Trouble With A Very Simple MOSFET Amplifier

**Repost from ABSE:

"/dev/phaeton"

Is the 2n7000 not the right choice for this circuit?

** No - it is not a " power mosfet".

Try something in a TO220 pack.

** It fails to work and the gate draws current.

....... Phil

FETs are voltage-driven, with capacitive gates and very little current flow. So, for enhancement mode MOSFETs, the gate should be a few volts "away" from the source (same polarity as the drain) to cause conduction between the source and drain. Find a 2N7000 spec sheet and look for the Vgs vs Id chart.

To test the biasing, remove the FET from the circuit and see what kind of voltage range you get on the gate circuit. You want something that puts the Ids in the right range for the voltage and resistance you already have - 9v, 30ohms, max 300mA, maybe something in the 200mA range or whatever your 2N7000 is good for. If you look at the Vgs/Ids chart, find something in the middle of the flat part of the curve (where small Vgs changes cause large Ids changes). I downloaded one, and it looks like anything from 4-8v gives you 400-1600 ma. To keep it under 300mA, you'd need 3.0 to 3.7 volts. Looks like you're shooting for 3.5 volts for the gate bias.

Use it as a switch. Put a LED/resistor combo off the drain to Vdd, source to Vss, and a 1k resistor on the gate. Resistor to Vdd, LED on. Resistor to Vss, LED off. The spec I downloaded had a simple test circuit in it.

Also, check for resistance between gate and the other pins. Should be essentially infinite.

That's not a power mosfet, although it should work for your demo if you don't let the smoke out. Something like an IRF510 would be more appropriate and you could get it at Radio Shack. You'll want a heatsink, which can just be some aluminum bolted to the tab. Beware that the tab is not tied to the source, it's tied to the drain.

The gate being 0V is strange. The whole point of the left column of resistors is to bias the gate so that the small AC voltage at the input falls in a nice linear range of the Ids vs Vgs curve. You can get a starting value from a datasheet, but I'm sure 0V is too low. Try turning it up with no input applied until the voltage at the drain drops below the battery voltage just a little. Then apply an input and see if you get a sound.

--
Ben Jackson AD7GD

http://www.ben.com/

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...

I'll try that tonight, *but* I'm also probably going to swing by Radio Shack and get an IRF510 (as suggested) to use instead of the 2n7000.

IIRC, I did that, and D to G was infinite, S to G was infinite, D to S was NOT infinite. I think it was in the 6MOhm ballpark, but I'm not sure.

really? I'm still a little fuzzy on the concept of 'power' and how much of it is needed to drive a speaker. What differentiates between a 'power' transistor and a 'signal' transistor, even if some of them run off the same voltage, and if some signal transistors can operate nearly rail-to-rail.

I'll hit one on the way home from work. I've been meaning to see if the local store is about to do the 'Everything 90% off' sale anyways, like some of them are doing now.

Right.. the 0V part was a red flag for me too... However, the left vertical column of resistors is NOT connected to the gate, as per the schematic in the book. It doesn't make sense to me either, so if it's wrong I wonder if the ink might have worn off that page there. I've biased BJTs this way so that's why I thought it was odd as well.

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).

Oh well.. let me get the right device in there first, then I'll measure more stuff.

Thanks for all the help and suggestions!

-phaeton

I don't have your image in front of me, but it *was* connected at the center pot connection (the wiper). In fact, since the pot is between two other resistances, you probably can't make 0V at the wiper at all.

Bad.

The Vgs to Id relationship is not linear, so the region you select will have an overall slope that will affect the gain. If you look at that graph, you'll see that the curve doesn't even start until Vgs(th), which on an IRF510 is somewhere between 2 and 4V. Below that, it's totally off. As you turn up the bias, you'll be class B initially (most of your input signal will, after biasing, fall below the threshold) and you'll conduct for more and more of your input signal as you turn up the bias. Eventually you'll have the entire input signal above the threshold voltage and be class A. If you keep going up (or your input is driven very hard), the gate voltage will reach the flat spot near saturation and your gain will go down and distortion will increase. Eventually you'll saturate the fet and let the smoke out (well, maybe not with a circuit based on a current limiting resistor). A lot of Class A, B and AB RF circuits using the IRF510 use a zener + 10R on the gate to absolutely prevent saturation (around 8V).

--
Ben Jackson AD7GD

http://www.ben.com/

In the case of MOSFET, a "power" mosfet just means it has a bunch of individual transistors all tied in parallel, so you can get bigger currents. The downside is the higher gate capacitance (i.e. slower switching times).

That's independent of the max Id current. You can have a small "power" mosfet, or a big signal mosfet.

As for the speaker, it's 8 ohms. If you want one watt of sound, you need about 1/R^2 amp (1/64 = 15mA). For reference, my subwoofers are about 1500 watts each (23 amps equivalent).

