Talk:Impedance matching

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Is there anywhere a definition of "L-section"??? If not this is quite unclear.

What it really needs is a schematic drawing. I think that would help considerably. I'll add it when I get a chance. Madhu 01:14, 4 November 2006 (UTC)

this should be generalized to impedances, not just resistances. i will do it eventually if no one else does, but i will have to review it. i forget how to match reactive loads. - Omegatron 19:10, Jun 24, 2004 (UTC)

Use a inductive load impedance to match an capacitive source impedances. In general, if the source impedance is (R+jX), you get the best matching with (R-jX), which is called the "complex conjugate". ... How can I say this in the article without making it sound far more complicated than it really is ?

Contents

[edit] impedance matching is a myth

The article currently says:

Whenever a source of power, such as an electric signal source, a radio transmitter, or even mechanical sound operates into a load, the greatest power is delivered to the load when the impedance of the load (load impedance) is equal to the internal impedance of the source. This arrangement is called impedance matching.

That is a common myth (although it is very close to being true).

Given a load impedance of 50 Ohms, the greatest power is delivered to that load when the internal impedance of the source is as small as possible (1 Volt at 0.1 Ohm delivers almost twice as much power as 1 Volt at 50 Ohm).

--DavidCary 01:16, 20 Jul 2004 (UTC)

It is not a myth. If the source impedance is unchangeable, then the maximum power you can transfer to the load is when Rload = Rsource. You can't usually change source impedances, unless you are magic. - Omegatron 03:26, Jul 20, 2004 (UTC)

Is so a myth. Naayaa Naayaa :-).

I can put a resistor on the power source, then use the other end of the resistor as my source. (see http://www.reed-electronics.com/ednmag/article/CA56674?pubdate=2%2F18%2F1999 for some good reasons for doing just that). Changing resistors now changes my source impedance.

 :-P So you can increase the source impedance, making the power transferred to the load lower? I see that it is useful for transmission lines when the source and load impedances are fixed, but the differences and similarities between impedance matching for reflections and maximum power theorem needs to be explained better. - Omegatron 22:39, Aug 14, 2004 (UTC)

You are quite correct for the case where I can change the load impedance, but not the source impedance or source voltage. The "myth" comes when people incorrectly apply the statement in the article to other cases.

Is there some way to change the statement in the article so that it is always true ? Or to emphasize which situations it is true, and which it is not true ?

--DavidCary 21:36, 14 Aug 2004 (UTC)

I added a comment that the maximum power theorem applies only when the source impedance is fixed. There might be some other subtleties that I have overlooked, so please correct me if I'm wrong. -- Heron 22:10, 14 Aug 2004 (UTC)

There seems to be no argument that for a given power source, with a given source impedance the maximum power into the load is maximum when the source and load impedances are equal: that has been well proved mathematically. In many cases the 'power source' impedance cannot be changed, ie radiation resistance of space, impedance of a transmission line etc, so you just have to match to it. Of course, good matching, is also important to minimise reflections and standing waves for hf signals. However, in many cases efficiency is much more important than matching, as in the domestic mains supply or a automobile battery for example. If power transfer is viewed from the efficiency point of view, it is then true that the lower the internal resistance of the power souce the more efficient is the power transfer to the load. I suspect that this is where the debate arises. Ideally, the source resistance in this case should be zero, so efficiency is maximum (with any load). In fact, for many power sources, certainly those mentioned, the load varies greatly with time and they do have an internal resistance approximating to zero (try dropping a spanner/wrench across your auto batt). Most electronic PSUs also have a near zero output impedance even at relatively high frequencies. Audio power amps are often designed to have an output impedance near zero for maximum efficiency and to aid loudspeaker damping (another debateable topic). Perhaps a change to the 'Impedance Matching' page would clarify the point about efficiency and matching, which to me anyway, is not clearly explained at the moment? - CPES 01:01, 10 Feb 2005 (UTC)
By the way, low source impedance is addressed in the article impedance bridging. I named it that, and there is probably a better name, like voltage bridging. Google returns few for either term. - Omegatron 01:38, Feb 10, 2005 (UTC)

