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Zimo 4th August 2005 04:09 PM

Need help with Giesberts HEXFET AMP
Hi everyone! :cool:

I have a project with this amp, i was nearly done with it, but then this problem came out! :bawling:

So i made everything ready and started to test the amp... Everything was fine until i put FETs fuses to the holders and bang.. every IRF9540N and IRF540N shorted and every fuse blew. Then I thought that the problem was gate voltage so I removed the wrecked fets and put multimeter over R24 and R27. I adjusted voltage as low as itīs possible, 2,8V in my amp.

I soldered a new fets in their places and put only 400mA source fuses in holders. Then I turned the amp on. Everything seemed to be under control, so I adjusted gate to source current to 200mA. It was very steady and thought that everything worked fine.

So i shut it down and put the normal 2.5A fuses back to their holders and turned the amp on again. BANG! every fuse blew and the fets were gone.

So I came here to look if someone else have samekind of problems with that amp. I found that someone here have the amp which is working and some have the same problem with their IGBT version.

I donīt have every value of the components just right because couldnīt find those and thought that those would work fine. Powersupply is 100% same as in original. Here is the values that I have used to countervail some components on the apmlifier PCB.

T12 IRF9540N (original IRF9540)
T13 IRF540N (original IRF540)

R6 82.0 ohm 2% (original 84.5 ohm 1%)

C3, C4 2.2nF 63V (original 2.7nF)
C5 470pF styroflex (original 330pF styroflex)
C9 1uF MKS4 100V (original 1uF polypropylene)

Ok I know that C5 should be really close that 330pF but canīt get a right component yet. And isnīt it just affect to maximum frequency of the amp. (so 470pF should decrease the maximum frequency?)


So if you guys could help me out of this situation i would appreciate that very much! :)

I put links to the schematics here:

Netlist 4th August 2005 04:20 PM

The problem is IMO not the different components you used.
If the amp died immediately after power-on I suspect a bad connection somewhere. Give it a rest and check your solderings thoroughly.
Use a lightbuld in series with the mains to reduce current if you don't have a variac at hand.

/Hugo :)

Zimo 4th August 2005 09:13 PM

Thanks for your reply!

I checked my solderings, but couldnīt find anything suspisious.. maybe because I used so much time to get good solderings :)

I try to get a variac from somewhere... let see what is says then. :devilr:

But i can only say that it was absolutely steady when i chanced the bias current from 0mA to 200mA and kept it there.
Thatīs why i thought that the problem would be at starting the amp.

Charles Hansen 4th August 2005 09:42 PM

Wacky circuit.

The output stage has gain, although I'm not sure why. Normally this is only needed if you are running an op-amp front end with +/- 15 volt rails. Seems goofy to me.

The feedback network provides the same gain at DC as at audio. This means that the amp has DC gain. There is a circuit at the input to inject some current to adjust the overall DC offset. Again it seems goofy to me. The maximum input offset current is only about 1.5 micro-amps.

Was a load connected to the output of the amp when it failed? If so, then maybe there is a DC offset problem. If not, then I would suspect parasitic oscillations.

Good luck...

Zimo 4th August 2005 09:50 PM

No there was not any load.. and that is just the strange thing why it burns those fets even without any load. :D

BTW I put some pictures of my project to internet and also the Elektor material of that amp.

ilimzn 4th August 2005 10:02 PM

Re: Need help with Giesberts HEXFET AMP

Originally posted by Zimo
I donīt have every value of the components just right because couldnīt find those and thought that those would work fine...

T12 IRF9540N (original IRF9540)
T13 IRF540N (original IRF540)
This is likely your problem - not that there is anything wrong with the FETs but some components around them need changeing, I will get to that a bit later on.


R6 82.0 ohm 2% (original 84.5 ohm 1%)
C3, C4 2.2nF 63V (original 2.7nF)
C5 470pF styroflex (original 330pF styroflex)
C9 1uF MKS4 100V (original 1uF polypropylene)
These substitutions are unlikely to be a problem, even the 470pF cap instead of 330p, and 2n2 instead of 2n7.

It is very likely you have an oscilation problem, and possibly a bias problem as well.
This amplifier has two problematic ponts - one major and one slightly less major:

The slightly less major one is the use of a CFP BJT-MOSFET pair with gain in the output in order to avoid the use of separate higher supply rails for the input and driver portion of the circuit. These sort of circuits can be prone to instability and require good PCB layout and decoupling techniques, even under ideal conditions.
Your substitution of standard fets with ones with the N at the end will make this problem quite a lot worse. The IRF540N is quite a bit faster in this circuit than it's non-N cousin, while the IRF9540N (which is an even worse complement to the 540N than the 9540 is to the 540!), is not THAT radically different compared to the 9540 without N. In particular, because the gate-source and gate-drain capacitance is reduced in the N versions (quite signifficantly in the 540N), the resistors in series with the gates are too low. I would go with at least 47 ohm even for the original FETs, more (100-150) for the N versions.
Furthermore, the 540N has a substantially lower threshold voltage than the non-N version, 9540N also has a lower threshold but not a lot compared to the 9540 without N. In order to bias these correctly, you will need to change R24 and R27 to a lower value (100 ohms or so), or, even better, R25 and R28 to a higher value (22 ohms or so). In any case, your lowest bias voltage between G and S of the MOSFETs whould be well below 2V.

