Impedance Matching

[Mazaag]

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

[n9xv]

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!

[chesart1]

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

[Ron H]

Originally posted by n9xv@Feb 16 2006, 02:43 PM
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!

Quoted post

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.

[n9xv]

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"!

[Ron H]

Originally posted by n9xv@Feb 17 2006, 05:30 PM
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"!

Quoted post

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?

[chesart1]

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

[n9xv]

Originally posted by Ron H@Feb 17 2006, 07:10 PM
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?

Quoted post

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!

[n9352527]

Originally posted by n9xv@Feb 19 2006, 04:12 PM
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!

Quoted post

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.

[n9xv]

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.