help!ab class mosfet amp

mbonus · 27th July 2005, 06:26 PM

hi!
I decided to build one power amp (with 2sk135/2sj50)
which works in AB class with power of 60w in 8ohms.
In croatia this mosfets are pretty expensive so I'm
thinking about replacing this mosfets with irfp240/irfp9240.
I heard that these hexfets need Vbe multiplier.
I'm beginner in electronics so I'll be grateful if
somebody can tell me can i just replace this mosfets or
help me what i need to change in my scheme so i can put
irf devices.
here is the scheme:

Craig405 · 27th July 2005, 06:44 PM

I am not sure but 2sk1058/2sj62 might do it, someone said they had used these in an amplifier that used the mosfets you mentioned and they were ok.
Check the data sheets first though.

Craig

tlf9999 · 27th July 2005, 06:47 PM

It is OK to use d-mosfets but with a Vbe or Vgs multiplier. It tends to be overly compensated with a Vbe multiplier and I usually use a small TO220 MOSFET as Vgs multiplier. You just need to be careful with the Vgs multiplier to make sure that you don't have a thermal runaway.

Steven · 27th July 2005, 07:36 PM

I think there is a fault in the circuit. I expect a connection from R14 somewhere to D7.

Steven

mbonus · 27th July 2005, 07:56 PM

you are right Steven.
my fault...
i corrected it now.

AndrewT · 27th July 2005, 08:03 PM

Hi,
What does D3 do? it seems it could be omitted.
V FETs must use a thermal compensator.
Drive to Q8 missing.
R1 is too big, try 680r and change C2 to 1nF to maintain rc time constant.
C7 & C8 could use a bigger decoupling cap in parallel, try 100uF+47nF.

amplifierguru · 28th July 2005, 12:49 AM

Hi Andrew,

D3 is needed to increase the drop from the 40V supply to bias the next stage AND to provide Tcomp of that following stage. Otherwise Vacross the 1K5 resistors is only 0.63V, not enough!
D3 raises this to about 1.25V enough with one Vbe Tcomp to provide a stable 0.6V for the 47R tail resistor of the diff'l Vas so defining the operating current as 6.4mA per leg.

Hi mbonus,

To answer the thread question, you will need a higher bias, variable up to 8V for the IRFP240/9240 pair and include source resistors of maybe 0R47. These MOSFETs have a +ve tempco of around 3 times that of a BJT but off a much higher Vgs so % tempco is actually about half the compensation needed to track for a BJT. That's why a std Vbe multiplier overcompensates.

You can provide this in a few ways -
1. A standard Vbe multiplier with degrading R in the emitter leg.
2. A Vgs multiplier.
3. Standard Vbe multiplier for half plus variable resistor for other half bias.
4. string of diodes in thermal contact for 1/2 + variable resistor for other half bias chain.

Hope that helps

Cheers,
Greg

mastertech · 28th July 2005, 01:25 AM

AndrewT

"Hi,
What does D3 do? it seems it could be omitted."

yes it could ,then youd need to adjust the colector resistors

cheers

mastertech · 28th July 2005, 01:27 AM

by the way,very nice design
job well done

mbonus · 28th July 2005, 06:23 AM

thanks andrew!
it will be much more easier to me
if somebody can draw in my scheme where
and how to connect this Vgs multiplier

so i can put irf-s.
(i tried to find 2sk1058/2sj62 but 2sj62
is also hard to find for me.)
thx!

27th July 2005, 07:36 PM	#4
Steven diyAudio Member Join Date: Oct 2003 Location: The Netherlands	I think there is a fault in the circuit. I expect a connection from R14 somewhere to D7. Steven __________________ The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973

27th July 2005, 08:03 PM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, What does D3 do? it seems it could be omitted. V FETs must use a thermal compensator. Drive to Q8 missing. R1 is too big, try 680r and change C2 to 1nF to maintain rc time constant. C7 & C8 could use a bigger decoupling cap in parallel, try 100uF+47nF. __________________ regards Andrew T.

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27th July 2005, 06:44 PM	#2
Craig405 diyAudio Member Join Date: Jul 2005 Location: Sussex	I am not sure but 2sk1058/2sj62 might do it, someone said they had used these in an amplifier that used the mosfets you mentioned and they were ok. Check the data sheets first though. Craig

27th July 2005, 06:47 PM	#3
tlf9999 Account Disabled Join Date: May 2005 Location: none	It is OK to use d-mosfets but with a Vbe or Vgs multiplier. It tends to be overly compensated with a Vbe multiplier and I usually use a small TO220 MOSFET as Vgs multiplier. You just need to be careful with the Vgs multiplier to make sure that you don't have a thermal runaway.

28th July 2005, 12:49 AM	#7
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Hi Andrew, D3 is needed to increase the drop from the 40V supply to bias the next stage AND to provide Tcomp of that following stage. Otherwise Vacross the 1K5 resistors is only 0.63V, not enough! D3 raises this to about 1.25V enough with one Vbe Tcomp to provide a stable 0.6V for the 47R tail resistor of the diff'l Vas so defining the operating current as 6.4mA per leg. Hi mbonus, To answer the thread question, you will need a higher bias, variable up to 8V for the IRFP240/9240 pair and include source resistors of maybe 0R47. These MOSFETs have a +ve tempco of around 3 times that of a BJT but off a much higher Vgs so % tempco is actually about half the compensation needed to track for a BJT. That's why a std Vbe multiplier overcompensates. You can provide this in a few ways - 1. A standard Vbe multiplier with degrading R in the emitter leg. 2. A Vgs multiplier. 3. Standard Vbe multiplier for half plus variable resistor for other half bias. 4. string of diodes in thermal contact for 1/2 + variable resistor for other half bias chain. Hope that helps Cheers, Greg

28th July 2005, 01:25 AM	#8
mastertech Banned Join Date: Mar 2005 Location: Australia	AndrewT "Hi, What does D3 do? it seems it could be omitted." yes it could ,then youd need to adjust the colector resistors cheers

28th July 2005, 01:27 AM	#9
mastertech Banned Join Date: Mar 2005 Location: Australia	by the way,very nice design job well done

28th July 2005, 06:23 AM	#10
mbonus diyAudio Member Join Date: Jun 2005 Location: Osijek	thanks andrew! it will be much more easier to me if somebody can draw in my scheme where and how to connect this Vgs multiplier so i can put irf-s. (i tried to find 2sk1058/2sj62 but 2sj62 is also hard to find for me.) thx!

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