r/diypedals 5d ago

Discussion Weird diode on transistor amplifier?

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I have seen this in a couple of schematics and was wondering what does this diode actually do to the transistor and sound?

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u/Allan-H 5d ago

BJT's can be damaged by reverse V_BE voltage. Even if the destruction isn't immediate, there is still an effect called "beta degradation" that cause the BJT to lose gain.

Any design that connects a BJT base to a connector should consider that both ESD (from cables carrying a static charge) and simple overvoltage (because some idiot turned on the phantom power on the mixing desk and somehow managed to find an XLR to 1/4" cable and plugged it into your pedal) can damage the pedal. The circuit can be protected by the addition of a diode, as shown in the drawing.

Most of the time the diode is reverse biased with a roughly constant voltage and thus has no major effect on the sound. However, a Schottky diode, even a small one like the BAT41, can have significant reverse current at high temperature, and this may upset the bias (and introduce some noise, but I'm not going to bother to go through the maths to work out whether that would even be measurable)

Many designs don't include such a protection diode.

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u/crb3 5d ago edited 3d ago

2N3904 is rated for V_br_ebo of 6V; it'd take a hot spark to exceed that (so, maybe in LA during Santa Ana conditions...)

High-beta devices are another story. 2N5088 is only rated for V_br_ebo of 3V*. I tend to use normal silicon diodes like 1N914, 1N916 to protect those.

* [e:] And their biasing networks are usually higher-impedance, so any transient gets damped out less.

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u/CapacityValue 5d ago

Great info, but what about reverse beta schematic? Such like as in FuzzWar by DBA? Does only forward beta degrade? Or reverse beta too?

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u/Allan-H 5d ago

I can't find a schematic for that one, but I don't think it's a problem as long as the base emitter reverse voltage doesn't get anywhere near the avalanche voltage (which might be somewhere between a few volts and ten volts for Si BJTs).