[Milsurplus] Germanium to silicon

[Bob Camp]

Hi

The Ft has to come from somewhere in the circuit model. In a hybrid  
pi model the sources are Cbe, Ccb, Cce, and the base diffusion  
resistance. Three of the four show up as a change in "C" in the  
circuit. Since the diffusion resistance is "in front of" Cbe it  
impacts the capacitance of a tuned circuit as well.

If you decide to do the solid state thing there are a couple of  
things you will find pretty fast. Tubes have a very low Gm compared  
to Fet's or especially to transistors. They "make it up" by having  
much higher input and output resistances, and much lower  
capacitances. A FET cascode with significant source resistance is one  
approach. Even that is still pretty low impedance compared to a tube.  
Obviously pentodes are even worse than triodes when it comes to being  
high impedance / low capacitance.

The germanium to silicon conversion is most likely to work if you  
look in the audio transistor bucket rather than the RF transistor  
tables. Most of the germanium parts looked more like a 2N3904 than  
like a 2N918. A lot depends on exactly what the circuit is doing.  
Each part of the schematic is likely to present different issues.

The LM / BC221 conversion to solid state does work, been there, done  
that. You can do it pretty easily with a couple of JFET's. If there's  
a "secret" to the conversion it's that you need a diode to generate  
AGC bias on the oscillator FET.  You do need a new calibration book  
after the conversion.

Bob
KB8TQ


On Aug 31, 2007, at 1:23 AM, Hue Miller wrote:

> Why would Ft have anything to do with capacitor values,
> particularly tuned circuits?
> I'd do one device at a time, trying to maintain same overall
> sensitivity and outuput. I think this is a path less taken,
> but there are probably lots of clues out there to help.
> -Hue
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