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Old 28th April 2006, 09:07 PM   #1
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Default Single mosfet amp

Hello everyone.
I don't know if this is the correct forum or not to post this, but here goes.

I'm more at ease with tubes, having built a few. So, this is my first excursion at SS amps, though includinga a tube.

I had this circuit in mind, using a battery tube that happily "sings" at 12V. Unfortunately, using it in a single-ended configuration, the estimated power output is a mere 3mW.
As such, i remembered using the tube as a voltage amplifier and a mosfet or similar as the current source. Of course, less than a watt is fine by me, it's more an exercise to learn.

Please, check if thie schematic is ok.

Thank you very much
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Old 28th April 2006, 10:09 PM   #2
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Forgot to tell that the tube stage puts out 10VRMS, so probably running the mosfet at 1,5A is enough, no?
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Old 28th April 2006, 10:16 PM   #3
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Yes this will work, but remember to AC couple it to the load since you'll have an offset at the MOSFET's output. Also an IRF610 isn't going to last long at that dissipation you are talking about. Try an IRF540 if it must be in a TO220 package.
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Old 29th April 2006, 04:16 AM   #4
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If possible, you might include the mosfet inside a NFB loop to help compensate for the non-linearities from the mosfet. Hexfets(like some of the IRF types) are quite non-linear at lower currents, and the capacitances inside the mosfet(Cgd, Cgs) change with operating point but certainly not in a linear fashion. A driver for the Fet is another idea. I'm not so familiar with tubes, but driving the Fet at higher frequencies will be more like driving a capacitor, so if the tube isn't up to that challenge, a driver would effectivly reduce this capacitance that the tube stage sees. Keep in mind that you have to drive the gate voltage up, and then pull it down to turn off the device....like a totem pole configuration or something similar.
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Old 29th April 2006, 10:33 AM   #5
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The tube voltage amplifier runs at aproximately 3mA, probably just enough to run the mosfet. Don't the capacitances change the low frequency response, not the high? Maybe a opamp buffer stage before the mosfet will do the trick is necessary.
I think i got the biasing wrong, right? The 4V are only enough to turn on the fet, from what i undestand; so i would have to bias it to 4V + 6Vrms to properly swing the input votage up and down..
Many thanks for your input and knowledge

Take care
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Old 29th April 2006, 01:18 PM   #6
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Default Generalisations...

A couple of things that are similar about a FET and a tube are:
1. The output current (Id) is related to the gate-source voltage.
2. There is virtually no dc current into the gate

Not so similar:
3. The output resistance (dVds/dId) id is much higher for a FET
4. The maximum useful output current and power dissipation is much higher
5. The parasitic capacitances, Cgs, Cdg, Cds are MUCH, MUCH higher in a FET (2 or three orders of magnitude) and are VERY non-linear.
6. FETs operate at much lower voltages, but characteristics start to go weird when Vds is low, meaning 5V or less.
7. The transconductance is probably higher for a FET at the Id you are likely to use but I'm not familiar with tubes.

If you are used to using tubes I would look into these characterisc differences very thoroughly. You can find this info on most datasheets.

I suspect the linearity and the capacitance differences will be the biggest shock. There is a tendency for people to think a FET is like some half-way device between a bipolar and a tube, but IMO this is far from the case and so just as much care is needed as if you were using a bipolar.
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Old 29th April 2006, 02:35 PM   #7
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Godot bought a decent watch this time, with vaccum inside for less drag and better eficiency
Seriously, i want to learn a bit about sand amps, and thought that some measely mW or so of a mosfet operating in SE fasion would be better than an integrated PP all feedback amp putting out several W of power, or a dedicated buffer chip like the BUF634 What's better, getting a plane from NY to LA, or take the road and know places?
thanks for the help guys
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Old 29th April 2006, 02:38 PM   #8
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Quote:
Originally posted by Simpleton
The tube voltage amplifier runs at aproximately 3mA, probably just enough to run the mosfet. Don't the capacitances change the low frequency response, not the high? Maybe a opamp buffer stage before the mosfet will do the trick is necessary.

I was not refering to the frequency responce as I was the drive required. The capacitance between the gate and source-drain channel is what the driver must drive. It is easier to drive a fet at lower frequencies because that impeadence is higher. At higher freqs, the impeadence drops and it takes more current to drive the fet, so I guess it would effect the frequency responce as well. With all of the different non-linear variables, sometimes placing the device inside the NF loop will allow the error correction of the circuit to help with this...but can also cause instability. This is why hexfets are popular as switches where they are either on or off like in SMPS type circuits instead of linear applications. That is not to say they can't be used, but it requires more complexity. There does exist lateral Fets that are more linear but they typically cost more. If you research the construction of a hexfet, you will see why they have non-linear properties.
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Old 29th April 2006, 04:44 PM   #9
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Simple circuits are the building blocks of the more complex ones.
I've split the thread, please continue with the interesting part.

/Hugo
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Old 29th April 2006, 06:44 PM   #10
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Thanks for cleaning this up NetList

Here's a new version of the complete circuit. Sorry for being so small, but I made it using paint and didn't realise how small it was :P
So, I have the tube grid biased for a 1Vpp signal, and at 25V supply voltage, and with an anode load resistor of 20K, the gain is 14, putting out 10,5Vrms. It is capacitor coupled to the gate of the mosfet (is this really necessary?), which has it's source 12V above ground, with a bias current of 1 Ampere (i could use a LM317 for this job to, i guess). The source pin is capacitor coupled to the speaker.

Do i have this right?
Thank you!
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