impedace matching differences at low freq & High freq

Hi,

In High frequency amplifier design we normally do the impdenace matching for maximum power transfer. According to my thinking this approach should be followed in low frequency circuits such as audio amplifiers...But in a lot of audio amplifiers voltage matching is done rather than impedance matching.i.e..(it is desired to have low output impedance of stage and high input impedance of next stage for maximum voltage transfer)....Why is this difference....can any body please elaborate...if i have to deisgn a transmitter at low frequency....what should i do...should i do the voltage matching or impedance matching to the antenna...

my 2nd question is also kind of related....In high frequency impedance is normally given in real plus imaginary parts...while in low frequency design i have seen impedance in only real parts e.g. impedance of speaker..why is this....and how i can calculate/measure impedance of a low frequency antenna/circuit....

if any body can answer thses questiosn it will be really great..
thanks

Re: impedace matching differences at low freq & High fre

viperpaki007,
Impedance matching is a source of confusion to many. The concept of maximum power transfer is this: Given a source voltage and source impedance, if you vary the load impedance from zero to infinity, the value of the load impedance that results in the maximum power delivered to the load is the value that equals the source impedance.
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However, maximum power transfer is not necessarily a necessary or desirable condition. An example is the case where you are driving an audio power amplifier by an audio preamplifier. In this case, you are not interested in power transfer to the input of the audio power amplifier. Another example is driving a speaker from the output of an audio power amplifier. Suppose the output impedance of the power amplifier is 0.1 Ohms (a realistic value). To get maximum power transfer, the speaker impedance would need to be 0.1 Ohm. Even if a 0.1 Ohm speaker were available, in order to deliver 100 Watts of power to the speaker, the amplifier have to supply 31.6 Amperes of current!
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You mentioned speaker impedance parameters. The specification sheets for all high quality speakers give both the DC resistance and the inductance of the speaker, as well as other parameters (the Thiele-Small parameters.) Google “Thiele-Small", and you will get lots of information on this.
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As far as transmitters are concerned, there is another consideration: In order to minimize the voltage standing wave ratio (VSWR) in the transmission line to the antenna, the impedances of the transmitter output stage (possibly reflected thru a transformer), the transmission line, and the antenna must match. If the VSWR is higher than one, excessive reflected voltages will appear on the output stage, and the transmission line will act as an antenna, a situation that should be avoided.
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I hope this helps clear things up.
Regards,
Kral

Re: impedace matching differences at low freq & High fre

Hi viper.
You can match impedances at one frequency, or at another. Not at both.

The flaw is in the person(s) who write an article or make a comment about an impedance value, in general the practice has been overseen so badly that nobody sees as wrong.

Anyone who talks about the impedance of a device, MUST express "at such frequency".
It does not happen. Marketing is who usually decides publications about a product and they do not care about proper technical terms.

The impedance varies with the frequency, by its nature. The formula "impedance = function of frequency"
Change the frequency, and the impedance changes.

A plain speaker with impedance rated at 8 Ω; should had said "8 Ω at 1KHz.
But nobody cares about doing things right and learning/understanding becomes difficult due to those omissions from careless people.

About matching :

Form a cotton ball; throw it as far as you can; it will land about 4 feet away.
Make a paper ball; throw it as far as you can; it will land about 25 feet away.
Throw a tennis ball as far as you can; it will land about 100 feet away.
Throw a golf ball as far as you can; it will land about 150 feet away.
Throw a baseball as far as you can; it will land about 200 feet away.
Throw a football as far as you can; it will land about 120 feet away
Throw a basketball as far as you can; it will land about 80 feet away.
Throw a bowlingball as far as you can, it will land about 15 feet away.
Throw a lead demolition ball as far as you can, watch your feet.

Now, have the same test done with a seven year old kid:
Distances reached will diminish accordingly to the strenght of the kid.-

What is going on ?

It is not the lightest nor the heaviest the one that got the farthest away.
It is not the lightest nor the heaviest the one that used up the full strenght
CAPABILITY of the arm muscles.

It is the one that MATCHES the arm strenght. There is one object
that flew the farthest for each person... you ; the kid.

When a rf or audio amplifier drive an antenna or a speaker or a
subsequent load, there will be an optimum FORCE -TO- LOAD ratio
that will deliver the most POWER. That load will not be a tiny nor a huge one.

A rated power output is the capability of a certain device AT a given
impedance load.

Unmatched driver-to-driven does NOT mean that there will be no action.
Your 400 watt stereo will burn yours ears off at full volume knob through
headphones but will be delivering only a couple of watts power, not
the 400 rated watts... because your headphones impedance is 600 ohms
instead of the at-rated 4 ohms... it is like the grown up throwing the paper ball.

As mismatch extremes, it is possible to push a ship by swimming,
or use a bulldozer to tow a bicycle.

I want to encourage readers to create and publish simple explanations for
everyone to learn and easily understand different topics.

