Simple mosfet amplifier

IRFP

I am using IRFP240 and IRFP9240.
As a matter of fact I have one board that I used 4 Mosfets (2 pieces of 240 and 2 pieces of 9240) and it works fine with +60 and -60V.
I will give it a try using 4 Mosfets for the new board two. I have even changed the PCB layout in order to use 4 output transistors.
Tekko,
I quite did not understood all of your message. Can you give me some more tips about the VBE multiplier or the diode string? Adding 2 mosfets to the output stage will do the magic? My electronics knowledge about amplifiers is not that good.
Regards,
Elton.
 

Tekko

Banned
2005-01-01 3:33 pm
If you look in this circuit:
acLsT.jpg


You see the transistor with two resistors connected between the gates of the output fets, that circuit is what you need to keep your non lateral mosfet amp from thermal runaway. The 895ohm resistor is a 5k multiturn pot in series with a 470ohm resistor.

Lateral mosfets are self regulating = they will not run away thermally as vertical mosfets will do without a bias compensation circuit.
 
Bias

Tekko,
Thanks a lot. It was very clarifying indeed. I will insert that part of the circuit on my amplifier and see how it works.
If you have a look on the original circuit, it has a bias adjustment (P2), but I guess it is not working so fine as it is not a so elaborated circuit.
The only thing that I am not quite sure about is that on the circuit you have posted, power supply is +35v and -35v. So for +60v and -60v which sort of bias adjustment should I be looking for?
Regards,
Elton.
 

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I am using IRFP240 and IRFP9240. My electronics knowledge about amplifiers is not that good.
when I joined this Forum just over 6 years ago I too had trouble with understanding that FETs come in different varieties.
jFETs was my most recent learning.
Lateral mosFETs and Vertical mosFETs came quite a bit earlier.

I am still not up to speed on Vfets, trench fets, Ufets, Hexfets etc.
But I will get there if I keep studying.

Stick at it and keep asking questions until it becomes clearer.

Post5,
Q8, R12 & R13 are the Vbe multiplier. A temperature compensating arrangement must be used with V mosFETs.
The resistor (P2) bias in post6 only works with Lateral mosFETs

BTW,
build yourself a mains light bulb tester. It would have saved your wrong schematic from blowing up.
 
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Hope it works.....

So guys, let see if I got it right.
I will replace the P2 of my original circuit by the VBE circuit (Q8, R2, R13 and C8) Tekko has sent. Is it right or shall I do any other changes?
As it is Sunday and I do not have a 2n5550 on my lab, I was wondering if I could use a MJE340, a TIP41C or even a BC337-25 instead. I have quite some of these.
Changing the output Mosfets was an interesting idea, but living in Brazil is not so easy to get any other ones that appear on the original list. So I guess I have to stick with the IRFP240 and IRFP9240 anyway.
Tekko and Andrew,
Thanks for your help so far. Without your priceless advices I would get literally nowhere.
Regards,
Elton.
PS. I have used the mains bulb "protection" circuit, but the output transistors blew up anyway.
 
Bulb protection

Hi Andrew,
I have conected the bulb lamp in series of the primary of my transformer.
I have used it just for the first time that the IRFPs blew up.
I will do the modifications and see how it works. I went thru some other schematics and saw that in some of them a MJE340 is used. So I will give it a try and see how it goes. I will post the results as soon as I run the tests.
I will be using the bulb circuit once again.
The extra information message I have sent was just to inform what happened before I started to look for modifications. I have not done any changes yet and have not turned on the circuit again on +60V and -60V without the modifications. The circuit now is as it was working with +30v and -30V. As I mentioned earlier on this post I will do the changes and test it once again. Hope it works fine.
Regards,
Elton.
 
Hi

Did you remember to include gate source protection Zeners? This is very important with any mosfet that does not have them included intrinsically within the device. An unprotected gate-source junction can destroy the device without it even being in the circuit, this is what makes them so static sensitive.

