Trouble With A Very Simple MOSFET Amplifier

[John O'Flaherty]

That's what I had thought. Jameco lists them as a 'power mosfet', but
I immediately noticed the case style. I looked at the datasheet they
supplied (which turns out to be from the wrong manufacturer anyways),
and since it was rated for "0.83W max" I figured maybe it didn't need a
thermal plate built in. (like the LM386 chipamp- no heatsink there for
500mW)....

So actually, this device is more in the 'small signal' neighbourhood
like the J201, BS170, etc, instead of anything you'd associate the word
"power" to?

That silly Jameco. Always pullin pranks... Anyways...

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}

Turning the pot has no effect on the reading I get at the Gate.
Furthermore, Forrest noted the pot as being a gain control, not a bias
control (the opposite of what I would have guessed by looking at it).

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.

 

[default]

The OP's posted circuit is THE ONE at issue here !!!

Is has a 8 ohm speaker with a 9 volt supply, so a current of over 1 amp is
possible.

Dissipation can be as high as 4.5 volts times 0.56 amps = 2.5 watts.

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.

 

[phaeton]

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

 

[Phil Allison]

"default"


( snip more absurd drivel)


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

The MOST basic rule in all design engineering.




.......... Phil

 

[Michael A. Terrell]

"default"

( snip more absurd drivel)

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

The MOST basic rule in all design engineering.

......... Phil

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


--
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

 

[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

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.

 

[Ken Smith]

probably good. A single ended class A with the speaker in the circuit
is not going to want a high DC offset on the speaker

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.