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February 21, 2012


William Ketel

Not only are diodes usable as variable resistances, and impedances, they can also be utilized as switches, by taking them out of the variable range and into either saturation or cutoff. The function in between, as pointed out, is far from linear. One more fact is that many diodes seem to generate a bit of noise while biased in the forward mode. Some generate more than others, but almost all diodes do generate some noise in the forward bias mode.

Alistair Macfarlane

One of the neatest applications of using diode dynamic resistance I ever saw was a variable cut-off ladder LP filter in an old analog sound synthesiser (Think it was a VCS3?) comprising two parallel chains of diodes with shunt capacitors, the diodes being run at a varying current to vary the cut-off frequency. Could even be run with sufficient Q to resonate giving a variable frequency sine wave, but not very stable with temperature...

Vladimir Doubovis

It is too complex. Usually diode impedance is defined by equation 25.8mV/I that complete match your calculations


I guess one should be carefull using it at high frequencies because the charge stored in direct biased the junction might be quite high and affect it behavior at high frequncies.
Also, I guess one can rewrite formula as dV/dI =(q/kT)/(I+Is),
where q/kT = 26mV. Often one can neglect Is (Is=0).

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