EMRFD Message Archive 14066

Message Date From Subject
14066 2017-06-26 10:00:41 Ray Anderson Re: Digest Number 2933


Looking at some spice models for typical Si and Ge diodes we find the following:


Rs (series resistance) is about 7 ohms for a germanium diode (from 1N34 model) and 0.65 ohms for a silicon diode (1N4148 model).


The series resistance in the device model would be the element responsible for determining the thermal noise of the device as determined by the classic equation


Vn = SQRT(4*kB*T*R*BW)


I haven’t any idea of what the equivalent series resistance of a vacuum tube diode is as it doesn’t seem to be specified in the typical tube datasheets.


Bottom line is that I’d expect the noise performance of a silicon diode to be better than a germanium diode.


Bear in mind that I’m not a device physicist (but am a signal integrity engineer).  Perhaps someone with more experience with device physics could comment.









Fri Jun 23, 2017 12:24 pm (PDT) . Posted by:


I was looking at the circuit diagram of my Heathkit IM-18 VTVM - noticed the 6AL5 dual diode tube.

The I remembered reading this issue of GE Ham News (July/August 1951) with the Signal Slicer phasing receive adapter circuit for SSB:

< http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2006%20No%204.pdf http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2006%20No%204.pdf >

On page 10, 2nd column, the author mentions that he considered using germanium diodes instead of the 6AL5 but used the tube because the effective noise resistance of Germanium diodes is high compared to a tube diode.

This would prompt some questions: is this also true of Silicon diodes ? What data sheet parameter might serve as figure of merit for effective noise resistance ? Would the possible lower noise of a vacuum tube diode be worth pursuing for 21st century mixer circuits ?

I know, measurements are in order - that brings me back to fixing that Heathkit :)


Chuck, WB9KZY


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14067 2017-06-26 12:00:24 Rick Herndon Re: noise: 6AL5 versus semiconductor diodes
[I am hoping to get the topic subject line back on top. Please, those who reply to Daily message collections, use the original subject line.]

Back when I worried about vacuum tubes and their internal resistance, I used the internal voltage drop and the normal maximum usable voltage to figure the internal resistance.

That worked for me, when I would replace vacuum rectifiers with one or two silicon diodes and a resistor. 

Rick Herndon K5FNI