Impedance Matching

Discussion in 'General Electronics Chat' started by Mazaag, Feb 16, 2006.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
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    Hey Guys,

    Quick Question.

    What is Impedance Matching ? and why is it important? I believe they are used in Audio Circuits , but are they used in other applications too ?

    Thanks
     
  2. n9xv

    Senior Member

    Jan 18, 2005
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    Impedance matching is (and should be) used anywhere your interfacing/transfering a signal from one point to another. A quick example;

    Mic to audio amp

    Antenna to transmitter

    preamp to driver, driver to amp, amp to power amp etc.

    You ideally want the output impedance of one device to "match" the input impedance of another device so as to have maximum power transfer. Maximum power transfer means minimum power lost due to a mismatched load and thus ultimately to heat.

    An audio microphone with an impedance of 600-ohms wont drive a 50-Kohm amp or have the predictable response as a mic with 50-kohm impedance.

    Impedance matching is very important!
     
  3. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    Hi,

    Impedance matching becomes very important at high freqeuncies. If the impedances of the source and load are not matched, the signal is partially reflected back down the line from the load to the source. The resultant waveform is the algebraic sum of the transmitted waveform and the reflected waveform. Hence the waveform the designer intends to send to the load and the actual waveform are different and may cause the circuit to malfunction.

    Here is a link to a National Instruments tutorial on impedance matching:

    http://zone.ni.com/devzone/conceptd.nsf/we...6256F2700114FC9

    John
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This is only partly true.
    Impedance matching is only important nowadays in certain applications. See what Shure has to say about microphones and audio, and the Wikipedia discussion of impedance matching.
     
  5. n9xv

    Senior Member

    Jan 18, 2005
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    Its true that the audio devices will seem to "work", but the best response will always be acheived when the impedances are matched. More drive may actually be required with matched impedance Vs mismatched impedance. At any rate thats speaking of the mic sensitivities at the input. The output if its of any significant power level, must be matched or it will "smoke"!
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Apparently you know more about this than a company that's been making microphones for many years.
    Do you also believe that audio amplifier output impedance should match the speaker impedance?
     
  7. chesart1

    Senior Member

    Jan 23, 2006
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    Do you also believe that audio amplifier output impedance should match the speaker impedance?

    Hi,

    I am aware that this question was not directed at me, but i have a few comments regarding it.

    Audio frequencies do not present the same problems as frequencies in the 30 Mhz range. Therefore I think that the only advantage to impedance matching would be maximum power transfer.

    The links provided by Ron H clearly explain the difference between maximum power transfer and maximum voltage transfer. It also explains why, in modern audio circuits, maximum voltage transfer is desired.

    John
     
  8. n9xv

    Senior Member

    Jan 18, 2005
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    I did'nt say it would'nt "work". I said the best response will be acheived with matched impedance. There's a difference between simply hearing it and getting the expected response from the amp in question.

    Yes an audio amplifier's output impedance should match the speaker impedance. Most people think thats not important because they see no ill effects at miniscule power levels. Even though you may not see any harm, the amp is not running efficiently as it would with the proper impedance. Try runing 50,000 watts into a 4-ohm speaker system, from an amp designed for an 8-ohm system or vice/versa. you'll have very short tube/transistor life lest it burn before your eyes. At high power levels impedance matching is crucial!
     
  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    I thought the amp would burn not because impedance mismatch, but because the output stage is overloaded. Assuming the voltage stay fairly constant, the amp had to provide twice the current at 4 ohm and dissipate close to twice the normal heat. Running an amp designed for 4 ohm with 8 ohm speakers, I think is not dangerous. You do get less power out, but nothing close to catastrophy.
     
  10. n9xv

    Senior Member

    Jan 18, 2005
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    When you change the impedance of the load, you radically change the limitations of the amp or device in question. Any audio device (especially high power devices) specifies the proper output impedance which must be seen by the output. For high end audio, there's a reason for that. The amp itself is designed starting with the load impedance and working back towards the power supply. When an amp designer sits at the drawing board, he starts with the load. Decides on a power level, at that point he knows what the power supply will be required to deliver. He then chooses the output devices that are best suited for this power level, tubes/transistors etc. Before an amp can be designed to deliver X amount of power reliably for X amount of time, you must know the impedance of the load it will be delivering all this power to! Impedance is very important to those who understand it.

