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Power Mosfet Biasing?

Started by will, May 14, 2004, 02:07:03 PM

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will

May 14, 2004, 02:07:03 PM
Hi,

Is there any secret to biasing a Power Mosfet? I just built a new experimental circuit. That uses a power Mosfet for the last stage. To hear if it sounds more like a pentode saturation alluded by RG. However I haven’t been able to get the last stage working.

I am using an IRF740 power Mosfet.  I am biasing the stage with a 500K trim pot decoupled with a 22uF cap and a 1M bias resistor to the gate. I used a 10K drain resistor and 1K emitter resistor bypassed with a 220 uF cap to max gain.

Observations: :(

I can adjust the bias from 0 to 9 volts, but the voltage really drops through the 1M bias resistor which seems odd as I thought Mosfet’s would not draw any current at the gate. Maybe it’s my volt meter.

No matter what the bias voltage is at the gate, the source voltage is 0 and the drain is at 9volts.

My thought is that I’ve installed it wrong. The data sheet says face up leads pointing to you. From Left to right the pins are Gate, Drain, Source.

Did I get a defective one? Or zap it? I know it will work with a BS170 or 2N7000.

Any Ideas?

Regards,
Will

gez

#1
May 14, 2004, 02:48:05 PM
The circuit you've described should work.  If you have 9V at the drain the MOSFET isn't on.

I don't have the data sheet in front of me, but you may well have the layout wrong, or an open circuit to the gate (maybe even grounded it by mistake?).
"They always say there's nothing new under the sun.  I think that that's a big copout..."  Wayne Shorter

R.G.

#2
May 15, 2004, 07:46:01 PM
There is no trick. You have the thing in with the wrong pinout or other problem. Check the pinout and try again.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

will

#3
May 15, 2004, 10:51:30 PM
Hi,

Thanks for your replies. I had a wiring error. :oops:  I put a 22uf cap that was to decouple the bias for the power mosfet at the wrong end of the 1Meg bias resistor. It sure dosent work that way.

The circuit sounds really good.  :D  I essentially built a modified blackface stage using a mosfet for the source follower with noiseless bias and I limited the gain of the bipolar transistor using a 100 ohm resistor in series with the emiter cap.  I tried Joe's idea of a Vulcan stage using a Fet after a gain pot and some tone shaping. Then went into a power mosfet.

It sounds very powerful and good. 8)

Got to go now.

Regards,
Will
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