🚀 go-pugleaf

In article <JON_SREE.91Jan21215810@world.std.com> jon_sree@world.std.com (Jon Sreekanth) writes:
>
>A rather curious query : what is the audio frequency impedance of 
>
>Specifically, for a small signal diode like 1N4148, between 0 - 70 C,
>with about 2.5 V or more of reverse bias, is the small signal audio
>impedance at least > 1 Meg ? 10 Meg ? I don't need an exact number,
>just a minimum, to see if it interferes with other circuitry. 

I checked my National Semiconductor data book, so here goes. 

The reverse capacitance for the 1N4148 is about 1.5 pF at 2.5V (don't
know the measurement frequency - probably 1 MHz.)  Assuming an upper
frequency limit of 20kHz for audio, the impedance will be approximately
equal to the capacitive reactance in parallel with the leakage impedance.
From Xc = 1/(2*pi*f*C), Xc is about 5.3 Meg.  Leakage current is harder
to determine (graph only goes down to 5 Volts, so I'll use that value.)
At 25 degrees C, leakage current is about 8 nA (the graph actually shows
8 mA, but the 'm' in 'mA' is a typo!).  The value decreases at lower
temperatures and increases at higher temperatures.  If we allow the 
current to double every 10 degrees, then from 25 to 75 degrees, the 
current will be increased by a factor of 32 (this is a common approx.
to the temperature coefficient of *reverse* biased silicon semi's), to
a value of approx 250 nA (32*8=256).  The impedance due to reverse leakage
is then, from Ohm's Law, E/I = 2.5V/250 nA = 10 Meg.
Applying basic complex math, the dynamic impedance is about 3.7 Meg.  
Based on the assumptions above, this should be somewhat of a lower limit,
so this can be treated as a minimum.
It is interesting to note that the diode's capacitance is so low that
the circuitry around it (wiring, etc.) is far more likely to affect
the reactance of the circuit's nodes.



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**   Jeff  Verive  |  If they ever stop making those little candy flowers **
**   259371048378  |  for birthday cakes, I shall lose my will to live.   **
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