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Dumb question of the week: Mis-matching & matching

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.
Reply to
billcalley
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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.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

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. 
--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller
Reply to
joseph2k

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