First power the circuit up without the output mosfets and tie the NF point to one side of the Vbe multiplier, don't worry too much about a little DC offset at this point. Then measure the voltages where the gates are to connect. You should be able to adjust the Vbe multiplier to a range with a minimum voltage below 2 times Vgs threshold, and be able to adjust it to ~1V above 2 X Vgs threshold. Fix the circuit so you get these results before adding the outputs. After including the missing Zeners and the missing output coil || 10R resistor, set the bias pot so the voltage between the gates will be at the minimum, i.e. below 2 times Vgs threshold, add the output mosfets and power it up with no load. Then adjust the Vbe multiplier pot until you measure ~100-200mA through R14 & R15, a respectable bias for these devices. Hopefully this will help keep you from needlessly destroying more mosfets.;)

As a more general side note, Trench and U-type fets do not have a very good SOA within the 'linear region'. This is primarily why they are not chosen for linear applications. It's not secondary breakdown per say, but is similar in destruction. They do have properties that make them better switchers though; class D, SMPS.
Also there are Planer Stripe Diffusion Mosfets, another type out there. Fairchild's Q-fet line is akin to these. They have a die structure forming a unified singular well as opposed to the 'Hexfet' types that have a discrete plural well die structure that forms multiple tiny transistors all connected in parallel. (see photo) From my experience the planer stripe mosfets are quite solid through the linear region and can take more abuse than the cellular types but they seem to exhibit more non-linear characteristics, particularly Gm that is a bit dependent on Vds. I believe the planer stripe mosfets can be better in terms of SOA but only if a form of local error correction is used to tame the Vgs vs Id distortion that is related to Vds, in addition to all the other distortions inherent to mosfets. (IMHO, no mosfet should be used as linear outputs without EC due mostly to their non-linear capacitances, not to say it will not work without EC, but I digress:rolleyes: )
I can crank out continuous 120Wrms @8R speaker load with very low distortion with only 2 pair of TO-220 Q-fet planer stripe mosfets.:D I have yet to try the TO-3P, I have not yet the need for that much power in my living room.:cannotbe:

All of the V-fet mosfets are designed by the manufacturer to be used as switchers and the datasheets are geared towards it but that doesn't mean they will not work as linear amplifiers. Some are very good if used properly.:cool:
 

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Helpfull tips

Dear CBS240,
Thanks for your tips. I was planning to do something similar to your advices. The questions are about the protection zener diodes and really how much voltage shall I have in each gate of the mosfets. I came across the circuit on the picture (wich I will eventualy also give a try), and was wondering if I could use the same VBE circuit and zener protection diodes.
Any quick help will be nice as I have to finish this project as soon as possible. My wife is driving me crazy about spending too much time on it.....
Regards,
Elton.
 

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I think the gate protection Zener must be at the gate. If it's before the gate stopper then when it passes the preceding stage can be overloaded.

The gate protection Zeners can be returned to the source or to the output.
When the Zener voltage is selected to be lower than the maximum allowed gate voltage you find it gives some overcurrent protection. Use this zero cost current limiting.
 
Zeners go directly between gate and source, I use two per transistor in series, each reversed so that the gate voltage is limited in both directions by the Zener voltage. Mosfets can oscillate at very high frequencies, the inductance of the physical pins can be significant at 40+MHz so mounting the Zeners as close to the device as posible will protect them in case of such oscillations. Under normal operation they present an open circuit, but set a limit to the Vgs in either direction. The datasheet states that Vgs max is +/-20V, any more and the oxide layer between gate and source breaks down. So 13-15V Zeners would be fine. Limit would be +/- Vz+0.6V. (to account for the forward bias one.)
 
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Yes, but Does it matter? -AK-KA- or KA-AK Think about it. If one Zener is reverse than it has the Zener voltage, the other is forward bias so it has the foward bias voltage of a diode, 0.6V, which ever way they are oriented. IOW, it is a voltage catch in either direction. You can get away with just one reverse Zener but the reason I use two in opposite direction is to allow the gate voltage to be pulled negative WRT the source to discharge the gate source capacitance as fast as possible in the event of very fast reactive transients. Besides, Zeners are very cheap. A Zener is basically a regular diode but with a very specific reverse breakdown voltage.


You may have to play with the emitter resistor, RVT7, to get proper thermal tracking of the outputs. Doubtfull it will track perfectly with a BJT Vbe multiplier, but if you set it up so that there is a slight negative temperature coefficient and set the initial cold bias a bit higher, as the outputs heat, the bias will reduce slightly but stay within an aceptable range. This will prevent thermal runaway.:hot::flame:
In my latest Mosfet amp the output bias starts at ~250mA per device and at the hottest temperature measures ~150mA. A little trial and error might be in store to dial it in to stable operation. Others might dissagree with this but it works for me.:)
 

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Lazy Cat

Member
2007-09-15 9:57 pm
Very nice sch to start with ... only that you should leave original suggestion for current source at LTP. Double transistors with RCR filtration will be some 20-30dB better in current stabilization at higher frequencies than only one CS transistor. Also thermal undependancy is improved quite a bit. This CS is very important, it is also the point where all kind of stability for the amp starts. It would be wise to put parallel zener to electrolytic capacitor. ;)
 
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