Germanium diodes- the definitive answer

Frequent question abouts LED and other types of diodes.
Lonkero
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Post by Lonkero »

What's the point anyway in putting same type of diodes in series? Does this raise the clipping voltage point?

I've always thought (eg two 1n1418 diodes in series ) would clip as usual on ~0.6V and just by putting two of them on a row has some subtle effect on tone since other diode may clip a few millivolts earlier or later, but nothing more.

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Post by bhill »

The second diode in series does not see any voltage until the first diode tries to conduct at around .6v. Then the second will block that voltage until the input rises to the point that BOTH conduct, at 1.2v. This gives you a higher output than a single diode will, but still lower than leds at 1.6 - 2.2v

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Post by bhill »

That last applies to si diodes, not germanium. For germs, cut that voltage in half, i.e. two germs in series = one silicon diode for forward voltage.

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Post by Seiche »

so what's up with putting diodes in parallel, but bands on the same side. like 2 one direction and 2 the other?

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Post by FiveseveN »

Good question. I'd wager the tolerances are high enough that the lower-threshold one starts conducting before the other has anything to say.
Probably why we don't see that configuration very often.
Except when a lower-threshold pair is clearly shunting another, and said pair is on a switch. That is rather popular.

PS: had a look at some figures on I-V charts. Devices in parallel means same voltage, twice the current over voltage if the devices were identical. As far as I can tell, if tolerances are tight enough, the result would be a sharper knee (to a maximum of 45°+tangent to the initial graph).
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Post by Seiche »

i just always assumed that adding diodes in parallel in the same direction doesn't do anything. But Joe Gore mentioned it in the Tonefiend beginner's guide and I figured there must be a difference. So that only works if they're the same type (tight tolerances assumed)?

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Post by rezzonics »

Germanium diodes start clipping at lower voltages 300mV, compared to 600mV of silicon diodes and 1.2V of LEDs, that means that for germanium diodes, clipping starts at lower levels and hence distortion happens in a wider dynamic range.
Germanium diode clipping has a more rounded I-V response than silicon and hence clipping is softer.

I recently did some research trying to find an alternative solution to germanium diodes.

Germanium diodes are expensive compare to silicon diodes and difficult to find in big distributors and out of specialized shops.
I was thinking if there is an alternative to replace germanium diodes while still having the same clipping response:
- clipping at low signal input levels
- softer clipping than silicon diodes
I am not sure if this is a subject that somebody has already raised but I think that I could have found an answer to replace germanim diodes that I would like to share with you. Let me know what you think.
A germanium diode could be replaced by a Schottky diode in series with a resistor to obtain very similar results.
I've been doing some LTSpice simulations comparing I-V response of both circuits and though exact overlap is not possible, the two goals previously mentioned can be met: clipping at low levels and softer clipping.
The figure below shows the I-V response of a 1N34A germanium diode (red trace) and the I-V response of a BAT54 Schottky diode with series resistors betwen 5 to 500 ohms (gree traces)

Image

BAT54 diode is an inexpensive widely used Schottky diode that can be found in a dual configuration in the same package for positive and negative clipping. SMD packages are also available.

The following figure shows the soft clipping response of both circuits:

Image

The most adequate series resistor value for soft clipping seems to be around 250 ohms (for a 1kohm opamp input resistor)

and the hard clipping response:

Image

The most adequate series resistor value for hard clipping seems to be around 14 ohms.

The schematics used for these simulations are shown below:

Image

Additional information can be found in my blog rezzonics.blogspot.com

These simulations must be confirmed by real experiements, though, but it looks promising.
Let me know your thoughts.

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Post by commathe »

Good work! Definitely promising. I wonder how well the resistor and schottky can emulate germanium in other situations though (to ground clipping etc)

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Post by Dirk_Hendrik »

FiveseveN wrote:Good question. I'd wager the tolerances are high enough that the lower-threshold one starts conducting before the other has anything to say.
Probably why we don't see that configuration very often.
Except when a lower-threshold pair is clearly shunting another, and said pair is on a switch. That is rather popular.

PS: had a look at some figures on I-V charts. Devices in parallel means same voltage, twice the current over voltage if the devices were identical. As far as I can tell, if tolerances are tight enough, the result would be a sharper knee (to a maximum of 45°+tangent to the initial graph).
Diodes in parallel in the same direction is always nonsense. Even when the 2 devices have excactly the same forward voltage one of them will always be the lowest (there's tolerance somewhere nehind the decimal point). That means that that one will always do all the work and conduact all the current.
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Post by DWBH »

How about note decay?
I've always had some kind of crackly/sputtery decay when using LEDs as clippers. Silicon/Mosfets/Ge were always smoother. Sharper I-V response?

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Post by DrNomis »

When playing around with various types of diodes in back-to-back diode clipping circuits, you will notice that the sharpness of the clipping will have a big effect on the tone generated by the clipping, the tone will sound buzzy if the clipping is sharp, and smoother/thicker/midrangey if the clipping is rounded, note that this is without considering the effects of the tone controls, or eq circuits...... :thumbsup
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Post by rezzonics »

DWBH wrote:How about note decay?
I've always had some kind of crackly/sputtery decay when using LEDs as clippers. Silicon/Mosfets/Ge were always smoother. Sharper I-V response?
LEDs clip at higher voltage, 1.2V or more, that means that low level signals don't suffer distortion and I would say that the I-V response is as sharp as a silicon diode but at a higher level. I think that what makes germanium diodes generate a nice distortion is a less sharp I-V response at very low levels, so you have less high harmonics but at lower levels, that's the goal of using schottky diodes with resistor in series: the schottky diode assures a low level clipping and the series resistor softens the clipping to the desired level.

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Post by tabbycat »

thanks to dr nomis and rozzonics for the graphs and explanations and whatnots. an interesting read.

rezzonics, am definitely looking forward to hearing your schottky shin-ei. don't keep us in suspense.

dr nomis, you posted this diagram of the jcm diode configuration you used to produce those oscilloscope readings.

https://www.freestompboxes.org/download/ ... &mode=view

it's pretty weird looking when all i've thought about re clipping diodes to date has been the classic 'pair' (and variations of using different types and values in series but keeping the basic restricted 'two way traffic' principle). what is the diode across the middle of your configuration doing? apologies if it's a newbie-type question. i still am in most ways.
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Post by rezzonics »

tabbycat wrote:thanks to dr nomis and rozzonics for the graphs and explanations and whatnots. an interesting read.

rezzonics, am definitely looking forward to hearing your schottky shin-ei. don't keep us in suspense.

dr nomis, you posted this diagram of the jcm diode configuration you used to produce those oscilloscope readings.

https://www.freestompboxes.org/download/ ... &mode=view

it's pretty weird looking when all i've thought about re clipping diodes to date has been the classic 'pair' (and variations of using different types and values in series but keeping the basic restricted 'two way traffic' principle). what is the diode across the middle of your configuration doing? apologies if it's a newbie-type question. i still am in most ways.
Actually you can hear it here:

Rezzonics version of the Shin-Ei Fuzz-Wah - Sound check (4/4)

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