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Vintage threads from the first ten years
Re: Tubes and transistors as diodes..
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 10/22/1999 10:34 AM |
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GFR |
Re: Tubes and transistors as diodes.. Fender has a patent on using triodes wired as diodes in a diode clipper, so I guess one of their amps may use this idea. |
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| 10/22/1999 11:41 AM |
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| Jimi2000 |
Almost certainly an invalid patent since prior art on using triodes as diodes, even as clippers, dates back to the 1950s or earlier. I saw a drawing with triodes used as signal limiters (clippers) in an elementary tube book in my local library and the copyright on it was 1959. J2K | |
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| 10/22/1999 3:18 PM |
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| Gus | I have seen an effect with the diodes as 1)1n4148 and an npn Si transistor collector and emitter connected the base as the other lead. |
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| 10/24/1999 5:19 AM |
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| David Manson |
Re: Question about diodes in general.. I saw this post and it interested me.. There are many other factors, like the "sharpness" of the diode characteristic, whether it goes from non-conducting to fully conducting right at the conduction voltage, or more slowly gets lower resistance as the voltage increases from say 0.500V to 0.510V, to 0.520V, etc. Soft "knees" are better for less harsh distortion. Are all diodes totally nonconducting when under their conduction voltage or is it a gradual leakage when it starts to approach it? I seem to notice that when I put 2 or three diodes in a series the signal strength gets weaker and weaker -- even if the conduction voltage hasnt been reached yet? any ideas? which diodes feature a totally on/off type of conduction and which are more gradual? I'm playing with biasing the cathodes in my preamp - kinda like the dual recto has... but this also applies to effects as well... |
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| 10/24/1999 2:51 PM |
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| R.G. |
Diodes leak a more-or-less tiny amount under all conditions, and they turn on a tiny amount that grows hugely right at the "turn-on" voltage. The sharpness of conduction is a characteristic of the material (Ge, Si, gallium-aluminum-arsenic-phosphide, indium phosphide, silicon carbide, - the list goes on) and of the doping profile of the semiconductor material. All diodes have a current-per-voltage characteristic that is an exponential function; that is, the current i is proportional to a constant times the voltage to some exponent times another constant. This accounts for why they seem to "turn on" suddenly. The current is miniscule until the "knee" of the exponential function is reached, and then the current skyrockets. The material involved sets the constants in that exponential equation. Diodes also leak cuerrent, and have bulk resistance, which is the resistance of the leads and semiconductor material on the way to the actual junction which is deep inside the chip of material. That means that on top of the exponential current-per-voltage, there is a temperature and voltage dependent forward leakage current and a resistive component that starts linearizing the diode at very high currents. So, taking this all together; Diodes vary a lot in how much they leak at first, and the details of how much voltage turns them on, and in series resistance. How big the leakage and series resistance are compared to the other resistances in your circuit and how big your drive signal is compared to the diode turn-on voltage are the things that determine how sudden the turn on seems. If you drive a silicon rectifier diode like a 1N4007 with a signal that is only 1V peak through a 4.7K resistor, the turn on will look very soft. If you drive the same diode through a 0.1 ohm resistor with a signal of 100V (that is, you use it as a rectifier), it starts to look like a perfect on/off switch. So it's the series resistances compared to the diode's internal resistances and the signal voltage level compared to the diode's turn-on voltage that sets the apparent knee suddenness. However, there are some things that you can say about the sharpness of the diode's knee compared to it's own turn on voltage; I think that we could get a better gauge of diode sharpness by dividing all of the voltages in the plot of current versus voltage for a diode by the nominal turn on voltage. This normalizes all the diodes to the same apparent voltage and makes for an easier way to compare sharpness. On this basis, germanium is much softer than silicon (that is, it turns on more, and earlier compared to its own turn on voltage). Schottky diodes are very sharp kneed on this basis. LED's are somewhat softer than Silicon, about like germanium. Zener diodes are sharper still. To tell the truth, I have no idea about GaASP or SiC, but these are very rare. I have a plot for electron tubes used as diodes somewhere in an old text. | |
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