[edit] An impedance matching example

In this circuit:

Image:Source_and_load_circuit.png

If Rsource = 50 Ω, Vsource = 1 V:

Rload = 1 Ω

Vload = 1/51 * 1 V = 0.0196 V

I = Vsource / Rtotal = 1 V / 51 Ω = 0.0196 A

Powerload = Vload*I = 0.00038 watts

Rload = 50 Ω

Vload = 50/100 * 1 V = 0.5 V

I = Vsource / Rtotal = 1 V / 100 Ω = 0.01 A

Powerload = Vload*I = 0.005 watts

Rload = 100 Ω

Vload = 100/150 * 1 V = 0.666 V

I = Vsource / Rtotal = 1 V / 150 Ω = 0.006666 A

Powerload = Vload*I = 0.0044 watts

[edit] Merge with Maximum power theorem

This should be merged with Maximum power theorem. - Omegatron 14:39, Jul 29, 2004 (UTC)


[edit] Conjugate Matching and Reflectionless Matching

THe term 'reflectionless matching' I have not heard before but I think both terms describe the same desireable outcome. That is, when a source and load are matched, no energy whatsoever is reflected. In order to do this with a simple resistive source it only requires that the source and load resistances are made equal.

In the case of complex sources and loads however, it is neccessary to employ the technique of complex conjugate matching to ensure max power transfer. I believe its done all the time to match the output of transmitters to their antennas. If the source reactance is inductive, then the load reactance must be capacitive. etc. This will result again in max power transfer. Whether this power is just as great as if you had purely resistive source and load, I'm not sure without referring to my library articles. This can be seen if you go thro the math. I not going to do that here!! Light current 16:35, 10 August 2005 (UTC)

If you don't understand the difference between them, maybe I shouldn't have directed you here. The "Reflectionless matching or broadband matching" and "Complex conjugate matching" sections describe the difference pretty well.
  • In one, you are matching impedances exactly to prevent reflections down a transmission line. You set the line and load to the same impedance and there will be no reflections at the line-load boundary. You set the source and line to the same impedance so that if there are reflections from the load, they won't reflect back again when they get to the source. If you have a capacitive impedance at the load, you have a capacitive impedance in the source and line, too.
  • The maximum power theorem is entirely different. You match the complex conjugates of the impedances of a source and load so that, if the source has a fixed impedance, you get maximum power dissipation in the load. If your source has a capacitive impedance, you have to "balance it out" with an inductive impedance at the load. I believe this is the same thing as power factor correction, but I could be wrong.
The only thing that this article needs is to be cleaned up and separated into two good articles.
In the case of purely resistive impedances, the two ideas sound like the same thing, which causes a lot of confusion. Please be careful. We're trying to make it less confusing; not mix them up more. - Omegatron 18:28, August 10, 2005 (UTC)
No Im sorry I disagree, I have rewritten the firt half of the article. Please have a look. I dont think it needs separating yet until we see what we have got in total. I am in the process of sorting it. Is it OK by you if I carry on? :-)Light current 19:08, 10 August 2005 (UTC)
It is true that for maximum power transfer from the source to the load, the load impedance should be the conjugate of the source impedance. Now, insert a transmission line between the source and load. What should the load impedance be for maximum power transfer? The answer is that the load impedance should be such that the impedance seen looking into the line from the source is the conjugate of the source impedance. This guarantees that maximum power is delivered to the line and, if the line is lossless, on to the load. Thus, if the load is matched to the line, maximum power transfer from source to load only occurs if the source is also matched to the line. In the general case where the source is not matched to the line, maximum power transfer does not occur from the source to the load even though there is a 'reflectionless' match at the load end of the transmission line. Thus, it seems reasonable that the ideas of matching for zero reflection coefficient (no standing waves) and matching for zero total reactive component (maximum power transfer) are worthy of separate discussions. Alfred Centauri 19:10, 21 August 2005 (UTC)

