John Larkin said:
There's loss along both transmission lines, both from resistive losses
in the inductors (or equivalent) and resistive components in series
with the fet gates and drains. And a lumped LC transmission line
doesn't behave like an ideal distributed line anyhow.
Is there any loss in a lossless amp? There can't be, if both lines are
lossless and there is no coupling (s12 = s21 = 0). That's never the case,
of course -- the most common exception being a hell of a lot of transfer due
to Miller C. But will that alone cause attenuation?
That's a pretty weird situation as it is, coupled transmission lines...
well, not so weird, such a structure can be expressed as, for instance,
differential lines over a ground. There's mutual and to-ground
capacitances, pretty basic. But the quirky part is you've stashed
amplifiers between them, so instead of coupling or loss, there's
unidirectional* gain. That's not your average twinax.
*Hmm, it's not strictly one direction, since there's a nonzero reverse gain
(s21)... 'anisotropic' would be better, but the word implies a spacial
rather than graphical relationship. Oh well.
With lossy elements, there will obviously be loss. Say, how lossy are
component capacitances, anyway? That's not something ever specified for
semiconductors. That, and inductor loss, sum up the lossiness of a real
distributed amplifier. It must've been pretty good back in the toob days,
even the early Teks had like 13 6DJ8s in a row. Tubes are fairly ideal
capacitors, which just so happen to have electrons flying around inside to
do Work. Tubes always give you an honest RC time constant regardless of
bias or amplitude. No voltage controlled capacitance, no storage time, no
slew rate limiting, just RC. Do the same thing with transistors and you'll
get all sorts of ugliness... and just imagine, someone tweaks the vertical
offset knob and your total capacitance goes up by 100pF... argghh!
Yeah, I think Hittite *earns* their money...
Tim