Maker Pro
Maker Pro

Dumb question of the week: Mis-matching & matching

B

billcalley

Jan 1, 1970
0
Hi All,

I have been trying to figure out this basic concept for quite some
time now, but without much success: When purposefully mis-matching an
RF stage (such as a PA) in a 50 ohm system, I sometimes see references
alluding to the fact that the next stage will *not* be able to see 50
ohms due to this mismatched stage. The part that really confuses me
is: Why not? Why can't the PA's output matching network not only
purposefully be designed to conjugately mis-match the PA's output (for
max Pout) AND also present exactly 50 ohms at the other end of this
same matching network? And if, for some reason, this is not
possible, then why not just add another pi (or L) network so as to
give the next stage exactly 50 ohms? Am I understanding this
correctly, or do I have it completely FUBAR'ed up?

Many thanks!

-Bill

Hi Guys,

From what I have been able to absorb now from all of your really
terrific responses is that it is totally and completely impossible to
LC match, even theoretically, a purposefully mis-matched active
device*, and then look back into that mis-matched network and see a
perfect 50 ohms for the next stage. However, the purposefully mis-
matched active device WILL get to see the impedance it wants to see.
And all this is due to the reciprocal nature of LC matching networks.
Thus, it would be wise, as you have all mentioned, to design any
non-50 ohm LNA or PA to be as close to 50 ohms as possible, or to
design the next connecting stage so that it properly works with
something other than 50 ohms -- 'cause there is no way to "fix" this
mis-match issue with an LC matching network. Is that correct, or have
I misunderstood something? (I can't believe I didn't know -- or
didn't understand -- this stuff from the get-go! A major glitch in my
knowledge-base, that's for sure.)

Many Thanks,

-Bill

*The mis-match created so as to optimize an LNA transistor's input for
NF, or a PA transistor's output for P1dB, to name two common reasons.
 
J

Joerg

Jan 1, 1970
0
billcalley said:
Hi Guys,

From what I have been able to absorb now from all of your really
terrific responses is that it is totally and completely impossible to
LC match, even theoretically, a purposefully mis-matched active
device*, and then look back into that mis-matched network and see a
perfect 50 ohms. However, the purposefully mis-matched active device
WILL get to see the exact impedance it wants to see. And all this is
due to the reciprocal nature of LC matching networks. Thus, it would
be wise, as you have all mentioned, to design any LNA or PA to be as
close to 50 ohms as possible, or to design the next connecting stage
so that it properly works with something other than 50 ohms -- 'cause
there is no way to "fix" this mis-match issue with any LC matching
networks. Is that correct, or have I misunderstood something? (I
can't believe I didn't know -- or didn't understand -- this stuff from
the get-go! A major glitch in my knowledge-base, that's for sure.)

That's pretty much the situation. But hey, it's not a glitch, we all
learn. I recently had a similar comeuppance with laser diodes where I
realized that the stuff engraved in my knowledge base was quite archaic
and needed some serious updating. And, like you, I got some of those
updates right here in the newsgroup.

To push a LNA close to 50ohms is possible but I wouldn't torture myself
too much. Above a few ten MHz the old trick of "noiseless feedback" via
transformers becomes nasty because you'd almost need stripline
techniques. Maybe you could compromise the BPF a bit instead.
 
J

joseph2k

Jan 1, 1970
0
billcalley said:
Hi Joerg,

But any filter at the output of the PA stage would have to 'see' 50
ohms in order to maintain its as-designed response...

Best Regards,

-Bill

OK. I am sure that someone has explained that the matched condition
provides maximum _power_ transfer. This is not always the most desired
property. It is frequent in audio work to have small impedances driving
large impedances, this provides best broadband (3 decades of frequency)
linearity and thus low distortion. Most audio power amplifier output
impedances are below 1 ohm, intentionally, to keep reactive properties of
the load from disrupting linearity.
In RF work, power transfer (to reduce losses before radiating from the
antenna) in the most important property. It also impacts operating economy
as transmitters operate above P(out) over 100 W.
For vacuum tube amplifiers the optimum matching impedance is the plate
resistance. Three (tunable) L sections can transform impedances as high as
50:1 for spot frequencies over the HF range (2 MHz to 30 MHz), i have seen
it in operation and worked on such gear. The RF final was two 4CX600J in
parallel each with a plate resistance of 2000 ohms.
In earlier bipolar transistor RF outputs (about 15 years ago) when per
transistor P(d) hit 300 watts but the operating voltage was not over 24 V
the collector impedances became very low, about 2 ohms in this case. With
modern mosfet output transistors operating at 100 V the characteristic
drain impedance was about 33 ohms and much more reasonable matching
requirements.
 
Top