[edit] Efficiency

If Rload is infinite, it dissipates no power (currrent is zero) so how does this give max efficiency? Or does the efficiency APPROACH 100% as the load resistance approaches infinity. Reminds me of perfect voltage sources and ideal short circuits!!! :)Light current 18:52, 10 August 2005 (UTC)

"Efficiency" depends on what you're trying to transfer.
Silly me. I didn't even see the Efficiency section; busy at work and such. In this case, efficiency means something very specific.
Yes, the efficiency approaches 100% as the load resistance approaches infinity, as it says in the article: "but tends to 100% as the load resistance tends to infinity". - Omegatron 04:29, August 11, 2005 (UTC)

[edit] No echos on audio systems

Even if you called someone very far away, the "echo" would only be a few milliseconds, which is generally considered inaudible. For instance:

(1 000 miles) / ((2 / 3) * c) = 8.05229063 milliseconds

I guess that could be considered an echo, but more accurately would be thought of as a delay-line filter. In reality, the length from source to load will never be that far, anyway; only as far as the nearest telephone exchange.

Regardless, they were originally matched for maximum power, not minimal reflections:

"For the proper operation on long lines, good impedance matching is needed to keep those reflections at minimum. In the real-life telephone subscriber lines the line wire is so short compared to wavelength in the telephone frequencies, that the cables not not have the "true characteristic impedance" on the voice frequencies (few kilometers of able is short line for frequencies below 4 KHz that have 50 km or longer wavelength on cable). The history for 600 ohms is that early telephone system typically used AWG#6 wires spaced 12 inches (305 mm) apart, which made their characteristic impedance exactly 600 ohms at voice frequencies."

"Balanced (for noise rejection) and impedance-matched (for power transfer) transmission lines were clearly necessary for acceptable operation of the early telephone systems, which had no amplifiers. Later, as the telephone network grew, amplifiers, filters and "hybrid" transformers were added to enable long-distance transmission. Proper operation of these components depended critically on rather precise 600 ohms impedances. This 600 ohm impedance is here still nowadays to stay." - [1] - Omegatron 19:02, August 10, 2005 (UTC)

So why are echo supressors used on long distance lines then?? Light current 01:02, 11 August 2005 (UTC)
Please do not delete this from the talk page like it was deleted last time.

I quote from the book "Understanding Telephone Electronics" by John L Fike Ph.D, PE. Adj Professor of Electrical Engineering, Southern Methodist University, Staff Consultant , Texas Instruments Learning Center. and George .E. Friend, Consultant, Telecommunications, Dallas, Texas and Staff Consultant, Texas Instruments Learning Center. Chapter 1, page 15.

"The amount of echo delay depends upon the distance from the transmitter to the point of reflection. The effect of the delay on the talker may be barely noticable to very irritating, to downright confusing. Echo also affects the listener on the far end but to a lesser degree. Echos are caused by mismatches in transmission line impedances which usually occur at the hybrid interface between a two wire line and a 4 wire transmission system. The effect of echo is reduced by inserting a loss in the lines." (Italics and bolding all mine). I rest my case, but an admission of error and an apology from Omegatron would be nice. Light current 02:02, 12 August 2005 (UTC)

One aspect of the operation of a hybrid interface that depends critically on a rather precise 600 ohm impedance is the return loss. But like Omegatron said, the length of the TP from the home to the DLC or to the CO is usually not great enough to generate what is usually considered a distinct echo. What I believe he failed to consider is that the mismatch might be at the far end of the call. In the case of an international call, the cumulative round trip delay could be substantial. Alfred Centauri 19:49, 21 August 2005 (UTC)
You're absolutely right. I didn't consider that... — Omegatron 19:29, 24 January 2006 (UTC)

"when a source is driving a load that is far away compared to the wavelength of the energy being sent" can more introductorily be phrased "when the signal changes quickly compared to the time it takes to travel from source to load" since newcomers will not know what "the wavelength of the energy" means. - Omegatron 21:54, August 10, 2005 (UTC)

