🚀 go-pugleaf

A reverse-biased diode will have an appreciable impedance at audio frequencies,
because the main mechanism is junction capacitance, which decreases as more
reverse-bias is applied.

You can measure it by connecting a DC source, audio generator, diode, and
a variable resistor, all in series. Set the audio generator to, say, 0.1v p-p,
and adjust the resistor until the voltage across the resistor is 0.05v p-p.
Measure the resistance; it equals the diode's impedance *at the measured
frequency*. Try varying the frequency. I've never measured the impedance
of diodes before, but I would expect it to be near a megohm at 1Khz.

Now forward-bias the diode and repeat the experiment. Neat eh ? Diodes
make nifty (but sometimes noisy) analog switches.

----> NOTE ! Use a scope with a 10X probe to reduce loading effects. You
should parallel the variable resistor with the scope (1M or 10M) when 
you measure it. Otherwise, you'll have to do some calculations (not hard).