Diodes from colector to emitter in power output transistor - diyAudio
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Old 26th February 2010, 07:14 AM   #1
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Default Diodes from colector to emitter in power output transistor

are they effective?

Explain, using simple words, how they work.

An orkut friend asked me and then i realised that i do not know.... or do not remember....also, after 50 years. if have not interested me, seems to me i could leave without them....seems not very important.

I can imagine protection if you invert polarity.....supply polarity and audio polarity, then it will drain,

I have tested in the simulator and was a disgrace!....seems something very bad that had more annoyng effects than the opposite.

Can you explain me?..... without complicate (my) mind with high level mathematics?....say...explaining in a such way people (average) can understand?

Yes..... i do not know everything, for sure there are things you can contribute... I was explaining, and proving, to my Orkuy group that V/I limiters are terrible for audio reproduction and then i have faced this question.

I am asking practical answer, the theorists, and also the book copiers or book text parrots, please, translate you complication in simple words,... here a chance to you to exercise the rigth to be friendly, cooperative, fair, kind and helpfull.

think on me as an audiot or a school children....so question was posted to you, as a chance to be frindly and to help, not to get another big chance to be snobish or bad.....evil guys, please..... Revertere ad locum tum (go back to the cemetery)

regards,

Carlos
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Old 26th February 2010, 07:23 AM   #2
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Default I am interested, also, to know the efectiveness, the increase or decrease of

efectiveness, efficiency, when you do not connect it to output rail (maybe to drain invertions, when inverted DC appear in the output)

I could see these two ways...diodes connected to the ouput line and diodes from colector to emitter.... are there others?

The best Brazilian audio group, the lovely Orkut communitty where all folks knows how to design amplifiers (not all about as you see) will be happy with your help to our improovement.

It is a rarity when people really wants to help... those last 6 years i could see majority of snobish guys, i could learn watching figths, when people forget to control themselves when want to show they are better than others..during some "smashing", some torture session...with vain, arrogance and pride popping all around!..heheheh...i have learned a lot while they were figthing... sometimes smashing fair, nice and good friends... a pity, at least i could learn some, and this is beeing multiplied in Brazil..now a days there are several guys that knows all i could learn here!

If you intend to to this here, better to give up, the double barreled is loaded, fair moderators are watching and i will react!

Explain for me please boys, to my understanding, not to show you are very good...i know you are, all i want to know is about these diodes, what do they do there?... also if you are really kind and cooperative.

regards,

Carlos
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Last edited by destroyer X; 26th February 2010 at 07:34 AM.
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Old 26th February 2010, 07:25 AM   #3
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Quote:
Originally Posted by destroyer X View Post
are they effective?

Explain, using simple words, how they work.

An orkut friend asked me and then i realised that i do not know.... or do not remember....also, after 50 years. if have not interested me, seems to me i could leave without them....seems not very important.

I can imagine protection if you invert polarity.....supply polarity and audio polarity, then it will drain,

I have tested in the simulator and was a disgrace!....seems something very bad that had more annoyng effects than the opposite.

Can you explain me?..... without complicate (my) mind with high level mathematics?....say...explaining in a such way people (average) can understand?

Yes..... i do not know everything, for sure there are things you can contribute... I was explaining, and proving, to my Orkuy group that V/I limiters are terrible for audio reproduction and then i have faced this question.

I am asking practical answer, the theorists, and also the book copiers or book text parrots, please, translate you complication in simple words,... here a chance to you to exercise the rigth to be friendly, cooperative, fair, kind and helpfull.

think on me as an audiot or a school children....so question was posted to you, as a chance to be frindly and to help, not to get another big chance to be snobish or bad.....evil guys, please..... Revertere ad locum tum (go back to the cemetery)

regards,

Carlos
Carlos,

Do you mean the diodes from output to the supply rail (in non-conducting direction of course )?
They are there to clamp the output to the supplies if the output would try to go beyond the supply as that might destroy the output transistors from overvoltage.
This could happen with pulses or high frequency signals into an inductive load. As you know, if you tried to quickly change the current that goes into an inductive load, the load 'kicks back' with a very high voltage peak because it tries to keep the current going (that's the nature of an inductor of course).