I'm sorry, I'm not going to cast any more 'pearls' amongst swine (New Testament)Light current 01:22, 11 August 2005 (UTC)
What do you mean? - Omegatron 03:19, August 11, 2005 (UTC)
Ask someone older
Let's try this again: What do you mean? (As in, "how can you be so persistently belligerent while claiming to be cooperative?") - Omegatron 07:39, August 12, 2005 (UTC)

"power transformers are not used for impedance matching but purely voltage transformation"

But why is the voltage transformed higher in the first place? - Omegatron 05:21, August 12, 2005 (UTC)
Ask someone in the electricity supply industry (like I used to be). You still dont want to take my word for anything. Do you?
You'll have to ask much more politely than that if you want an answer.
Which part of my question was not polite?? You're breaching assume good faith all over the place.
Stop responding to every question on every talk page with "go find out for yourself". If you don't know the answer or don't want to provide it, don't reply; someone else will.
It was a somewhat rhetorical question and note to self, anyway, to explore later, and which I see is irrelevant in retrospect. The voltage is kept high so that they can use a small amount of current for the same power output, to minimize resistive losses in the line. The resistive losses are not related to line impedance as far as matching is concerned, though.
I would imagine there are several impedance mismatches between the high voltage lines and residential lines, however. - Omegatron 07:39, August 12, 2005 (UTC)
Sorry, I thought you were asking me, as you seem to move stuff to the talk pages. My (sufficient) answers are provided in the material I write on the article pages. Unfortunately, some people are reluctant to believe me. With reference to my (apparently to you) cryptic comments -- why not try looking them up in a'pedia. I've heard Wikipedia is quite good. Just enter the words in the search box and pick the answer you think fits.Light current 13:22, 12 August 2005 (UTC)
I think you have answered your own question quite correctly. Bad matching does not really have much effect at 50/60 Hz. Now you are starting to think! Light current 22:52, 12 August 2005 (UTC)
Which cryptic comments are you referring to? You really need to try being more cooperative. - Omegatron 14:30, August 12, 2005 (UTC)

[edit] Sanity check

As I implied in my recent additions to the External links, Hyperphysics says that impedance matching is used in audio:

http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html

As far as I know (and I really should know), impedance matching is never used in audio anywhere except speaker distribution systems, but I just want to make sure I'm not missing something. Maybe it is used somewhere else that would have influenced their page, like alarms or something? Where the output power is more important than the distortion? I doubt it though. (Especially considering the output power would be lower than a regular audio amp in the system they describe.) — Omegatron 19:27, 24 January 2006 (UTC)

Do you remember our conversation about telephones many moons ago? I dont know if you want to call phones 'audio', but we did come to the conclusion that phone systems/lines were matched to stop echoes etc.
Also, are not hi-quality mics passed via 600R cable to 600R inputs on the mixing desks in recording studios? In fact most of the recording studio stuff is 600 R balanced and these circuits are usually matched. Attenuators are 600R etc. I also think that most broadcast audio systems are 600R balanced.

Ref: Audio Systems, Julian L Bernstien. Pub J. Wiley & Sons Inc.1966. Lib of Congress Cat card 66-17326 So you were wise to check your sanity!. Verdict: see a shrink! :-) --Light current 23:07, 24 January 2006 (UTC)

Thanks for putting me down. You're always so pleasant to interact with...
Anyway, yeah, I wasn't including telephones in "audio". I'm talking pro and consumer audio equipment; recording, PA systems, home stereo systems, and whatever.
Impedance matching of microphones like you described isn't done. In the situations it is/was used, it's not really "impedance matching" for maximum power or reflection; just "optimum loading". It definitely deserves mention in the article, though, because of all the confusion it generates.
I don't know about the broadcast stuff. Can you dig up some references? Can you think of anything else? — Omegatron 03:23, 25 January 2006 (UTC)

