>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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