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On Topic: Distributed amplifier?

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Tim Williams

Jan 1, 1970
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How much phase shift is necessary between stages before "a shitload of amps
in parallel" is called "distributed amplifier"? And what is the role of
that phase shift, what does it actually accomplish? Like, why not put a
bunch of amps in a circle and feed their outputs radially towards the center
and skip the phase shift BS?

Tim
 
M

mike

Jan 1, 1970
0
Tim said:
How much phase shift is necessary between stages before "a shitload of amps
in parallel" is called "distributed amplifier"? And what is the role of
that phase shift, what does it actually accomplish? Like, why not put a
bunch of amps in a circle and feed their outputs radially towards the center
and skip the phase shift BS?

Tim
Phase shift is not the objective, it's the consequence.
Each amplifier has some input impedance.
If you drive them all in parallel, you get LOW impedance.
If you make them elements of a distributed transmission line,
you make the whole thing easier to drive. Ditto on the output.
 
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Paul Keinanen

Jan 1, 1970
0
How much phase shift is necessary between stages before "a shitload of amps
in parallel" is called "distributed amplifier"? And what is the role of
that phase shift, what does it actually accomplish? Like, why not put a
bunch of amps in a circle and feed their outputs radially towards the center
and skip the phase shift BS?

As already pointed out, the input and output capacitances would be in
parallel. The failure of a single component could take down the whole
system.

A similar way to avoid this kind of problems is to use the Wilkinson
power divider to split the input signal into several paths to be
amplified and combining them back again with an other Wilkinson power
divider. This has been often used to combine a large number of medium
power transmitter modules to produce similar power levels than a big
transmitter tube.

The failure of a single module only drops the total output power with
that amount, but it does not affect the operation of the other
modules.
 
C

Charlie E.

Jan 1, 1970
0
We tried building our own wideband (roughly 5 GHz) distributed amps
using discrete GaAs fets. Results weren't very good, especially step
response, which we needed to be very clean. We concluded that it would
take a lot of work to do it right, and then we discovered the Hittite
parts.

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.

John

When we were building these back in school, with our hand construction
techniques, the law of diminishing returns hit around four FETs.
Beyond that, the little inaccuracies in spacing starting digging into
your bandwidth...

Charlie
 
T

Tim Williams

Jan 1, 1970
0
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
 
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