Balanced microphone technique Im sure is still used especially in studios. Microphones o/ps are transformed to 600R output impedance with transformers to drive the cable which is always 600R screened balanced cable. Im sure youll agree in these case that its sensible to terminate the cable at the studio desk in 600R. Whether this is actually done, I dont know but if I was designing such a system. I would termninate just to be safe (Its only a 6dB voltage loss). Its more important maybe on 'OB' setups where mic cable runs can be a few hundred yards. Again , whether you would notice any difference between terminated and unterminated cables (apart from the level) at audio, Im not sure (depends on cable length). But of course OB audio used to be sent over phone lines so this would make it more necessary to terminate properly. THe only real reference I have at the moment is the one I have given. Its a good book on the basics tho', and you would probably be interested to read it.

Generally speaking though, I dont think matching is that important at audio unless you have long cables (miles)--Light current 04:57, 25 January 2006 (UTC)

Of course balanced microphones are still used. They're not 600 Ω → 600 Ω, though. They use a voltage bridging connection, with maybe 3 kΩ at the input to the mixer and maybe 100 Ω output impedance from the mic.
  1. Matching microphone and pre-amp impedances to the same value (power-matching) reduces both the level and the S/N ratio by 6dB and is not a technique that is used for those reasons. For dynamic and condenser microphones, the preferred preamp input impedance is generally about ten times that of the microphone output; normally around 1.2kO or 2kO. [2]
  2. If a microphone has a low impedance of 200 ohms, a typical mixer mic input or preamplifier should have a low impedance value of between 5 to 10 times or higher that of the mic impedance giving a value of between 1000 and 2000 ohms or higher for best results. [3]
  3. The electronics in the Active Series ribbons provides a perfect load on the ribbon element at all times, meaning that R-122’s are able to deliver 100% of their full sonic potential regardless of the input characteristics of the following mic-pre. Due to the low-impedance output, Active Series mics can also be used with extremely long cable runs with minimal signal loss.

    A good impedance match is critical with ribbon microphones because impedance mis-matching “loads” a ribbon improperly, resulting in loss of low end, diminished body, lowered sensitivity and overall compromised performance. With the Active Series ribbons, the ribbon element lives in a perfect world; it sees an optimum impedance match at all times regardless of the following equipment, so its performance will never be compromised by the effects of improper loading. In addition, the ribbon element can’t be damaged by phantom power, electrical glitches or miswired cables.     [4]
  4. For audio circuits, is it important to match impedance?

    Not any more. In the early part of the 20th century, it was important to match impedance. Bell Laboratories found that to achieve maximum power transfer in long distance telephone circuits, the impedances of different devices should be matched. Impedance matching reduced the number of vacuum tube amplifiers needed which were expensive, bulky, and heat producing.

    In 1948, Bell Laboratories invented the transistor - a cheap, small, efficient amplifier. The transistor utilizes maximum voltage transfer more efficiently than maximum power transfer. For maximum voltage transfer, the destination device (called the "load") should have an impedance of at least ten times that of the sending device (called the "source"). This is known as BRIDGING. Bridging is the most common circuit configuration when connecting audio devices. With modern audio circuits, matching impedance can actually degrade audio performance.     [5]
  5. Impedance matching went out with vacuum tubes, Edsels and beehive hairdos. Modern transistor and op-amp stages do not require impedance matching. If done, impedance matching degrades audio performance.

