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Old 3rd May 2005, 04:14 PM   #1
bengt is offline bengt  Sweden
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Default Another N-channel

Hi,

I am not soo good at this but very intrested and trying to learn about amps. I found a schema on a HEXFET amp, using IRF540 /9540 as O/P. Im trying to make this as an n-channel amp, using some small ideas from Zeta (IRF9640 as an inverter). I have not simulated the construction but I have made a real, working prototype.
Well, sound is comming out but is seems to be difficult to set the bias correct. If I set the bias to 150mA it seems to decrease quick to a lower value of 50-70mA with the BD139 mounted on the heatsink of O/P. If i take it away from the heatsink the same happens but slower. I think this circuit is not good optimised.
It works but with all knowledge here maybe someone can give me some advise to make the circuit more stable and correct!

Regards,
Bengt
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Old 3rd May 2005, 04:15 PM   #2
bengt is offline bengt  Sweden
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Old 3rd May 2005, 04:29 PM   #3
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Bengt: This one will work fine. However your BIAS circuit is over compensated, you don't need a pure VBE multiplier.

Also you don't need the 0R33 Source resistors. Just leave them out, and connect the source directly.
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Old 3rd May 2005, 05:02 PM   #4
bengt is offline bengt  Sweden
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Thanks for your reply mr Clausen

So I take away the BD139. I am not so familiar with n-channel but as the original O/P was IRF540/9540 and it was impossible to bias the circuit without BD139 VBE. In this one I am using IRFP260 and the whole circuit seems to behaive "different".

So, I will continue and play music from my ZAP Player.....

I can not have a better source I guess....


regards,

Bengt
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Old 3rd May 2005, 05:33 PM   #5
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No no don't take the BD139 away, reduce the vbe multiplication factor.

But take away the 0R33 resistors.
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Old 3rd May 2005, 06:23 PM   #6
bengt is offline bengt  Sweden
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Thanks for your answer Mr Clausen


Ok, I took away the 0R33.

I missunderstood you, I think that only in amps with lateral mosfet you don't need "BD139" on the heatsink to prevent thermal runnaway as they have negative tempkofecient (?!). In this circuit BD139 will also compensate for heat and reduse the current to prevent thermal runnaway, did I understand this correct???

I will decrease 2k2 to 1k and increase 100 to 470ohm if I understood correct. BD 139 is needed for thermal compensation, right ?

Learning all the time....

Is it a good choise to use IRFP260 ? I have some of this transistors...

Regards,
Bengt
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Old 3rd May 2005, 06:42 PM   #7
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The schematic shows the IRF9640 upside down?
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Old 3rd May 2005, 06:51 PM   #8
bengt is offline bengt  Sweden
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Default corrections

Yes, you are right, Drain and Source have to change in the schematic, on my testboard it is in the right place, else it would be fireworks perhaps....

I will change it.

Bengt
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Old 3rd May 2005, 06:58 PM   #9
bengt is offline bengt  Sweden
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Default corrected

Corrected

Bengt
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Old 3rd May 2005, 08:44 PM   #10
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You might like to try adding a 100 ohm resistor in the source of the IRF9640. This may bring you a little better stability.
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