Understanding Impedence

[VTgreen81]

Okay so let's see if I've got this right:

Impedence (Z) which is measured in Ohms is the resistance to voltage passing thru a circuit. If the input or load Z of a device is 10 x the output Z of of the signal it's receiving then the system is bridged and if they are equal it's matched.

If the load Z is too low for the output Z it draws voltage too fast and causes a multitude of problems.

So it makes sense to me that the load impedence should NEVER be lower than the output impedence, but how do you know if thesystem should be bridged or matched or somewhere in between?

I could also use a brief explanation of the effect Z has on wattage.

Thanks for the help.

 

[Robert D]

Okay so let's see if I've got this right:

Impedence (Z) which is measured in Ohms is the resistance to voltage passing thru a circuit. If the input or load Z of a device is 10 x the output Z of of the signal it's receiving then the system is bridged and if they are equal it's matched.

If the load Z is too low for the output Z it draws voltage too fast and causes a multitude of problems.

So it makes sense to me that the load impedence should NEVER be lower than the output impedence, but how do you know if thesystem should be bridged or matched or somewhere in between?

I could also use a brief explanation of the effect Z has on wattage.

Thanks for the help.

A couple of points: Voltage doesn't pass through a circuit, current does. Similarily, a load doesn't draw voltage, it draws current. Voltage is like water pressure, and current is like the flow of water through a pipe. Resistance is the size of the pipe in the analogy. The load could be a faucet, and how far it is opened. This analogy works best with DC, which is much simpler to understand.
Now impedance is a bit different than straight resistance, and involves things like capacitive reactance and inductive reactance, which are AC considerations. Explaining it all here is not possible. For simplicity, let's just say that impedance is the ac equivilant of resistance.
Back to our water analogy, current is a measure of how fast the water is flowing through the pipe, and watts is a measure of the actual amount of water (voltage x current) passing through the pipe. Because lower impedance means higher current, it also means higher wattage.

I'll let someone else take it from there........

 

[Phyl]

Impedence (Z) which is measured in Ohms is the resistance to voltage passing thru a circuit. If the input or load Z of a device is 10 x the output Z of of the signal it's receiving then the system is bridged and if they are equal it's matched.

If the impedance of the signal source circuitry and the receiving circuitry are equal they are matched, if not they are mismatched. I haven't heard the word "bridged" use before in relation to impendance matching, but it's been a long time since I graduated.

.If the load Z is too low for the output Z it draws voltage too fast and causes a multitude of problems.

The load circuit doesn't draw voltage too fast from the source. The load circuit simply presents too little resistance, this causes the source circuit to try and provide more current than it is designed to do.

So it makes sense to me that the load impedence should NEVER be lower than the output impedence, but how do you know if thesystem should be bridged or matched or somewhere in between?

In a perfect world the impedance of the output device and input device will always be the same. In reality, designers allow for a certain amount of mismatch between source and load. For instance, instrument level inputs are typically spec'ed at 1 Meg Ohm, but not every guitar and bass out there has exactly a 1 Meg Ohm output impedance.

You will keep yourself out of trouble by following simple rules like only connecting line level sources to line level inputs, low impedance sources (mics) to low imepedance inputs (balanced), instrument level sources to instrument level (high Z) inputs.

I could also use a brief explanation of the effect Z has on wattage.

Bingo. This is how is was taught to me in college, and it makes sense since it's related to the conservation of energy principle taught in physics.

Energy is neither lost or created, it's simply transformed from one form to another. Example: An output source produces 1 watt of power. If the impedance of the output and input circuitry is matched exactly, that one watt of power is transferred with no loss. If the two are mismatched, some of that energy ends up being reflected back towards the source, and eventually is lost through...who knows...hear disappation, spurious emmisions, etc.

The other important point about impedance mismatch is that if the mismatch is great enough, it will cause a noticeable degradation of signal fidelity (distortion).

Hopefully someone will offer a better explanation. I had to go through all this stuff when I got my EE degree, but never did work in analog design so I'm a little rusty.