hope this helps,

jd
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Old 26th February 2010, 07:29 AM   #4
Mooly is offline Mooly  United Kingdom
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Hi Carlos,
They are a "safegaurd" against inductive loads causing a transient that may cause secondary breakdown of the device or failure by a sudden reverse bias again caused by inductive loads . They clamp the output (in an audio amp) to the rails, any excess energy being safely absorbed by the PSU and rail decoupling.
A bit academic I think to be honest for audio. If you are driving motors etc then fair enough, but a speaker in normal use won't generate such back emf's.

That's always been my understanding.

And how fast are these diodes anyway... you remember the old BU508D etc TV line output devices with flyback diode built in... now they are fast (the diode)

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Old 26th February 2010, 07:36 AM   #5
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Default Oh!..... thank you dearest fellows....i am gratefull to you two dear friends

Now i know and i can help my boys.

thanks a lot...really thanks a lot!

Will eat less meat dear Mooly!

MP3 audio answer is beeing prepared, rigth now, to my young students.

Uncle Charlie feels very happy, very proud, when see friendship, cooperation, and education in our lovely forum.

Thread will continue opened...ahahahah..we gonna see from now on!

Carlos
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Last edited by destroyer X; 26th February 2010 at 07:42 AM.
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Old 26th February 2010, 07:51 AM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by destroyer X View Post
Will eat less meat dear Mooly!
Carlos
Lol

Yesterday was fish, today is chicken (Thai chicken) and tomorrow ? maybe moo steaks
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Old 26th February 2010, 07:53 AM   #7
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Default Man!...in the simulator i have increase the input signal till i have distortions in

the output..square wave naturally....then, when i switch these diodes one...you cannot believe the mess these diodes done to the waveform ....also to the distortion measured.

Seems they are not a very good idea.

Am i wrong?... was my testing method wrong?

regards,

Carlos
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Old 26th February 2010, 08:09 AM   #8
Mooly is offline Mooly  United Kingdom
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They shouldn't affect normal operation at all... you can try for real of course on one of your amps too. Nothing bad happens.

I think it's something with the simulation, have no idea why though...
Did it do it at low output levels ? What load... resistor ?

Below clipping should have no effect
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Old 26th February 2010, 08:16 AM   #9
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Default Was already above the threshold....maximum power...when the square wave amplitude

does not increase when you increase signal.... entering sinus and distorting till have a nice square wave, but not further beyong that input level.

Otput was combination of capacitive, resistive and inductive, Ostripper load to substitute a speaker.... i think i have made something wrong Mooly....will eat fish today Mooly, because of you, to save Cows..ahahahahah..Hake (Merluza) will be the fish.

Early morning here dear Mooly, the air is filled with a lovely smell.... bread toasted.... people here do that, they put butter and they fried the bread..sometimes they use a two piece metal thing to cover the bread that goes directly to the fire...bread is inside the metal capsule...this smells good... tastes a good Coffee with Milk.... i know English loves Tea...we love Coffee with milk, dark brown coffee with suggar...hummmmmmm, will have a tosted bread too..you are invited dear Mooly.

MooooooooooH!

regards

Carlos
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Last edited by destroyer X; 26th February 2010 at 08:45 AM.
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Old 26th February 2010, 08:48 AM   #10
godfrey is offline godfrey  South Africa
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Hi Carlos

"Why are diodes needed to protect output transistors?"

Guess what - it is because VI limiters mess up so badly!

When the VI limiter switches off the output transistor, the amplifier loses control of it's own output.

Then an inductive load can cause the amplifier's output voltage to shoot right up above the supply voltage. The diodes are needed to stop that otherwise an output transistor will be damaged.

If you do not use a protection circuit, then the diodes should not be needed.

Regards - Godfrey
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