There are a lot of magical properties attributed to diodes. They’re frequently used to introduce clipping in audio effects, and there is a lot of hyperbole and emotion out there about the way different types of diodes clip and how that impacts the sound.
Some folks from the Audio Electronics DIY community tested a bunch of diodes to see exactly how they behave. Their data was captured here, including visualizations of the forward voltage (VF) curves—plots that show how current starts to be let through as the voltage increases. I thought that I could contribute by making their plots mildly interactive and more user-friendly for the web, but due to technical limitations of Trevor’s site, he wasn’t able to incorporate the code. I have therefore reproduced their data (and much of their explanations, verbatim) here with their permission.
Trevor, Dylan, and David (and perhaps others) performed tests on many different kinds of diodes to get definitive answers to questions like “Why do some people prefer germanium diodes over silicon?” or “What is the effect of multiple diodes in series?” or “Why would anyone use the PN junction of a MOSFET like a diode?”
Tools and Equipment:
- Two multimeters (for voltage and current)
- Stopper resistor (1k)
- Variable resistor (100k)
- Power supply: 9V battery or lab PSU 0-20V (recommended)
- Breadboard and jumper wires with alligator clips
By varying the power supply or RV1 it is possible to alter the current through the diode under test. The testers measured and noted the voltage across the diodes at 0.1mA, 1mA, 5mA, and 10mA (although to make the charts prettier on this page, I’ve skipped the 10mA test points, which tended to de-emphasize the interesting part of the curve and are probably beyond the point where most clipping circuits clip). When possible, three or more diodes of the same type were measured and averaged, since there can be quite a bit of variability.
This is all of the diodes that were tested, including various general purpose silicon, germanium, Zener, and other semiconductors.
Germanium and Schottky
Germanium (Ge) and Schottky diodes are characterized by having a low VF of around 100-200mV. What makes germanium diodes more interesting than silicon (Si) to many people is that while they allow current through at lower voltage (and can therefore dump more of the tops of waves to ground for more clipping), they have "a soft knee" and engage more slowly, which may make an audible difference in some circuits. The "softness" is visible in these plots by the steepness of the curve, with shallower curves indicating slower engagement.
In these tests, some of these turned out to be fake, such as the 1N34A from Ebay, which exhibits the steeper curve of a silicon diode, and appears to actually be a 1N60 (silicon) Schottky with its markings rubbed off. On the other hand, the BAT41 is proudly a silicon Schottky, but has a curve much more like a germanium.
Also included in this view are a couple diode combinations. Two 1N34A Ge diodes in series results in double the VF but also an even shallower curve than either on their own. If someone wanted high VF (resulting in a higher output signal) but also with a soft curve, it could be achieved with a combination of a Ge and Si, as shown by the combination of a 1N34A and 1N4148 in series.
Silicon is so important to semiconductors that they named a whole valley after it.
Silicon diodes were considered a drastic improvement when they were developed, although what makes something good from an electrical engineering standpoint doesn't necessarily mean that they sound good when used in an effect. Their electrical properties don't change with temperature as much as germanium, and in general, they have a steeper curve. The Schottky diodes (1N5817, 1N5819, 1N60, and BAT41) have a low forward voltage, although the BAT41 once again shows that it might as well be regarded as a germanium diode with its soft knee. Also of note is that 1N4148s are essentially the same as 1N914s.
A Zener diode is a type of diode that is meant to allow "backward" current at a specified voltage (the Zener voltage) and is commonly used to stabilize the voltage in a low-power circuit. When used in forward bias, they behave like a standard silicon diode, but in reverse they have a shallow curve where current ramps up slowly until the Zener voltage is reached.
In the above graph a common 1N4148 silicon diode is used as reference. Zener diodes in reverse bias have a rather soft curve, but also need a lot of voltage before they start allowing current through. There were two types of 1N4728 tested. These are both supposed to be 3.3V Zener diodes, but the one of dubious origin went all the way up to 4.3V, indicating that it was either faulty or mislabeled. Note that one Zener can theoretically be used to clip both sides of a waveform, since they operate in both forward and reverse bias.
Light Emitting Diodes
In the normal world, outside of pedal zaniness, light emitting diodes (LEDs) are primarily used for their...light-emitting properties. They're also popular in audio circuits for their electrical properties: they have a relatively high VF that allows more signal through before clipping. Sometimes they even light up when being driven hard enough. They're what makes a Turbo Rat "Turbo."
Every once in a while, I try to remind myself how LEDs work, but it's ridiculous: something about how when electrons jump across the PN junction of a semiconductor they go from a high orbital to a low orbital and release energy in the form of photons? In most diodes, that jump is so short that the photons' frequency is too low to make visible light (although they probably are making invisible light). That gap can be tuned, however, to emit photons of other frequencies, including those in the visible spectrum. That means that LEDs are not optimized for being ideal diodes in the electronic sense, but for emitting a certain color of light—and sub-optimal components are often of great interest to pedal makers. An LED's VF is related to its color, so different colors can be used for different amounts of headroom in a clipping circuit, although red is probably the most commonly used. Red LEDs clip at lower voltages than other colors, so they're more reliably audible. Some colors have such high forward voltages that they'd rarely clip in a 9V circuit, at least in any obvious way.
MOSFET and JFET
Some pedal builders thought it would be interesting to use the PN junctions inside of FETs as clippers.
The 1N34A Ge and 1N4148 Si are included again as reference. In practice, the body diodes of FETs behave like normal silicon diodes. The forward-biased PN junction of these FETs result in shallower curves.
Again, this is not my work. I modified the text to be closer to my voice, but the testing and analysis was all done by others from the Audio Electronics DIY community. It is reproduced here with their permission.
- Original Content: https://therepaircafe.wordpress.com/2019/10/24/forward-voltage-of-various-diodes
- Community Discord Server: https://discord.gg/bMuhX4TkZM
- Additional Contributions: https://bentfishbowl.wixsite.com/electronics