Schottky diode as RE?

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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
 
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?
 
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...
 
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.
 
lumanauw said:
...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.


lumanauw said:
...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.
 
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
 
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?
 
Possible reasons for not seeing small-signal Schottky diodes used much: They have lower availability and higher price than junction diodes. The main advantages (low forward voltage drop, lack of reverse recovery glitches) are of low importance. The downsides (high reverse leakage current, high capacitance) can be more important.

LEDs likely behave most similarly to normal junction diodes since they are also made of a PN junction, but that's something you would have to measure yourself, as datasheets won't tell you stuff like that.

Personally I like 1N4448 for small-signal use. It's the same as 1N4148 but lower capacitance.
 
I've never tested the turn-on and reverse recovery characteristics of LEDs

Diodes in audio circuits are usually employed to generate voltage drops and to compensate for Vbe drop of bipolar transistrors. Typical >1A 40V schottky diodes produce too small a voltage drop [<200mV for small currents] so they are of little use and hardly seen on circuits except when such a small voltage drop is required

For signal clamping applications the 1N4148 is superior to 1A Schottkys since it features much smaller capacitance

Also I tried a 1A 40V Schottky for Vbe compensation some time ago and I think it showed too low temperature coefficient to be practical
 
Hi, James,

My book is 1st edition, maybe it don't include that chapter. What is he writing in essence that you can quote about schottky for RE?

Hi, EVA, Mr.Evil,

The reason I ask so much about diodes, because when I first think of this, I search for the most suitable diode. I came up with schottky type, but while searching, I read much about diode types, and I just realize I know so little about diode types. :D
Offcourse I won't put IN4148 or LEDS for this application here, and I asked the questions to know more about diode types.

I got another "laughable" question. In diodes like IN4148, we usually note Vdrop=0V6. What happens when Vdrop is only 0V1 or 0V3? Does it fully turnoff or does it passes very little current? The graphs I can see in the datasheet starts with 0V6.

The graphs for diodes and transistors are they reverseable? Like in condition of Vdrop=0V61 for one diode, in the graph the current is 1.5A. Could we force 5A current while the drop is still the same 0V61, or the drop needs to be more?

Also, how good or how bad schottky in comparison with ordinary diode (IN4002) because schottky do not experience reverse recovery phenomena?

Hi, Mike,

I see a very big problem using diodes instead of Re, in normal EF-type one outputbjt is typically reversebiased while the other is heavily conducting. This mechanism is used to reduce crossoverdistortion, so by using diodes you would kill this mechanism...
If you have a never switchoff topology, this is different of course !
:D Yup, that's it ! In the picture that I attach here : ......

Figure A shows what we usually do. For simplicity, I assume the VBE of transistors is one value, 0V6. In steady condition, where the bias current is 100mA passing thru 0R5 degeneration, so the VBE multiplier value will be (0V6+0V6+0R5x100mA+0R5x100mA+0V6+0V6) = 2V5. This 2V5 will be the same value in every condition, right? So point A will be sitting at +1V25 and point B will be sitting at -1V25 if we assume the output is attached to speaker who is negative is 0V. All transistors are on in this condition.

Figure B shows when there is quite some current passing thru the speaker (in frequencies when speaker's impedance drops very low, in uS time). The current that is needed for the speaker is 10A. So the drop in RE1=10Ax0R5=5V. That makes point A is sitting at +6V2. The VBEmultiplier only gives -2V5 steady difference towards pointB, so pointB will be sitting at +3V7 instead.

Lets see the output voltage node (assume 0V) towards +3V7 in pointB. TR3 and TR4 are PNP's. Their bases is more positive than their emitors, so in this condition TR3 and TR4 is heavily reversed OFF.

This is what happens in classB amp. While one side of transistor is turning ON, the opposite side will be turning OFF/reversed. This makes ON-OFF-ON-OFF-ON-OFF-ON-OFF-ON-OFF in both sides transistors.
The ON-OFF will generate high-order distortion, because abrupt state change from ON-OFF.

You can try simple experiment like figureC. Put ordinary diode (IN4002) in place of D1-D2, then play the same music.
You will hear the trebles will have "LESS" resolution (compared when you dont have D1-D2 in that position). I think this "LESS" treble resolution is the right sound, because in normal (more) resolution, what we actually hear is not a high resolution, but it is a high order distortion being feedback to differential, regenerate by output stage(+another high order distortion), re-feedback to differential, (+another high order distortion by output stage), re-feedback to differential, and so on-so on.

When you put the diodes, the ON-OFF is reduced, feedback signal that is received by front end differential has less of this high order distortion and so on, the result is "LESS" resolution highs, but actually it is less high order distortion.

This happens in the 3 stage topology class(A)B amp that we know all this time. This is caused by 3 factors :
1=Feedback (if you built non-feedback amp you won't hear it),
2=classB output stage (if you built full classA output stage, you also wont hear it)
3=VBE multiplier (if you can get with biasing technique that is sliding perfectly) you wont hear this also.

We all cannot get too far with no.1 and 2.
NP known about this, he came up with patent #3,995,228 to deal with no.3. He also came up with fig.C. (but now he seems cannot get away with classA :D )
Mark-Levinson also knows about no.3, they came up with "Dynamic bias" with their biggest amp, ML-33. Krell makes "sustained plateau".
They all tries to makes all transistors not starting cycle from deep OFF/reversed but rather from always-on condition, so the high order distoriton is less.

I think putting schottky for RE like this thread also will reduce it, and we can still uses classB output stage + feedback like we usually do.
 

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lumanauw said:
Hi, James,

My book is 1st edition, maybe it don't include that chapter. What is he writing in essence that you can quote about schottky for RE?

There is some text about the use of diodes in the first edition. It was expanded (a little) in the 3rd. In the first edition see Chapter 5, Large-signal distortion (distortion 3a), especially Figure 5.13 and the text above it on page 108.

James
 
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