RadioBanter - RF amps: tuned load in Class A?

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On Mon, 08 Sep 2003 10:23:36 -0700, gwhite wrote:

That is clever -- you want me to "declare" something is true that I've
made no reference to. You are quite the inventor. Face it: you had an
incorrect notion about linearity. All the rest of your words are
twisting, squirming, and turning to try to save face after you acted
condescending (and still do) about a very simple matter. That's all.

I have to say I've been wrestling with this attitude problem of kev's
as well. Earlier this evening I postulated to myself that the reason
for all these contradictory posts and arguments over semantics might
possibly be due to Kev's impared usage of English. If the guy's
dyslexic or has some other comprehension problem, he might very well
be an electronics genius but we'd find it hard to tell because this
veil between he and us muddies the water both ways. You view as
expressed above is a very much more cynical one, but I'm forming the
view that one or t'other must explain it. But Kev has admitted to
English not being his strong suit elsewhere on the group and if that's
the case and he really *does* know what he's talking about then I feel
sorry for him. It must be pretty ****ty and exceedingly frustrating
for anyone in that position. I know; I've met a few and it's ruined
their lives.

--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill

Kevin Aylward wrote:

In summary, there are differing concepts of what linearity is being
understood to mean in the real world.

No again. Those who are confused about it, and can't admit they are
simply wrong about what amounts to a widely accepted definitional
matter
[snip]
You nor anyone else need take my word for it: it is in *all* the
Signals, Systems, and Communications texts I've ever opened up -- they
are wholly consistant with each other; check for yourself. Your
"definition" is not in any of them (af(t) = f(at)???). So I feel
justified in simply saying you are flatly wrong. If you could at least
post a citation from a text that has your definition and a worked
mathematical problem/solution (no "Circuits" junk), then at least we
could say it was all a grand misunderstanding.

herewith a self-confessed doesnt-know-it-all's analysis:
IF
y(x) = mx+c (even KA cant argue with the linearity (and time-invariance) of
this....LOL)
THEN
y(ax) = max+c
AND
ay(x) = max+ac
Elementary Sesame-Street Theory (one of these things is not like the other)
clearly shows this "definition" of linearity to be rubbish.

gwhite most certainly has it correct. KA does not.

as a slight aside, I have read H&H about 8 times, and will continue to do
so - it is one of the more useful books on electronics I have ever bought
(and I have about 600 of them). If you do not have that book - GO AND BUY
IT!! I even met WINFIELD Hill at an MIT junkfest once a few years ago, and
had an interesting discussion with him about my work on high-speed PMSM
energy storage flywheels and giant SMPS. That guy is really smart - I
suggest anyone reading this forum should pay close attention to win's
postings (i sure do). As far as being an "academic" - well, go read H&H -
its beauty lies in its practicality, unlike most texts. Just because someone
works in academia, doesnt mean they are useless (although to be fair, its
usually not a bad first guess). Likewise I have met plenty of blithering
idiots out doing "real" engineering (its a good thing - competent people end
up being well paid to fix their screw-ups). The worst ones tend to work in
sales (I presume its because they cant get real jobs)

Really this entire thread has done little more than allow Kevin Aylward to
appear like a pompous idiot, with a somewhat limited understanding. A BSc
and half-a-dozen MSc courses (one A - wow. I remember those - they are what
you get if you dont do well enough for an A+) simply makes for a failed MSc.
Of all the pomposities, I just loved this one:

In all honesty, there is not much I don't know about general analogue
design, although, obviously, I don't claim to know it all.

It kind of makes one wonder just how KA knows there isnt much he doesnt
know.

why do I post on these newsboards? am I being selfish? I dont think so. I
have had questions answered for me, so it is only fair that I answer those
that i can (quid pro quo). I also get a bit of a buzz when i can "show off".
And reading posts from others can be very informative. Some stuff is
downright hilarious (thanx KA). I could "listen" to Jim Thompson & Win
reminisce for hours - riveting stuff.

Frank Raffaeli wrote:

gwhite wrote in message ...
[snipped long diatribes]

Dude, you are responding to one of the shorter messages.

Class A works just fine in multipliers/modulators -- "non-linearity" of
circuit elements is not required. Maybe you can analyze the old
MC1496. That would be enlightening to you. But more important and more
simple (it will save you loads of time), just apply *the* linearity test

[snip]
Hmmm .... you may be mistaking the (sometimes linear) current steering
effect for the mechanism within the transistor that makes current
steering possible: the relationship between gm and Ic ... or from
another POV, the change in rbb with respect to bias current. These
effects are non-linear.

