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Trouble With A Very Simple MOSFET Amplifier

With a 22 ohm drain resistor and 8 ohms dc resistance in the speaker, the max power in the FET would be at 4.5V drop and 4.5V/(30 ohm) current, or 150mA. That would give you a max dissipation of 0.675W. The transistor might have needed a heat sink, but it should have been alright. To get maximum symmetrical swing out of this circuit, look at the extremes of full off and full on. When the FET is full on, the speaker voltage is 8/(22+8)*9V, or 2.4V. That means the speaker output should be biased for 1.2V This would be at 150mA where the transistor dissipation is .675W. Then, any audio you put into it only decreases the dissipation.

{snipped}

It does affect the bias, though. With a different transistor, you might need to change R1 or R3 to accommodate a different cutoff voltage for the FET. If you can adjust those resistors for 1.2V output at the speaker, it will just _have_ to amplify.

-- John

Reply to
John O'Flaherty
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Really? I didn't see the speaker impedance listed. With a half amp of DC on a speaker that circuit wouldn't fly - at all or very well, and a small fractional watt speaker would just smoke.

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Reply to
default

Well, ok...

Turns out it *does* work with the IRF510. However, it's just not powerful enough to drive a speaker. After switching a few speakers around (went from 8ohm to 4ohm) i got brave and plugged a set of headphones into it. It amplifies, but it's only 'audible' with headphones. I put a single BJT booster stage in front of it, and while it distorted a little bit, it didn't really get any louder, so it's a 'headroom' thing, i presume.

Would increasing the supply voltage from 9V up to say, 12V or 18V give it more 'power', or should I look elsewhere? Will higher voltages change the biasing requirements?

I expected this to be roughly the same volume level as the LM386 1/2W chipamp (a fun little device, IMHO). Is that about right?

Thanks.

-Phaeton

Reply to
phaeton

"default"

( snip more absurd drivel)

** This f****ng PITA IDIOT had not learned about "Murphy's Law" .

The MOST basic rule in all design engineering.

......... Phil

Reply to
Phil Allison

You're right. Phyllis will show up to disrupt a thread with her foul mouth, at the most inopportune time. :(

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Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

There are much better designs than that one. You can't just keep pushing more DC current through a speaker and expect it to sound good.

In addition to the LM386, National has several higher power amps that aren't too hard to apply, and at least one or two TO220 case five lead power op amps with pretty good specs (lots of gain and power output).

I believe a one or two transistor device could easily amplify your 100 mv signal - but with single ended class A you need to match the impedance to the speaker better (like with a stepdown transformer - which also keeps the DC off it) or at least a design that uses a coupling cap to keep DC off the speaker.

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In article , default wrote: [....]

Many a speaker has been operated with a DC bias. At low powers it doesn't matter much. I'd reverse the 22 Ohm and the speaker. That way the cicuit would not depend on the DC resistance of the speaker. You'd give up a fair bit of gain doing this unless you add a bypass capacitor.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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