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Tue May 27 17:05:43 EDT 2014

Typical C / I for diode filters

```It seems that dynamic resistance is quite low for the current range
1mA - 1uA which seems most useful for this type of circuit.

(EDIT: this was a bad intuition. I've seen these circuits with LEDs
that light up[1], so at least there should current in the mA range.
Typical range of C in circuits is around 20nF.)

Diode resistance is V_T / I with V_T approximatly 25mV, this gives R:

25 Ohm  @ 1mA
25 kOhm @ 1uA

Actually, this maps fairly well into the nF capacitor values.
e.g. the "audio units" mA kHz V uF kOhm

f = 1 / 2 pi R C
R = V_T / I
=>
f = I / 2 pi V_T C

Here are some ratios between I and C that would work to get the high
point at audio range.  Low point will just be 1000x less.  Here V_Tf
is the "thermal frequency voltage" 2 pi V_T corresponding to V_T=0.255

f = I / V_Tf C

V_Tf is about 0.16V or 6/V

mA      V    uF         kHz
(/ 1    (* 0.16 0.330)) => 19
(/ 1    (* 0.16 0.220)) => 28
(/ 0.5  (* 0.16 0.100)) => 31
(/ 0.1  (* 0.16 0.033)) => 19
(/ 0.1  (* 0.16 0.022)) => 28
(/ 0.05 (* 0.16 0.010)) => 31

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