Input Impedance Matching

[DigiTan]

Regarding amplifier inputs, I remember someone saying something to the effect of: "Matched input impedance gives max power transfer. Max input impedance gives max voltage transfer." Is that accurate?

 

[ahmedragia21]

Zinpt must be high value to not attenuate the incoming signal to be ampilified, and to let the ampilifer gives max power to the load , Zinput= Zout*conjugate , and Zout should be small value...

 

[Torben]

Regarding amplifier inputs, I remember someone saying something to the effect of: "Matched input impedance gives max power transfer. Max input impedance gives max voltage transfer." Is that accurate?

This is the way I understand it:

If the output impedance of the source circuit matches the input impedance of the load circuit, power transfer is maximized. However, the load then loads down the source somewhat, reducing the voltage and the signal transfer.

If the source impedance is lower (rule of thumb: at least 10x lower) than the load's input impedance, then signal transfer gets better. The way I think of it is: if it's harder for the incoming voltage to get to ground than to the input of the load circuit, then more voltage will wind up at the input.

First you figure out the output impedance of the source (which you of course tried to make nice and low), then try to make the input impedance of the next stage 10x higher. Say you're making a preamp for an electric guitar. The pickups usually have quite high impedance, so the impedance on the preamp input is usually very high (10Meg is not uncommon).

An electret mic pushes much harder, so you don't need that much input impedance. Apparently in RF work you often try to match impedances, but I haven't done that yet.


Torben

 

[Nigel Goodwin]

The quote is correct, but you need to understand what it means!.

 

[dknguyen]

And neither necessarily means maximum efficiency.

I get confused about the max power transfer thereom. Since if the source impedance is zero ohms, then it is most efficient but it's not maximum power transfer (you would also think that the load would be getting the most power then as well but apparently there is some difference between the two). I have not been able to entirely wrap my head the difference between the two though.

It's weird, but Wikipedia says that the maximum power transfer thereom only applies when the source resistance is fixed. WHich I kind of understand when the source is something like a transducer. WHere I realy get confused is when the source is something like an actual power source like a battery- but I don't bother about impedance matching then, I just use lowest source impedance possible.

https://en.wikipedia.org/wiki/Maximum_power_theorem

Maximum power != maximum current != maximum voltage != maximum efficiency (necessarily).

 

[ecerfoglio]

And neither necessarily means maximum efficiency.

I get confused about the max power transfer thereom. Since if the source impedance is zero ohms, then it is most efficient but it's not maximum power transfer (you would also think that the load would be getting the most power then as well but apparently there is some difference between the two). I have not been able to entirely wrap my head the difference between the two though.

........

Maximum power != maximum current != maximum voltage != maximum efficiency (necessarily).

You don't get maximum power with maximum current or maximum voltage:

With a high load impedance you get the higher voltage (with an infinite impedance you get the maximum voltage = Thevenin's voltage), but with little (or cero) current. So power = voltage times current (times power factor) is low or even cero.

With a low load impedance you get the higher current (with a short circuit you get the maximum current = Norton's current), but with low voltage. Again this gives low power.

When the source's impedance is the same as the load's, the voltage is one half of the maximum voltage, and the current is one half of the maximum current, but the power into the load is the maximum that this source can give. In this condition, one half of the power goes into the load and another half into the source's internal impedance, giving an efficiency of 50 %.

 

[RadioRon]

Regarding amplifier inputs, I remember someone saying something to the effect of: "Matched input impedance gives max power transfer. Max input impedance gives max voltage transfer." Is that accurate?

Yup, that's accurate alright. Of course, in this context, the word "matched" is jargon having the specific meaning that the load impedance is the complex conjugate of the source impedance.

 

[bloki]

Term Matched Input Impedance is related to maximal power transfered to the load. Only when Rinput=Rsource or Rload=Rsource load can get maximal available power from source and thus max power gain. This is always needed in RF field.
Commertial audio amplifiers, for example have missmatched inputs because there is in question voltage gain and not power gain. Maximal voltage on input is achieved if input impedance is much higher than source impedance.
Such amplifiers have also missmatched outputs. Rout should be 0 ohms regardles the load impedance.