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-   -   Schottky diode as RE? (http://www.diyaudio.com/forums/solid-state/54357-schottky-diode-re.html)

lumanauw 28th March 2005 09:33 AM

Schottky diode as RE?
 
In EF output stage, usually we use 0.1ohm-0.5ohm/5watt resistor as RE/Rsource degeneration. Has anyone tried to put schottky diode (TO-220pack) in this place?

About diodes....IN4148, is it schottky or what? I cannot read it in its pdf datsheet.

nemestra 28th March 2005 09:46 AM

Hi lumanauw

Douglas Self mentions the use of Schottky diodes for this purpose in his books and also in an article in Electronics World. His conclusions were that it would work but provide no benefit over resistors (if I remember correctly).

1N4148 are not Schottky, they are standard diodes (and not of use for Re replacements as their current handling is very limited)

James

lumanauw 28th March 2005 09:58 AM

Hi, James,

I'm thinking about less variation in VBE multiplier if we use schottky in the place of RE.

John Curl writes that in uS time, current flowing in RE can reach more than 20A. If RE is 0.22ohm, it has 4.4V accross it alone (VBE doesn't have that value, the other transistor and its all driver must be off/heavily reversed at the time 20A passed).
If passed 20A , diodes wont give drop as big as 4V4, isn't it?

Inspired by NP who uses ordinary diode in parrarel with RE, so I think why don't just use schottky in this place, without any R? Schottky has very smooth characteristic compared to ordinary diode.

Also thanks about the info about IN4148. If it is ordinary diode (not fast recovery or soft recovery) why is the datashee, the TRR only 25nS, like fast recovery diode? Or is IN4148 is a fast recovery small diode?

Oh one more question. What kind of diode is LED? Is it fast/soft/schottky or ordinary diode?

ilimzn 28th March 2005 10:00 AM

Perhaps the only benefit would be that you could easily cool them if they are TO220 style packages. But I do find the idea interesting - what would be the principle behind it? Reduction in losses compared to resistors? It would certainly make I-V protection difficult to design...

lumanauw 28th March 2005 10:08 AM

The main principle is that we have (much) less drop than resistor when big current passing to speaker. Speaker impedance usually written as 8ohm. But if we see the impedance curve, especially in 2 way or 3 way speakers driving real music, the noted 8ohm impedance is not flat. There are frequencies which the impedance is more than 8ohm, but there are also frequencies which impedances drops very low.

But the question : RE with resistor usually needed in parrareled output stages to balance the heat between each transistor (when output to speakers is big, heat needs to be spereaded evenly between each output transistors). If we put schottky in the place of RE, the balancing act is still there or not? If not, then it is dangerous using schottky for parrareled output transistors.

Mr Evil 28th March 2005 10:45 AM

Quote:

Originally posted by lumanauw
...Also thanks about the info about IN4148. If it is ordinary diode (not fast recovery or soft recovery) why is the datashee, the TRR only 25nS, like fast recovery diode? Or is IN4148 is a fast recovery small diode?..
It's a small-signal diode, so it's bound to be quite fast compared to big rectifier diodes, if that's what you're thinking. 1N4148 is the standard small-signal diode. If you see a diode somewhere, chances are it's a 1N4148.


Quote:

Originally posted by lumanauw
...Oh one more question. What kind of diode is LED? Is it fast/soft/schottky or ordinary diode?
LEDs are LEDs; they're not quite the same as other types. They aren't intended for rectification, so you'll never see their recovery characteristics specified. I expect Google will reveal information on how the various types of diodes work.

Eva 28th March 2005 12:56 PM

According to some datasheet the 1N4148 is a small signal Schottky diode featuring 4pF capacitance and 4ns recovery time. Its only drawbacks are their limited voltage and current capabilities [75V and 300mA] and its high Vf drop [>650mV]

Anyway, I think it's a nice and very useful diode model for low current applications

Mr Evil 28th March 2005 01:27 PM

Are you sure? The datasheet(s) I have for 1N4148 call it a silicon switching diode (indicating a normal PN junction diode). The forward voltage drop graph certainly looks like a junction diode.

lumanauw 28th March 2005 01:57 PM

Hi, James,

I just re-read my Doug Self book. I cannot find where he writes about schottky in the place of RE. Where / in what chapter is it?

Hi, EVA,

In some schematics, the signal are passed through IN4148 or LEDs. But I never see small signal passing thru Schottky. Why is that? ?Is schottky worse than IN4148 or LED in passing small audio signals?

In your opinion, LEDS similiar to what diode? schottky?fast/slow recovery or what?

nemestra 28th March 2005 02:07 PM

Quote:

Originally posted by lumanauw
Hi, James,

I just re-read my Doug Self book. I cannot find where he writes about schottky in the place of RE. Where / in what chapter is it?


Hi lumanauw,

see Audio Power Amplifier Design Handbook, 3rd Edition, pg. 131-132.

James


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