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Old 27th November 2010, 07:51 PM   #1
Eltongp is offline Eltongp  Brazil
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Default Simple mosfet amplifier

Hi everyone.
I have done the amplifier of the web page ampli mosfet simple.
It works fine if a use a power supply of +30v and -30V, but when I switch my 110v/220v key on my power supply, which means that I have +60v and -60V, the output mosfet transistors blow up. Can some one help me on recalculating components for this circuit to work with +60V and -60V?
Regards.
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Old 27th November 2010, 08:36 PM   #2
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What output devices are you using? The circuit will only work with lateral type not IRF (even though it says IRF in the parts list).

When you increase the voltage do you turn bias down before doing so?
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Old 27th November 2010, 08:55 PM   #3
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It needs one more transistor to work with IRF type mosfets, a Vbe multiplier or a diode string mounted to the heatsink.
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Old 27th November 2010, 10:54 PM   #4
Eltongp is offline Eltongp  Brazil
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Default 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.
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Old 27th November 2010, 11:21 PM   #5
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If you look in this circuit:
Click the image to open in full size.

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.
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Old 28th November 2010, 11:15 AM   #6
Eltongp is offline Eltongp  Brazil
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Default 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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Old 28th November 2010, 11:26 AM   #7
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The bias circuit is the same no matter the input voltage, it just need to be adjusted for proper bias current through the output devices.
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Old 28th November 2010, 11:42 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Eltongp View Post
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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Last edited by AndrewT; 28th November 2010 at 11:46 AM.
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Old 28th November 2010, 04:49 PM   #9
Eltongp is offline Eltongp  Brazil
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Default 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.
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Old 28th November 2010, 05:07 PM   #10
Eltongp is offline Eltongp  Brazil
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Default Extra information

I have just forgotten to mention that when I turn the circuit on, it works for a few seconds with some output voltage (around 5V or 6V) and then it blows up the output IRFPs.
Elton.
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