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Old 14th May 2007, 08:35 PM   #21
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Quote:
Originally posted by Nelson Pass

Also in both schematics, it is wise to put the zener diodes on
the outside of the gate resistors, otherwise you will occasionally
encounter parasitic oscillation.


VERY good point, Nelson.

Bob
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Old 14th May 2007, 08:42 PM   #22
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Quote:
Originally posted by Iyremenko
Sakis,
Donot shout

Listen carefully what i mean to say that this circuit is not a reliable one to drive mosfets, if you want to put zeners than put them across gate to source terminals, but again they would not protect your amp when overdriven, because the overdrive would eventually destroy the zeners also. Regarding IRFP240, there is no gate diode in it but all the Hexfets exhibit inbuilt body diode which is antiparallel in termination.

If you put the zeners on the "outside" (as desirable for better suppression of parasitic oscillation), you want to have a gate drive circuit that has some degree of current limiting, so that in the event of an output short the driver does not try to drive the load through the zener diode. This can be as simple as some additional small resistance between an emitter-follower bipolar driver and the place where the zener is.

Bob
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Old 14th May 2007, 08:54 PM   #23
fotios is offline fotios  Greece
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Again the usual para-philology! Each one expresses costless criticism. As we say in Greece, each one says his long and his short. Sakis knows what means that please Sakis can you translate what means this in English? In this and in many other cases we need SOLUTIONS and not COMMENTS. Please, mercy on us! Well Sakis, did you notice the second reply of N. Pass? First he is confirmed – something which i had said to you in our contact by phone – that the use of the 2 zeners between G-S it is enough for protecting mosfets from spikes. Do you have check it? Second, he gives a solution to the negative offset that appears in the amplifier’s output. “Drop D1 and the output swings more positive”. Above i quote a revision of your circuit according an old application of RCA which seems to be very close to it. Also change this MJE340 in the VAS with a TIP47. These MJE340 and 350 used as pre or main drivers. TIP47 is a better device in this place. Trust me. And C9 is a Miller compensation device. I thing there is no need for the extra 100pF around VAS. If you have oscillations after the VAS try to increase the value of C9 to solve it. And you may experiment hardly with these solutions offered to you. Nothing it comes without labour. It is not our work to make this circuit. Make your circuit safe; don’t leave it at its luck. Çave you thought ever what it could happen if your amplifier it was found near dimmer packs with hard RF emissions from TRIACS? I wish you success.
Fotios
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Old 14th May 2007, 09:01 PM   #24
fotios is offline fotios  Greece
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Quote:
Originally posted by Bob Cordell



If you put the zeners on the "outside" (as desirable for better suppression of parasitic oscillation), you want to have a gate drive circuit that has some degree of current limiting, so that in the event of an output short the driver does not try to drive the load through the zener diode. This can be as simple as some additional small resistance between an emitter-follower bipolar driver and the place where the zener is.

Bob
Mr. Cordell can you propose any corrections needed in the last drawn that i have posted above? Because i am not sure that the 2N5401 (the phase inverter) it is connected properly. Also for the emmiter resistor value of 2N5551.
Fotios Anagnostou
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Old 14th May 2007, 09:38 PM   #25
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Wait for a moment please! Just now, I took answer from the author of this project Mr. Borivojie Jagodic in a question that I placed to him. His answer I believe that it reverses what we discuss up to now regards to the output stage.

This is my question to Mr. Jagodic
Mr. Jagodic, because so many fuss exists the last month about the practical application of your LEGEND amplifier by our friend Sakis from Greece, i tried to give a help by redrawing your original plan which posted to me by Sakis before one month. I ask your comprehension for my intervention in your intellectual property, but the drawn under this manner it is a standard in audio amplifiers theoretical design as you know, to be easy comprehensible. I hope to not be angry with me, and i ask - if you have the kindness - from you to confirm that the values of components and the circuit nets and nodes are connected correctly in my redraw.
Kind Regards
Fotios Anagnostou

Click the image to open in full size.

And this is the answeer from Mr. Jagodic
Hello Fotios!
I can confirm that your drawing is correct except that you added 4 resistors of 0,18 ohms (not existing in my schematic) of which two are correctly positioned in sources of upper units, but two lower ones are in drains ?! They won't be of much use there. Visual analogy to complementary configured output stages is not enough to provide functionality and right purpose.

Regarding all that "fuss" you mentioned, I decided from the first moment not to comment anything. My friendly extended hand and help I gave from my heart from the first contact, were "turned" upside down, many things were misinterpreted and published that way, my design abused without even minuscule intention to ask for my permission (at least!)....... Hence, I don't see a point of any further involvement in any way!
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Old 14th May 2007, 09:40 PM   #26
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Quote:
Originally posted by fotios
Second, he gives a solution to the negative offset that appears in the amplifier’s output. “Drop D1 and the output swings more positive”.
Unfortunately I was referring to the maximum output swing,
and not the offset at output.

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Old 14th May 2007, 10:15 PM   #27
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Quote:
Originally posted by Nelson Pass


Unfortunately I was referring to the maximum output swing,
and not the offset at output.

As said to me Sakis by phone the amplifier presented negative offset (about 200mV in static condition i.e. without signal in input) and i supposed that it is translated in more negative swing in output under dynamic condition i.e. with signal in input. According to this thought i translated what you wrote for the “Drop of D1”. That is to say it will resolve also the problem of dc offset. However now mr. Jagodic said to us that the 2 resistors in the drains of the lower Mosfets they are placed wrong and it should connected in sources. This puts the things somehow in their place.
Fotios
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Old 14th May 2007, 11:06 PM   #28
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Default Why the need of source resistors?

I want to express a query because I do not know many things about Mosfets except from my inverter welding machine. Why they need resistors in sources (corresponding to the emitter resistors of BJTs)? In BJTs we know that the emitter resistor offers thermal feedback to stabilize the current flow. Mosfet as we know it has by itself automatic current balance according to its case temperature. Well why there is the need of thermal feedback by a resistor?
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Old 14th May 2007, 11:13 PM   #29
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Default Re: Why the need of source resistors?

Quote:
Originally posted by fotios
Mosfet as we know it has by itself automatic current balance according to its case temperature. Well why there is the need of thermal feedback by a resistor?
Except that it doesn't. Mosfets will increase their current with
temperature until you reach rather high currents, so a Source
resistor is commonly used to stabilize this and to help make
parallel devices share more evenly.

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Old 14th May 2007, 11:40 PM   #30
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Default Re: Re: Why the need of source resistors?

Quote:
Originally posted by Nelson Pass


Except that it doesn't. Mosfets will increase their current with
temperature until you reach rather high currents, so a Source
resistor is commonly used to stabilize this and to help make
parallel devices share more evenly.

O.K. i get. I believed that the increase or the reduction of current is linear. On the contrary as soon as his temperature reaches in a limit, then the Mosfet it drops abruptly the current that flows through it.
Thanks
Fotios Anagnostou
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