Also, just because a transistor is rated for N amps, doesn't mean it stops at N amps. It means it melts at N amps. So it's up to you to ensure that the circuit won't try to overload the transistor.

The circuit should have +9v - resistor - potentiometer - resistor - ground. ^ | gate

That way, like any old resistor divider, it gives you a range of voltages off the pot. The two fixed resistors can be chosen such that the pot's whole range covers the useful range of gate bias voltages.

Check the resistor wiring.

FET curves are non-linear. Depending on where on the curve you are, the slope varies. Since in an AC coupled amp, slope == gain, the gate bias determines the gain of the transistor.

This is a little milliwatt amp intended for a 9 V battery a 2N7000 is 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 . . . for that one needs a transformer, or resistor (or constant current source) replacing the transformer and a large electrolytic to couple to the speaker.

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The gate should have about one to four volts with respect to the source. Disconnect the gate and see what the gate divider is reading adjust for ~1+ volts. Reconnect to the gate and it should start pulling some current down through the speaker (AND YOU SHOULD STILL SHOW THE ONE VOLT ON THE GATE- or the device is fried) - one volts isn't carved in stone, but should put the 2N7000 in the linear region with a 9 volt supply.

If you download the data sheet for the 7000 you'll see the operating bias curves.

That should work with reasonable volume - increasing the current capability of the mosfet might not be a good idea. That design is called single ended Class A. Increasing the power supply and transistor current might put an unhealthy amount of DC on the speaker.

One can make some pretty good sounding simple single ended mosfet amps with a current limiter in the source (even a 3 terminal regulator will work) If you want more power check out Nelson Pass' site

"This is the second installment of a trilogy of construction projects centered around the performance obtainable from absolutely minimalist circuitry. Part one described the Zen amplifier, a 10 watt single-ended class A power amplifier using a single MOSFET gain stage."

The Zen amplifier looks like the Forest Mims circuit on steroids

It is fun to read about even if you don't build anything - if you do build ; it won't break the bank, will sound great (rivaling some "audiophile" stuff) and is simplicity itself.

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"default" "Phil Allison"

** Nope.

** A fragile 2N7000 may easily wind up damaged in that circuit.

Looks like the OP's already is.

A "power mosfet" is specified and wisely so.

......... Phil

2N7000 is good for 200 ma and 400 mw dissipation, should be plenty for 50 + milliwatts out with a suitable high impedance speaker and enough voltage drive - this ain't no stereo system.

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"DJ Delorie"

** Huh ????????????

What sort of twisted thinking is this ?

Power = I squared R.

So I = sq rt P / R

1 / sq rt 8 = 0.35 amps.

The bias current needed for 1 watt of audio into 8 ohms with a simple class A stage is actually 0.5 amps.

Signal current swings from 1 amp to zero giving an 8 volt swing.

....... Phil

"default"

** You are simply not following the point.

THAT particular circuit will allows a 2N7000 he mosfet to exceed it ratings !!

** You are an asinine, MORON.

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.

........ Phil

Sigh, I had I R squared. No wonder the results seemed incorrect. I'm blaming my arthritis. Yeah, arthritis, that's it.

Based on the diagnostics he's sent in so far, I think he just wired up the bias resistors wrong and the circuit just hasn't done anything yet.

He only want a few mW......

Don

And you may be missing the point - someone needs to go back and find out what speaker the original plans call for. I see the speaker as a problem - half amp DC continuous through the speaker is likely as not going to cause distortion (not that someone with an electric guitar might object to that). The speaker should be up around a 30-100 ohms.

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"default"

** You are an asinine, f****ng MORON.

Drop Dead.

........ Phil

Sorry that this caused such a controversy, everyone :-(

I guess I failed to note on my diagram that the speaker is 8 ohm.

Last night I stuck an IRF510 into the circuit instead of the 2n7000 (observing different pinout, of course). Now the speaker at least crackles a little bit when I touch parts of the circuit and/or plug in the guitar, so it's a little more alive. I still didn't get any guitar sound out of it, but the voltage measurements were much closer to what has been discussed here. It seems that turning the pot changes the drain and gate voltages and their relationship to each other. I moved it all around and got no sound out of it, still.

I did find that the cap was leaking, so I changed it. I measured the resistance of the speaker to test for continuity (7.5 ohms was what i read). I poked around the circuit with an audio probe* and got guitar signal on all 3 pins, with the drain being the loudest. In fact, i got it everywhere, including where the speaker connections are, just no sound out of the speaker. Ran out of time before I could change speakers, but if the speaker had continuity and was roughly in the expected range, why wouldn't it do something? :-P

I'll have to piddle with it s'more. Thanks for all the suggestions thusfar.

-Phaeton

• Audio Probe: essentially a piece of wire plugged into a known, working amplifier, isolated with a 2.2uf capacitor in series.

Don't pay any attention to Phyllis. She's on the rag, as usual.

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