Help on Source Follower output

AudioGeek · 4th June 2011, 03:35 PM

Just wondering how to design a source follower output stage. (Class A SE) with a resistor as the load and not a CCS?

I understand that it would be extremely inefficient.

Looking for about 10W into 8ohms. Probably cap couple to load.

1.5A bias current or so.

Thanks for any help or any links to how to work out the values i need.

lineup · 4th June 2011, 03:50 PM

What output transistors do you have or think of?

lineup · 4th June 2011, 05:09 PM

It takes a lot. For 10 Watt.

10 Watt is SQR(10*16) = 12.65 Volt peak
Most effective would one 8 Ohm power resistor.

Supply need to be more than 3x12.65V. Probably 42VDC.
Current needs to be more than 3.16A Say 3.40A.
Across power resistor would be ~27-28 Volt
27V x 3.40A = 91.8 Watt 8 Ohm resistor.

You could use 4 resistors 8.2 Ohm
Each should be rated more than 25 WAtt

You can see in my simulation how one could do.

lineup · 5th June 2011, 09:14 AM

With constant current source
there will be slightly smaller heatsink.
Trafo 2x22 VAC for rectified 30VDC

AudioGeek · 5th June 2011, 03:55 PM

would it be easier to do as a BJT or a Mosfet?

I was thinking mosfet.. as an example a IRF240N or something.

Like i said i dont care how inneficient it is.. just 10W 8 ohms and nice gradual clipping etc.

Thanks!

AudioGeek · 5th June 2011, 04:08 PM

i did some sim and i came up with

45vdc supply. 8 ohm resistor for load. 3.75A bias

using IRF240

can get about 14v peak into 8 ohm load at clipping.

12v peak into 4 ohm load at clipping.

can u confirm this?

ends up being like 35v bias for the 30v drop across the 8ohm resistor load.

hope that makes sense. lol.

lineup · 5th June 2011, 05:40 PM

Yes.
All the figures make sense.
But it must a h**l of a power resistor.
30v x 3.75 = xxxx Watt !!!!!

But if you have 4 power resistors rated 50 Watt each
then you can use those four 8 Ohm resistors together.

Difference by using MOSFET or BJT.
Both can be used.
But we get more dropout voltage with MOSFET
0.7Volt and 4v is a difference 3.3v in favour of BJT
So teoretically MOSFET need at least 3.3v higher power supply.

Bigun · 5th June 2011, 05:54 PM

This would be my first choice for what you have in mind:

SEWA - Seven Watt Amplifier

4th June 2011, 03:35 PM	#1
AudioGeek diyAudio Member Join Date: Jan 2004 Location: Minnesota	Help on Source Follower output Just wondering how to design a source follower output stage. (Class A SE) with a resistor as the load and not a CCS? I understand that it would be extremely inefficient. Looking for about 10W into 8ohms. Probably cap couple to load. 1.5A bias current or so. Thanks for any help or any links to how to work out the values i need. __________________ It is not you... you are it.

4th June 2011, 03:50 PM	#2
lineup diyAudio Member Join Date: Dec 2005 Location: the north	What output transistors do you have or think of? __________________ lineup

5th June 2011, 03:55 PM	#5
AudioGeek diyAudio Member Join Date: Jan 2004 Location: Minnesota	would it be easier to do as a BJT or a Mosfet? I was thinking mosfet.. as an example a IRF240N or something. Like i said i dont care how inneficient it is.. just 10W 8 ohms and nice gradual clipping etc. Thanks! __________________ It is not you... you are it.

5th June 2011, 04:08 PM	#6
AudioGeek diyAudio Member Join Date: Jan 2004 Location: Minnesota	i did some sim and i came up with 45vdc supply. 8 ohm resistor for load. 3.75A bias using IRF240 can get about 14v peak into 8 ohm load at clipping. 12v peak into 4 ohm load at clipping. can u confirm this? ends up being like 35v bias for the 30v drop across the 8ohm resistor load. hope that makes sense. lol. __________________ It is not you... you are it.

5th June 2011, 05:40 PM	#7
lineup diyAudio Member Join Date: Dec 2005 Location: the north	Yes. All the figures make sense. But it must a h*l of a power resistor. 30v x 3.75 = xxxx Watt !!!!! But if you have 4 power resistors rated 50 Watt each then you can use those four 8 Ohm resistors together. Difference by using MOSFET or BJT. Both can be used. But we get more dropout voltage with MOSFET 0.7Volt and 4v is a difference 3.3v in favour of BJT So teoretically MOSFET need at least 3.3v higher power supply. __________________ lineup*


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5th June 2011, 05:54 PM	#8
Bigun diyAudio Member Join Date: Jan 2009 Location: K-W Ontario Blog Entries: 2	This would be my first choice for what you have in mind: SEWA - Seven Watt Amplifier __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

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