Non-linearity is *not* required to create DSB-AM out of transconductance
type multipliers like the gilbert cell. In fact, *non-linearity is
specifically something that designers hope to minimize* -- just like in
any linear device. The standard linear approximation practice ensues:
that is, the taylor expansion of exp(x) is done and the linear term is
the desired one and *it is all that is required or wanted for this
linear multiplier*. The rest of the terms are unwanted and that
includes the multipliers usage as a frequency translator, which I proved
to be a fundamentally linear operation. No one needs to take my word
for it. Anyone interested can idealize the gilbert cell to only the
linear terms and the device will still be able to produce DSB-AM -- IOW,
non-linearity is *not* required.

The *linear terms* on both the inputs of a class-A biased gilbert cell
are exactly what are anticipated in the higher level system drawing, the
non-linear terms are *not required* for DSB-AM to occur (to the extent
higher order products exist they are unwanted products). Driving the LO
port into the switch mode is notwithstanding. For example:

The System
+---------------+
| |
in | /¯¯¯\ | out
x(t) O--------( X )---------O y(t)
| \___/ |
| | |
| | |
| O |
| cos(wc·t) |
+---------------+

The linear terms are *exactly* what make the gilbert cells "linear
multipliers." Linear means linear.

Here is a system which uses the above linear system, and another "linear
circuit" to produce large carrier AM:

System 1 System 2
+---------+ +---------+
| | | |
in1 | /¯¯¯\ | | /¯¯¯\ | out
x(t) O----( X )------( + )----O y(t)
| \___/ | | \___/ |
| | | | | |
| | | | | |
| O | +--- | ---+
|cos(wc·t)| |
+---------+ |
|
in2 |
cos(wc·t) |
O-------------------+

That these two systems can be (and have been) made with "linear"
circuits is indisputable. The output is indistiguishable from large
carrier AM so we can just as well call it that.

Now it is interesting that as far as the x(t) - y(t) concern _alone_
goes, this "total" system is _not_ linear (apply the linearity test to
see how/why). But that has absolutely nothing to do with any circuit
issues like exp(x) and does has everything to do with the definition of
linearity as it is taught in the EE curriculum. No "non-linear" circuit
element was required to result in a non-linear system with regard to the
x(t) - y(t) transfer. The "non-linearity" is the sole consequence of
the fact that this "total system" is a Multiple-Input-Single-Output
(MISO) system. It is interesting that System 2 is the MISO system.
With regard to either of the inputs, and its respective output, the
response of System 2 is linear -- identical in manner to the simple
op-amp summer. But with regard to the total output of System 2, when
neither of inputs are zero, the system is non-linear _with respect to a
single input_ by the EE definition. That ought to make KA's blood boil:
a "linear" circuit reduced to non-linearity by definition.

In Summary:
Non-linearity in terms of circuit elements is not required to "make
AM." The concept of linearity in the EE curriculum is consistant across
courses. That includes Signals and Systems, Circuits, and Electronic
Design. That some are not up to speed on the linearity property is
notwithstanding.

Anyway, with all this talk of AM modulators I recall some papers I
read back in the early 1980's on vector modulation ... or the use of
the sum of two phase-modulated signals at high power, with a
compensating signal at low power such that the sum yielded a perfect
amplitude modulated wave ... very little power required from the
actual linear "AM" section. The technique is used to generate an AM
signal of many kilowatts, whilst using only a few hundred watts in the
required linear [c(t)] stage.

If the audio signal is expressed as a(t), and the phase-modulated
signal is limited to a p-p deviation of slightly less than pi/2, and
the compensation signal is c(t), then the crude diagram is as follows:
______
PM(-a(t)) -----| \ ______
| T1 0--------| \
PM (a(t)) -----|_____/ | T2 0----- "AM" out
c(t)--|_____/

T1, T2 = 90-degree hybrids

IIRC, many AM broadcast transmitters now employ this technique.
Hopefully, someone out there has a better recollection than I and can
provide a reference.

You might hinking of the Chirex Outphasor, or a similar idea (I don't
know how c(t) fits into the Chirex). Yes, it can be used to make an AM
signal. The thing is it *is* a non-linear method and you aren't calling
out the amplitude to phase mapping that is required for your +/- a(t)
signal.

Paul Burridge wrote:

But Kev has admitted to
English not being his strong suit elsewhere on the group and if that's
the case and he really *does* know what he's talking about then I feel
sorry for him.

I don't dispute at all that he knows a lot about EE. Rather, I would
say he does know quite a lot. But no one knows it all and all people
make mistakes here and there. As best I can tell, and for what I
learned (and I worked pretty hard at it), he looks to be flat wrong on
this detail (and a detail is about all it is).

The good news is that a misunderstanding of what linearity *strictly*
means will not likely prevent an otherwise competant engineer from
building a very effective AM or AM/PM system.

It must be pretty ****ty and exceedingly frustrating
for anyone in that position. I know; I've met a few and it's ruined
their lives.

Now in thinking of those relationship and communication issues, I am
looking forward to the next Survivor show.