    Example of the 50,000-watt amp;

    50,000 watts into an 8-ohm load is 79-amps. 50,000 into a 4-ohm load is 112-amps. When this magnificent amp is "maxed" out at its "designed" output of 50,000-watts into 8-ohms (79-amps of current), you are going to ask it to deliver an extra 33-amps into a 4-ohm load when maxed! Thats asking quite alot from an amp thats maxed at its design limits already! The power supply alone would cook.
    You would have distortion due to clipping (MPP exceeded) etc.

    As I said earlier, it would work for a while at lower power levels and up to a point. But if your running any real power, its a good way to destroy an amp.
     
  11. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    I think you've just proved that it is not matching the output impedance of the amp to the impedance of the speaker that's important, but observing the drive capability of the amp by selecting an appropriate speaker impedance so the amp is not overloaded during operation. Using an 8 ohm speaker with an amp designed for 4 ohm, although also constituted as an impedance mismatch per your example, would not be dangerous because the amp is not overloaded and per your example above would only deliver half the output current (although heat dissipation on the output power stage of the amp wouldn't necessarily be halved).
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    It seems to me you are not talking about output impedance of the amp, becase if it should match the load, you would have to add 8 ohm 25,000Watt resistor in series with the amp, because audio amplifiers have out put imp. very close to zero.
    Then the output impedance would be 8ohms, matched to the load. But it would seriously affect the amp´s damping factor...
     
  13. n9xv

    Senior Member

    Jan 18, 2005
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    oops!
     
  14. n9xv

    Senior Member

    Jan 18, 2005
    329
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    The following is an exerpt from Shavano music online that re-enforces my point about the importance of proper impedance at significant power levels.

    "Your goal is usually to load your power amplifier somewhere between 4 and 16 ohms (refer to your power amplifiers documentation to be absolutely certain of its limitations and never provide a load lower than the system was designed for). "

    As I stated earlier, you should never run power into a load that is lower than the amp was designed for.

    Going the other way, if you connect the power amp into a load that is higher than what the amp was designed for, your preventing the amp from delivering the amount of power the amp "could" deliver if it were connected to a matched load.

    An amplifier's power rating is based on a specified load impedance. Without a specified load impedance, the rating means absolutely nothing.

    Shavano Music Online
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't think there are any here who would dispute that. My understanding of the OP's question was, do you have to match (meaning equal) source and load resistances. There are definitely places where you need to do that: Transmission line terminations (not always necessary, but sometimes), antennas, etc. There are many other source/load combinations where it is definitely not the best thing to do - including audio power amplifiers.
     
  16. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    As Ron H stated above, this is not an impedance mismatch problem, but more of using the correct load impedance to achieve the designated performances. Your excerpt from above seems to support this argument.

    Audio amp output has low impedance, much lower than the designed speaker impedance of 8, 4 or 2 ohms. There is no doubt about this. Although this is classed as impedance mismatch, it is not detrimental to the amp. This is just how it was designed. Using a correct impedance as the load (speaker), however, is necessary. But this is nothing to do with impedance mismatch, more of suitable designed load rating problem.
     
  17. n9xv

    Senior Member

    Jan 18, 2005
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    With the possible exception of "front end" entertainment audio, the impedance of a load must match its source for maximum effeciency and proper operation. This is critical in RF work as well. A random wire antenna that yeilds 500-ohm impedance connected to a solid state tranceiver with 50-ohm impedance is going to be very ineficient and destroy the output power transistors without a means of tuning/matching the antenna to the transmitter. 500:50 is an SWR of 10:1. An SWR of 10:1 translates to a reflected power of 66.9-%. At an SWR of 3:1 (150:50) the reflected power is 25-%. In RF work, an SWR of 1.5:1 (.04-% reflected) is considered acceptable. The average guy running a "thumper" car stereo is probably running it into a somewhat mismatched load. Unless he's running it full bore 24/7, he'll probably never realize the error of his demize. If however, he were to do a case study and document the "life" of his power amp with a mismatched load Vs a properly matched load, He would realize a bit more "milage" from the properly matched scenario.
     