    Modern solid-state devices transfer voltage between products, not power. Optimum power transfer requires impedance matching. Optimum voltage transfer does not. Today's products have high input impedances and low output impedances. These are compatible with each other. Low impedance output stages drive high impedance input stages. This way, there is no loading, or signal loss, between stages. No longer concerned about the transfer of power, today's low output/high input impedances allow the almost lossless transfer of signal voltages.     [6]
  6. It has often been claimed that a 600 Ohm microphone should be matched to a 600 Ohm input for best performance. This is simply wrong, and microphone manufacturers specifications will support me on this. [7]
  7. How about impedance matching in audio applications? There are a few applications where it?s important, but perhaps not as many as you might think. Because audio signals are quite low in frequency, it?s generally only where they have to be sent over quite long cables that transmission-line effects make it necessary to perform impedance matching to prevent reflections. And in most cases, we can get quite efficient signal transfer simply by arranging for the output impedance of our audio source (such as an amplifier) to be much lower than that of our load (such as a loudspeaker). [8]
I believe the only reason a low impedance would be connected to a microphone is as in #3; it's just an optimum load for the mic element so that the transducer can move current around, and is internal to the mic. You still connect it to a high-impedance input.
There might also be something involving vacuum tubes, as implied in #5? I'm not sure why the maximum power theorem would be important there.
We should also mention in the article that sometimes when people say "impedance match" (as in #3), they really mean "optimal impedance", and it refers to a bridging connection. "Matching" up the source and load for best voltage transfer, which doesn't mean making the source and load impedances equal.
What is an "OB" setup? — Omegatron 04:04, 26 January 2006 (UTC)

OB is outside broadcast. Of course the other place matching is used is loudspeakers being matched to the acoustic impedance of the air for high efficiency. For this purpose, large exponential horns were used. You can still se them on some PA bass bins and on hf horns. As regards #5 , I would have thought you still need a transformer to match the hih anode impedance to a low impedance LS.--Light current 16:41, 26 January 2006 (UTC)

Dude, impedance matching is definitely an issue in a professional audio environment! Not so much because of reflections (although those are a problem over longer runs), but more because Low-Z microphones and instrument outputs change their frequency response when connected to higher loads. You lose the characteristic sound of a mic or a pickup when you plug it in to a Hi-Z input. And vice-versa, if you take a Hi-Z output and plug it into an amp that's got a lower input impedance, you drive the amp differently and it sounds different. Admittedly, part of that is because of difference in line levels, but impedance still must be dealt with. There are TONS of products on the market to help recording engineers deal with these problems - DI's, line transformers, etc.

That's not impedance matching. Impedance matching sets the source and load impedances equal. You're using the colloquial definition covered in the Impedance matching#Terminology section. — Omegatron 21:40, 14 September 2006 (UTC)

[edit] Suggestion

I suggest that we move the discussion of dynamical analogies in the two entries above to the discussion at the analogical models page, and maintain a link to analogical models from impedance matching. Anyone strongly disagree? Sholto Maud 22:32, 4 May 2006 (UTC)

I agree!--Light current 23:17, 4 May 2006 (UTC)

It shall be done. Sholto Maud 02:32, 5 May 2006 (UTC)

[edit] Engines, road wheels and gearboxes

The discussion about this topic and analogical applications of impedance matching and maximum power may now be found at the Talk:Analogical_models page. Sholto Maud 02:36, 5 May 2006 (UTC)


[edit] Merging

The other page seems to be about other meanings of the word 'impedance', so Im not sure if a merge is wise. May I suggest a disambiguation page called Impedance matching?--Light current 21:33, 25 July 2006 (UTC)

Impedance mismatch is about impedance matching. It should be in this article. — Omegatron 13:35, 26 July 2006 (UTC)

Yeah but look at all the other crap it contains. What do you suggest doing with all that? 8-?--Light current 15:39, 26 July 2006 (UTC)

Ah. In my edit summary I said "merge the electrical and radio into impedance matching, copy the rest to articles that already exist, delete the essay on humans".
So the computer stuff would be moved to an article about computers, with a link to here, since their use of the term is derived from the electrical, and the human consciousness/communication stuff will be deleted as original research editorialism. — Omegatron 17:47, 26 July 2006 (UTC)

The term "impedance mismatch" is a common software term (different from the electrical meaning). I think it is best to have a "impedance mismatch" article. - Thebithead 15:53, 16 August 2006 (UTC)

Yes, but the software term is borrowed from the electrical term. I think what you are looking for is Object-Relational impedance mismatch. A disambiguation page might not be a bad idea, I suppose. Madhu 21:39, 16 August 2006 (UTC)
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