There are people of great knowledge and people with great teaching abilities,
but too few have both , and from those few, even fewer show up for us to learn better.
Miguel

Re: impedace matching differences at low freq & High fre

There are a couple reasons why impedance matching is important.

One reason why it is important to impedance match a load to its source (which will probably have a transmission line connecting the two) in high frequency circuits is because if they do not match (or actually the source does not match the transmission line or the load does not match the transmission line) the voltage signal will reflect back to the source as a ratio of the mismatch. This is usually not desired. If you are trying to drive a lot of voltage on an antenna for high transmitting power, these reflections can exceed the dielectric breakdown in the antenna cable or ruin the power amplifier.

The other reason you also mentioned is the maximum power transfer. High-frequency circuits are often required to amplify very small voltages, such as the voltage signal that comes from an antenna. And the antenna represents a voltage source with a source impedance (for example, 50 ohm) that is a non-adjustable characteristic. To transfer the maximum power from the source to the receiver requires the receiver impedance to match the voltage source impedance.

You also pointed out:

in a lot of audio amplifiers voltage matching is done rather than impedance matching.i.e..(it is desired to have low output impedance of stage and high input impedance of next stage for maximum voltage transfer)....

Click to expand...

The maximum power transfer condition does not get the maximum voltage from the source to the load, in fact it causes the delivered voltage to be half of the source voltage. So why use a power match, especially if it attenuates the voltage signal that is already very small?

It would be possible to use a high impedance load (like in the audio amp) and obtain the full source voltage at the load, but the power in the signal received at the load would be very small, P=(V²)/R (we might double the voltage level but decrease the power level by 1000.) Which brings up the issue of noise sources. Any resistive element generates thermal noise: resistors, semiconductors, wires, etc. and these noise generators are power sources (not voltage sources, per se) that compete with the received power signal at the input for amplification. By matching the source and load according to the maximum power transfer theory, the transferred signal has the best chance to overpower the competing noise "signals" at the load. The voltage source and the noise sources all deliver their power into exactly the same load impedance. If the signal power delivered to the load is greater than the noise power delivered to the load, so too will the signal voltage at the load be greater than the noise voltage at the load. Even though the absolute value of the load voltage is half of the source voltage for maximum power transfer condition, the impedance match helps overcome the effect of noise at the inputs.

(This explains why high-frequency circuit design is interested more with power measurement than voltage or current.)

Using the maximum power transfer theory makes less sense in other applications, like the audio amp, because maximum power transfer condition is not a power efficient condition (the zero source impedance, infinite load impedance is the efficient condition.) In low-freq circuits, strong signals are available and are far greater than the noise generators in the circuit. Voltage sources are often very low impedance by design, and we can treat the voltage sources as capable of supplying infinite power (by that I mean we often neglect that the voltage source has a source resistance.) So, using the maximum power transfer condition between source and amplifier would waste a lot of power, when it is only necessary to transfer the voltage signal level from source to load. Of course the maximum power transfer condition will apply at the final output stage where it is desired to transfer power from the amp to the loudspeaker and get real work done.

Real thanks for theses nice explanations...But i still have one question that needs explanation...

I am trying to make a transmitter at 27MHz for Radio controlled application. How can i measure the input impedance of antenna connected to the transmitter for impedance matching....Normally i have seen the antennas connected to such RC circuits as only small wires...Its kind of hard to predict their impedance.....

For the speakers, it generally not 8ohm @ 1khz. 8ohm is just a good approximation. generally approximating a speaker in a ported box, possibly with a second speaker and a crossover network.

It is not a precise measure, and will generally be listed as 4ohm, 6ohm, or 8ohm. sometimes 6ohm speakers will be listed as 4ohm.

because the speaker is an electro-mechanical device, changes in mechanical properties (box, room, ect...) can change the electrical properties, like impedance.

for audio, the opposite is also true. Changing electrical parameters like source impedance can change the mechanical properties (reduced damping), resulting in a different sound.

The most comfortable way to measure antenna impedances is to use a vector network analyzer (VNA). For 27 MHz, also classical impedance bridges should work. Basic configurations of wire antennas have well known impedances, that can be found in text books.

Re: impedace matching differences at low freq & High fre

viper :
To measure antenna impedance, I have this animal, and it works fine :
---> **broken link removed**
You see the change when lenghtening, shortening, or bringing objects near.

A typical speaker impedance plot, for all the audio spectrum :
--->

There is one more question also...Impedance matching at low frequencies is normally done without taking care of complex parts of impedances ..For example using a transformer for impedance matching converts real impedances...why this is the case?...

Re: impedace matching differences at low freq & High fre

viperpaki007,
An ideal transformer multiplies both real and complex components of an impedance by a factor equal to the square of the turns ratio. At high frequencies, you must account for parasitics such as inter and intra winding capacitances.