  18. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    That's what we've been saying all along, for RF amp it is absolutely necessary as you've so clearly explained above. However, for audio amplifier and speaker this is not the case. You argued that high power audio amplifier requires matching output stage impedance and speaker impedance, but this is not the case. Your previous excerpt shows that it is only necessary to select suitable load impedance, not matching it to the amp output stage impedance. Several other members had pointed out that the output stage impedance of audio amp is very low, much lower than the common 8, 4 or 2 ohm speaker impedances.

    I would be very surprised if an audio amp designed for 4 ohm would have a shorter "life" when operated with an 8 ohm speaker.

    There are cases where matching impedance is necessary and there are cases where it is not.
     
  19. n9xv

    Senior Member

    Jan 18, 2005
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    What I am saying about impedance matching in regards to audio power amps is based on the indicated or published output impedance of the amp. If a high power amps manual says connect output to 2, 4, 8, or 16-ohms etc, then thats exactly what you should do if you want to acheive expected published results. If you connect the amp to a higher load impedance than that specified, you'll denie the amp of its potential to deliver the rated power. If you connect the amp to a lower load impedance than that specified, you'll be overworking the power supply in order to deliver the "extra" current to the lower load impedance. In other words, think of a "lab test" if you will. Take the high powered amp and run it full bore into its "specified" load impedance. Take the same amp, run it into a load impedance of half the specified value. The amp running into the lower load impedance will tend to clip the signal peaks due to the power supply not being able to support the designed MPP levels any longer. So we have distortion and increased heat dissipation and soon much damage will occur. Running the amp into a higher load impedance wont "hurt" it per say, the amp is just not reveiling its potential in that case. It cant deliver the "rated" power because the higher load impedance "limits" the available current to the speakers.

    I'am trying to point out the facts about the importance of impedance matching but all I've seen as a rebutle is that the output impedance of an audio amp is very low so it does'nt matter what you connect to it. That wont "hold up" in the court of science.

    When I say matched impedance, I mean "matched" to the specified output impedance of the amp in question. If an amp has a connection on the rear pannel labled 8-ohms, then one understands to connect an 8-ohm load to that output. The load is now "matched" to the output and the amp is happy. It does'nt matter what you "think" the output impedance of the amp "really" is, all that is necessary for the amp to operate properly is that it sees it's specified load impedance. Why do you think the impedance matching phenomena is different for AF than RF? It all boils down to efficiency. You can run power into a grossly mismatched load, and, as long as you keep it cool enough it will operate. The efficiency will be close to nothing but if kept "packed in dry ice" it will indeed operate. The designers knew what they were doing when they designed the amp. They gave it ratings based on certain criteria etc. For an audio amp that criteria is load impedance. To say an amp can deliver X-watts is only true if the amp is connected to the specified load impedance. If the load impedance is different than that specified, then X-watts is no longer true. Again I'am speaking of "real" power levels. This is true and applies to any power level but the "damage" is prominant at the higher power levels. I'am not arguing that 1 watt into a walkie-talkie speaker will destroy the amp if it is "mismatched" to the load.

    Load impedance < the specified value = heavier burden on the power supply, increased heat dissipation, clipping/distortion etc.

    Load impedance > the specified value = amp unable to deliver the available power to the load.

    How much of a "problem" this all is depends on the power levels involved.
     
  20. Gadget

    Distinguished Member

    Jan 10, 2006
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    Of course, not being a resistive load, an 8 ohm speaker is only 8ohms at a small range of frequencies and can vary from 3 or 4 ohms up to 16 or so ohms (even with the best crossovers and drivers) over its operating frequency.
    I have no problem with using a higher impedence speaker then quoted for most amps, as long as I am not after maximum power. I would NEVER load an amp with less however, as that would certainly shorten the amp life, with the output devices and PSU trying to deliver more power than their ratings.
     
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