How do I learn more about "impedance" ?

[petercom]

Originally Posted by Global

Here's a simplistic view of the relationship between input and output impedance.

Imagine two resistors in series, one large and one small. Now apply a voltage across the pair. If you now measure the voltage across each individual resistance you find that the applied voltage is divided in the ratio of the two resistances, so that most of the voltage appears across the larger resistor.

This is what happens when you couple an output to an input. If the output impedance is low (desirable) and the input impedance is high (likewise), most of the signal voltage appears across the input, which is what you want.

A cathode follower has a very low output impedance so can deliver signal into moderate input impedances without significant loss.

Mark

That's nicely put but I think a more accurate version is still to think of the system as two resistors and a voltage but consider it like this:

Imagine the output impedance of your source is a series resistance.

Imagine the input impedance of your load is a parallel resistance.

The two then form a voltage divider.

If the output impedance, i.e. series resistance, is high and the input impedance, i.e. parallel resistance, is low then most of the voltage appears across the series resistance and hardly any across the parallel resistance or load.

Now if your output impedance (series resistance) is low and the input impedance (parallel resistance) is high, nearly all the voltage will appear across the load.

To transfer DC resistance to AC impedance like this doesn't quite follow but it does hold generally true for the level of capacitance and inductance that are encountered at audio frequencies. So the capacitance and inductance of interconnect cable is relatively low, as is that of the source and load, so to all intents and purposes the system behaves as it would at DC. In other words there is no necessity to match impedance.

However at higher frequencies, for example RF, this level of capacitance and inductance begins to have a big effect and can cause transmission losses in, amongst other things, the connecting cable. Here it makes more sense to match impedances of source, load and cable, to reduce transmission losses and reflections from the load.

[Global]

"Education is a process of diminishing deception".

Much diminution is occurring here!

Mark

[Pedro]

Thanks guys

- this is great , and making a great more sense to me now.

Why do none of the leading popular texts really articulate this so clearly ?
Or am i not reading diligently enough ( dont answer that ! )

Keep it coming if anyone can add more to this.

Is there an oPportunity to build a "learning zone" on the forum for key topics which BBs feel arent covered so clearly elsewhere ( especially for hobbyists like me, rather than trained professionals) ?

So, to summarise so far - when I'm adding stages or designing from scratch am I basically looking to maximise input impedance and minimise output impedance in each of my "building blocks " ?

Rgds
Pete

[Ianm2]

Originally Posted by Pedro

Thanks guys


Why do none of the leading popular texts really articulate this so clearly ?

Pete

bcos most people can't either articulate it properly, or don't understand themselves and just copy it from other peoples writings, as goes on in many many gardening books.

however, a good well written textbook is worth its weight in gold, some of the american writers of academic books are incredible.