[Amps] TS-440S Update

[Richard]

>Rich said;
>
>>Simply add a low-ohm chip-R in the emitter or source (FET) to create 
>RF-NFB.  
>
>
>Hmm, I'm not sure that is the only answer. NFB, yes, (despite what the
>audiophools would have us believe!), but there are methods using transformer
>feedback which have certain advantages. 

€  Especially in broadband RF applications, transformers have frequency 
dependant phase-shift.  What is negative feedback at 2MHz can be positive 
feedback above 20MHz -- just what is needed for oscillation.  Chip 
resistors are less prone to this malady. 

>Although an emitter resistor does help
>thermal stabilisation.
>
€ and increase driving Z.

>The logarithmic transfer characteristic of the bipolar transistor does 
>mean that it isn't as as good as a MOSFET for linearity, although I have 
never done the
>mathematical expansion to determine the actual level of the coefficients as
>compared to a tube.
>
Without RF-NFB, tubes are not wondrously clean.  6146Bs are c. 15x 
cleaner with feedback.  

>Small geometry MOS can be very linear, although it isn't exactly the stuff 
>for high power! Because of charge carrier velocity, channels of sub micron 
length
>tend to a linear transfer characteristic, rather than the classic square 
>law of the FET, but the break down voltages are too low for power (above a few
>milliwattts) use.
>
>But these points are somewhat esoteric, and get away from the original point
>about clean tx's.
>
When I was working at Raytheon, an engineer said that that if we had been 
using solid-state since Marconi and someone invented a miraculous new 
device that would operate at 250ºC case-temp, it (i.e., the vacuum-tube) 
would be hailed as a techological breakthrough.  
>
cheers, Peter

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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