The first major problem this amp has is also the reason for the problem not being obvious - the fuses are in the source line of the MOSFET, so they act as source resistors, lowering the gain of the MOSFET. This is why you could use 400mA fuses but not 2.5A fuses - the 400mA have a larger resistance at 200mA, and you get a lower bias current. You would have not seen such a discrepancy with the original FETs, the N versions have a lower threshold and higher gain so the problem is magnified.
Putting fuses in the source line is, IMHO a rather bad idea. Fuses are quite non-linear, and in this application they represent local feedback for the FETs. Granted, there is a secondary feedback between the FETS and the BD139/140 drivers, still, why make things worse? Further, some slow blow fuses are constructed as small coils. Keeping in mind that these MOSFETs can easily osvillate at tens of MHz, putting any kind of inductance into the source line is asking for trouble. I don't know what the layout of the PCB looks like, C11 and C12 should be very close to the FETs, and IMHO they need a good quality non-inductive foil cap in parallel. At the frequencies where the FETs can oscillate, electrolytics alone may as well not be there - they will provide almost no decoupling unless they are special low ESR kinds, and even them it will be inferior to a non-inductive foil (not to mention tantalum or ceramic, but I would rather not use these here).
Still, the resistor changes should do the trick. It should be noted that you really cannot rely on any kind of idle current measurement in this circuit, if you take out a fuse and measure in it's place. The fuse type and rating is one of the things that determines the idle current in this design, which i find a very bad idea.

MikeB 4th August 2005 10:04 PM

Hi !

Charles, it's completely correct that the outputstage has gain,
this way this amp can deliver nearly rail to rail outputswing.

Zimo, this c5 increased to 470pf is very critical, this can bring
the amp into oscillation which will blow up everything...
But you should follow hugo's suggestion to recheck every
connection and so on. Just for testing, check if you get a ceramic
330pf, or maybe only 220pf. If it stops blowing...
As you reduced c3 and c4 also the amp already had less compensation.
The reason c5 is so critical, is because this cap reduces closed
loopgain for higher freqs and thus increases feedbackfactor.
This can lead to a point where openloopgain is still above
closedloopgain at the critical resonancefreq, then the amp

It's possible that the resistance or inductance of the smaller
fuses prevented the amp from oscillating as they are directly
connected to the sources of the mosfets.

I think this is a nice circuit !


Charles Hansen 4th August 2005 10:05 PM

The MOSFET has only two failure modes:

1) Too much current (power) will raise the die temperature to the point where it melts. This could happen if the bias were too high.

2) Excessive gate-source voltage will puncture the oxide insulation. This could happen if it oscillates.

So it has to be one of these mechanisms. Since it worked fine with the small fuses, it may be an intermittent problem, perhaps due to a bad connection. Or it could be related to oscillations. Sub-optimal layout can cause it to oscillate.

Good luck...

Zimo 4th August 2005 10:32 PM

WOUH!!! :eek:

THANKS to everyone of you guys!! :) :) (specially to you ilimzn, I think youīre really a guru with amps :D)

I would never have a glue that those IRF9540 and IRF9540N (and 540/540N) have so much differences... And I was quite sure that they would fit well. But hey thanks a LOT of that!

So now I will change definedly those resistors and that 330pF styroflex in the feedback.

And i forget to put here the link to layout of that amp. So here it is:

I change the components in few days and will be back here to tell how it works after these modifications.

Be back soon ;)

ilimzn 5th August 2005 01:01 AM


Originally posted by Zimo
[B]WOUH!!! :eek:
THANKS to everyone of you guys!! :) :) (specially to you ilimzn, I think youīre really a guru with amps :D)
Not a guru for sure, just had similar experience with IRFxxx and IRFxxxN, IIRC so did MikeB but in class D...
To be perfectly honest, neither IRF540/9540 or 540N/9540N strike me as the ideal devices for this amp, especially since the layout offers a possibility of using larger ones. My favorites for this sort of application would be IRFP240/IRFP9140, without N at the end. Similar current handling, as good a complementary maych as you can get within the IRF series, and more robust. They do cost a bit more - about 2-3 EUR normally.
The layout strikes me as rather sparse, long tracks tend to be problematic with MOSFETs. One small suggestion: although it may seem unimportant, it may help to exchange places of R26 and the jumper next to it. With MOSFETs it is always good practise to put gate stopper resistors as close to the MOSFET as is feasible.


I would never have a glue that those IRF9540 and IRF9540N (and 540/540N) have so much differences... And I was quite sure that they would fit well. But hey thanks a LOT of that!
It is a good idea to download the relevant datasheets from and compare the Vgs vs Id @ Temp curves. It is not always easy as they are seldom in the same scales, but even a few points will give you an idea. Next, compare capacitances in the parameter table. The problem with this amp is the gain, which svales up the reverse transfer capacitance by the gain, so at first glance low value gate stopper resistors are a good idea. However, in the N ch N suffix versions, Crss is lowered quite a lot, so the gate stopper may become ineffective. In order to truly get the proper value, you would need to experiment, which implies access to a signal generator and scope :(

MikeB may also be right, though at first glance, I don't see the capacitances being a huge problem, unless the design is not overcompensated at all (designs usually are overcompensated a little, at least by the tolerance of the caps, though doing it JUST right improves performance, but requires said signal generator and scope).

The problem of gate overvoltage is a very real one in this amp. There is no protection, and if the amp oscilates or has a shorted output, the gate voltage may become excessive. At some point you may want to add zeners across G-S of the MOSFET. 6V or so will also limit maximum current somewhat, but in this instance, the gate protection is of more importance.

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