Method to set FET bias and supply voltage

Post by amplifiednation »

Awesome!! Thanks again for the great instructions!

TC
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Re: Method to set FET bias and supply voltage

Post by vibratoking »

This is a great tip for those that don't understand fully how to bias the FET. One small comment if I may. The drawing of the test set shows four resistors installed, but does not show that the FET should be installed. It should be, correct? Without it, there is no real current path through Rd and Rs. I suspect there may be some confused questions as a result.
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Re: Method to set FET bias and supply voltage

Post by martin manning »

vibratoking wrote:This is a great tip for those that don't understand fully how to bias the FET. One small comment if I may. The drawing of the test set shows four resistors installed, but does not show that the FET should be installed. It should be, correct? Without it, there is no real current path through Rd and Rs. I suspect there may be some confused questions as a result.
OK, there (updated .pdf)... happy now? ;^)

I was thinking just show the pertinent parts, but in fact you are right, the board should be more complete, especially the input resistors so that the gate is ground-referenced.
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Re: Method to set FET bias and supply voltage

Post by ChrisM »

Adjusted the 150K voltage divider to get 18V at the top of the drain resistor. Ended up a 150K/10K voltage divider.

I would normally keep the source resistor constant and vary the drain resistor to get half the supply voltage at the drain. You see this on many designs with the use of trimmers on the drains. So your method seems a bit odd to me.

Changing the source resistor changes the gain and frequency response of the stage quite a bit.

That said my FET isn't working right now. That's why I'm bring this thread up. Got a 8K2 source resistor and 18V supply. Gotta adjust the drain resistor now, hopefully that does it.
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martin manning
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Re: Method to set FET bias and supply voltage

Post by martin manning »

You won't have an 18V supply after you get the FET biased correctly since its current draw strongly affects the voltage drop from B+5. This is the advantage of the battery method- the Vcc is held constant while the DC operating pouint is dialed-in. 10k is a good load line for this FET. Choosing that first and then adjusting the source resistor to center the operating point on the load line is not at all odd.

I agree that some attention should be paid to the low-end frequency response after the DC operating point has been found, so thanks for bringing it up!

The 2N4416 that many people are using seems to want a ~2k Rs, and that in combination with the 4.7u bypass cap puts the boost -3dB from its peak at about 77Hz,. If an 8k8/4.7uF combination was used in the HAD ODS, then taking the bypass cap up to 22uF is appropriate. That puts the -3dB dB gain at about 17Hz.

I'll add a note about this to the .pdf.
Last edited by martin manning on Sat Dec 01, 2012 12:39 am, edited 1 time in total.
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ChrisM
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Re: Method to set FET bias and supply voltage

Post by ChrisM »

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.
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Re: Method to set FET bias and supply voltage

Post by bluesfendermanblues »

Brilliant adjustment instruction, Martin!


Neat trick using the "backward" battery in order to get +/- 9v 8) Never entered my mind :idea:
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