After all this time I'm still somewhat biased ...

 

Here is an example in impedance matching.
Suppose you built an audio amplifier with an output impedance of 5kΩ and wanted to connect this to an 8Ω speaker. You can use an audio matching transformer with
turns ratio = sqrt(5000/8) = 25

Suppose you wanted 500mW output = 1/2 W.

2V output would give 2 x 2 / 8 = 1/2 W.

To get 2V output at the speaker your amplifier would have to generate 2 x 25 = 50V output!

Power generated at the 5kΩ output = 50 x 50 /5000 = 1/2 W
same as power at the speaker, i.e. maximum power transferred.

Excellent example.
I've been reading about impedance matching and it's amazing how many different techniques there are.
The transformer was one of the more common ones mentioned.

Again, thanks very much.

 

Check this out guys, there is a thread here on the topic of impedance matching and there were arguments about it's necessity. One guy argues you only have to worry about it at high frequencies or depending on line length etc...and others say it is important for power transfer, but it is a different concept! Yes max power transfer is always good! Always needed obviously. There is a different name I found on the two different things, read this from wiki:

"As cables between line output and line input are generally extremely short compared to the audio signal wavelength in the cable, transmission line effects can be disregarded and impedance matching need not be used. Instead, line level circuits use the impedance bridging principle, in which a low impedance output drives a high impedance input. A typical line out connection has an output impedance from 100 to 600 Ω, with lower values being more common in newer equipment. Line inputs present a much higher impedance, typically 10 kΩ or more."

So impedance bridging is what we are talking about and it gets mixed in on discussions of impedance matching and transmission line effects. The two things are different.

 

Okay guys, so the pre-amp drives the output stage with enough voltage, but low current since output transistors have high hfe and require little current to bias them and thus they have high input impedances to draw little current(but just enough to bias them properly). The voltage from the pre-amp is the signal that drives the output transistors. The current seen at the base actually varies proportionaly to the voltage input correct? The output of the pre-amp has a relatively low impedance so that most of it's voltage is droped/inputed to the output stage for signaling?

So when we go from stage to stage we need to use opposite values that we will be amplifying? For instance: The output stage amplifies current and thus takes in little current and high voltage. The pre-amp can take a small microphone voltage and it get amplified, the input impedance beeing relatively low compared to the mic impedance? And does it opposingly draw more current from the mic, but see little voltage from the mic? Basically like transformers do due to the conservation of energy.

 

The typical arrangements are:

A low impedance microphone will be a coil type. The current is fed to an inverting opamp with fairly low input impedance to produce a higher voltage output.

A high impedance microphone will be connected to a high impedance, non-inverting opamp to get a higher voltage.

 

...

The pre-amp can take a small microphone voltage and it get amplified, the input impedance beeing relatively low compared to the mic impedance? And does it opposingly draw more current from the mic, but see little voltage from the mic? Basically like transformers do due to the conservation of energy.

Yes the preamp can amplify a small voltage, but the Input impedance of the pre-amp must be at least equal to but preferably higher than the impedance of the microphone.
As in the bridging scenario above, 600 ohms source can connect to 10K line input, but you cannot connect a 10K source to a 600 ohm input without out 'loading down' or losing most of the signal.

Why do you think that would be?

 

Yes the preamp can amplify a small voltage, but the Input impedance of the pre-amp must be at least equal to but preferably higher than the impedance of the microphone.
As in the bridging scenario above, 600 ohms source can connect to 10K line input, but you cannot connect a 10K source to a 600 ohm input without out 'loading down' or losing most of the signal.

Why do you think that would be?

Because the little voltage the mic produces will stay with the mic since higher resistance series connections hog the voltage in a voltage divider when the ratio is a large enough difference, correct? The current will be higher on the input since a parallel resistor is used from input to ground and the lower resistance hogs most of the current due to the ratio aswell. So current and voltage are sort of inversed in these type of circuits since inputs use a parallel resistor from input to ground but this same shunt resistor is in series with the mic? Something like that?

 

Bump for this thread...