Impedance matching questions

The 50% power transfer results from matching the impedance of the source and the load. This is true for all circuits.

The question is why match the source and load impedance? When you only have control over the load impedance, the maximum load power is obtained by adjusting the load impedance to this point. But if you have control over the source impedance you can get higher power levels in the load by minimizing the source impedance no matter the load impedance.

But there are other reasons for matching the source and load impedance. When working with AC signals there may be issues of transmission line effects. That will depend on the details of the signals and the circuit design. At lower frequencies you don't need to consider transmission line effects. At very high frequencies they completely dominate the design process.

Sounds like your impedance of the load may be complex rather than purely resistive, no? If the source and load are mostly resistive the impedance matching should be wide band. If you are actually concerned about 5 GHz, then there are a great many aspects of the design you need to consider, not just the load impedance.

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Rick

daku

** In audio, equal value impedance matching is never used. Normally, the load is made 10 times or more the impedance of the source.

Egs:

The measured output impedance of a typical audio amplifiers may be anywhere from 0.1 ohms to over 100 ohms - but the rated load remains 8 ohms for maximum power transfer.

The output impedance of common audio op-amps may be under 1 ohm, but the minimum load is about 1000 ohms for max output voltage and power.

** There is no such animal.

** All wrong.

Equal value impedance matching is used in very few places ( cos it wastes half the power) and only works for *passive circuits*.

When the source uses some form of negative feedback to achieve a low output impedance, then all bets are off regarding the optimum load.

.... Phil

In audio, you usually want the source impedance (much) higher than the load impedance.

No! The general method would measure the mean of instantaneous current x instantaneous voltage. There are shortcuts for some special cases, such as sinusoidal signals. Vrms x Irms may be right in some special cases, but in general, it isn't.

Matching for maximum power transfer is almost never done. Wideband matching when the source or load has a non-zero imaginary component is hard. The most common case where matching is important is when you need to terminate a transmission line, and even then only when its length is significant w.r.t. the signal wavelength.

Jeroen Belleman

** Wanna try that again ??

** Transmission lines are neither sources nor loads so the example is irrelevant to equal value power matching. ** Silly, ham radio think fallacy - yet again.

I am heartily sick of debunking it here and elsewhere.

** Get your info right.

.... Phil

I slipped. It's the other way around, of course.

Don't, then. I wasn't addressing you anyway. You know your way around well enough.

Jeroen Belleman

Nonsense.

If you can only vary the load impedance (the source impedance is fixed), the best power transfer occurs when the load impedance matches the source impedance.

If you can vary the source impedance, its best impedance is ZERO!

[snip] ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

I am sure you meant True RMS or is that now the way True RMS is referred to now as just RMS? I used to have both RMS and True RMS meters in my lab. And yes I do know why.

Wow, thanks Jim. This "somewhat silly" impedance matching question is a recurring one. The above is a very succinct way to explain it.

George H. (oh unless it's a current source :^)

Nope. Think about it. If the source is a current source, its impedance is (near) infinite, so the most power transferred to the load occurs when the load impedance is (near) infinite. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Grin, sorry that's what I meant a current source wants infinite impedance not zero. But for a fixed output impedance current source the equal impedances works.

is absurd.

Why in hell do you want to waste 50% of the power?

Don't bother. You *don't* want maximum power transfer.

Hint: forget the whole subject.

For RF transmission lines i.e. connection longer than a significant part of a wavelength, you have to match the source impedance to the transmission line impedance and again match the transmission line impedance to the load impedance.

The free space wavelength at 20 kHz audio is 15 km and 50 Hz mains

6000 km, so in practice not an issue in most cases. In special cases, as the first trans-Atlantic telegraph cables, this can be significant.

As an anecdote, some early researchers actually believed that source and load impedance matching with DC generation was important, some DC dynamos actually used thin wires in the generator, but of course burned half of the mechanical power in resistive losses. I have seen this claim on a poster in a museum close to some of the earliest Edison dynamos (which already had thick conductors and low source impedance).

Why would you want to match at the source end?

Jeroen Belleman

There is only one definition for RMS. The True RMS thing came about to describe meters that can measure the RMS of an almost arbitrary waveform vs averaging meters whose readout is calibrated in RMS for a sine wave with low distortion.

If the load is not matched to the line, say due to tolerances, a matched source resistance quenches the reflections, resulting in a much flatter frequency response at the load.

Hint: Spice simulations are wonderful.

Kevin Aylward

- SuperSpice

Most audio circuits are voltage coupled, not impedance matched. The source impedance is A Lot higher than the load impedance.

By "general communications circuit" you mean "transmission line", yes? Those depend on the "characteristic impedance" .

50 ohms is common, as is 75, 150....

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Les Cargill

Don't you mean the source impedance is a lot *lower* than the load impedance? A high source impedance would be like a current source.

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Rick

So, how do I match the impedance of my ferrite rod antenna to the input of my AM radio for best signal to noise ratio?

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Does your AM radio have a variable capacitor across the ferrite rod antenna? If so it is already matched when you tune it.

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Rick