[Milsurplus] Germanium to silicon

[jzonfrelli at comcast.net]

If somebody has a schematic, I can re-engineer the device. Germanium has a Vbe of approximately
0.3 volts; A silicone divice has a Vbe of 0.7 volts. If the gain is too high, I would use negative feedback. This increases the input impedance, decreases the output impedance, reduces distortion and increases the bandwidth.
Anybody have a schematic?
Joe
afa0w

-------------- Original message -------------- 
From: Bob Camp <ham at cq.nu> 

> 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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