Diodes Was: Re: opinions on HIFLI...

[Rene Schmitz]

At 11:11 17.04.1997 +1000, you wrote:
>>
>>And you're right about that voltage drop. The situation here is different,
>>you control the current. You don't care about the voltage drop.
>>There are Ge and Si types whose U/I curves seem the same but only 400mV
>>shifted apart. The principle was the dU/dI (dynamical resistance) varies
>>with the current. And the forward voltage drop doesn't matter, since it
>>disappears when differentiating. We're after dU and not U.
>>
>>Hope this makes sense!
>
>I'm no expert on analog stuff :(  but I follow what you are saying. At what
>sort of current range does this occur?
>
>Ken

>From the diagrams I have here (u/i plot of a 1n4148 (si) and a AA143 (ge)) 
I would say the dU/dI changes most quickly at about 500uA.
Actually this effect occurs over the whole range. Since were dealing with a
voltage divider here, it is not possible to say a specific value.
The attenuation depends on both the resistance of the diodes, and of the
Resistor.
But to give you an impression try the following: 
Apply a signal thru a 100k to the center of two diodes, feed a current of
0..2 mA 
tru both diodes to ground. (We certainly don't need the symmetrical version
here!)
Set the signal levels with no current so that there is no distortion,
monitor the output.
Raise the current. The signal should disappear.
You can vary the values of the resistor and the current according to your needs.
I personally would not use a current source but a resistor of 4.7k, and a
variable 
voltage in the range of 0..10V.

Hm, I even tried this with two LEDs and it seems to work.
Could this be useful to lower the parts count in a diode-based phaser?
Any Opinions?

Bye 
    René