Need Help On Push Pull Amplifier

[azira]

Originally posted by djk

"* There's a lot of oppinions about thermal runaway with MOSFETS. I don't understand a lot of it but if you're planning on your Vgs multiplier following your output devices then you might consider swapping the BJT for a small FET instead. It'll should work the same but will have more similar thermal tracking. On the otherhand you might not need to track your output devices at all."

I always wonder why people make remarks like this. Pull the data sheet and look. Then there is less chance you will make an inappropriate remark. By accident you almost made a good remark, but it shouldn't be a small FET, it should be of the same type the outputs are.

Hmm... after having posted the logic behind my suggesting a FET, what makes it such an "accident"? Not everyone on here agrees what "thermal runaway" is or if MOSFETs suffer from it...

What I am saying is straight forward:

IF you are of the classification that believes FETs suffer from thermal runaway then a FET Vgs multiplier might be better than a BJT one.
On the other hand, iF you are NOT of that classification, then it doesn't matter what you use...

Really, how many BJT amplifiers do you see use the same device for the output device AND Vbe multiplier?

Not the Citation 12, Not the Leach amp, Not P3A

[djk]

You still don't get it do you?

"IF you are of the classification that believes FETs suffer from thermal runaway then a FET Vgs multiplier might be better than a BJT one.
On the other hand, iF you are NOT of that classification, then it doesn't matter what you use..."

Whether it needs it or not is not determined by anyones' opinion, but by the construction of the part.

The manufacturers data sheet will show the tempco of the RDS and the Vgs.

"Really, how many BJT amplifiers do you see use the same device for the output device AND Vbe multiplier?"

If you use the same part then the Vgs tempco will be the same, what does a TO220 FETcost? Maybe $0.50?

On CFP amplifiers I always use the same exact transistor I use as the driver, diode connected and in contact with the driver.

Look at a Mark Levinson ML3 schematic, that's where I got the idea from.

[azira]

Originally posted by djk
You still don't get it do you?
...
Whether it needs it or not is not determined by anyones' opinion, but by the construction of the part.

The manufacturers data sheet will show the tempco of the RDS and the Vgs.

Yes, I do. You are one that doesn't believe MOSFETS can have thermal runaway. Or atleast what you interpret thermal runway
to be.

If that was the case, why bother putting a heatsink on it?

Here's a link that states that power MOSFETS can thermally runaway.

http://www.smpstech.com/tmos0000.htm

Here's a link to a thread I had a while ago in which the participants couldn't agree what thermal runaway WAS.

Will a true class B have thermal runaway?

[djk]

At the risk of repeating myself:

"Lateral MOS types are the only easy ones to use in a class AB amplifier, they are easy to spot because the Source is connected to the case. Vertical MOS are easiest to use in class A designs, the Drain is connected to the case."

That should make it simple for those who can't find or read the data sheet.

[basiliou]

Dear all

Your replys were a great help for me.
thank you all for you comments and for your help.
I will let you know for the results of my amp as soon as possible.

Regards

George Vasiliou

[ekaerin]

Yo are right about this. Contrary to public believe FETs run away.
I have experienced this many times at work when we constantly monitor
the losses in different transistors during evaluation.
The mechanism, however, is different from BJTs but the bottom line are
always exploding transistors.

This almost deserve a new thread I think.

/ Regards / Mattias