electronics low/high impedance

For signals encoded as voltage levels you want a HIGH INPUT impedance and a LOW OUTPUT impedance. For signals encoded as currents, you want the opposite.

For voltages, if your source is high impedance then your device input should be even higher impedance. A 1 MegOhm input is NOT high impedance if your source impedance is also 1 Meg.

The question you ask is really so broad that at least a monograph could be written in answer, but the stuff above should supply you a start on it.

LOW OUTPUT impedance.

be even higher

also 1 Meg.

written in answer,

Original question (I deleted it) Circuits are described as having either low input impedance or high input impedance. The articles imply that their circuit is somehow better because of it. Can someone explain when it's beneficial to have low input impedance and when it's beneficial to have high input impedance?

Kevin, Thanks for the response.

If an input is "high impedance" it means that is easy to "drive" i.e. easy to push around, in the sense that powered steering is easy to handle, you don't need much muscle power to move it.

So generally speaking, inputs will tend to be high impedance because then they will have more tendency to do what they are told as opposed to a lower impedance input e.g. unpowered steering.

A "high impedance" output is a "weak" output, a weak drive. If you used a high impedance output to drive a low impedance input (old man trying to turn non-powered steering) then the movement transferred will be reduced or to use the jargon, the ouput will be loaded by the input, and subsequently the signal will be loaded and reduced in size.

So it is usual to have e.g. the output of a HiFi amplifer (0.0001 Ohms) drive a loudspeaker of 4 Ohms. By this design, the loudspeaker will do exactly as it is told being driven by an impedance very much lower than its own.

Now a big confusion is "matched impedances". When a strong guy turns powered steering, he does not expect to get tired and he won't because his low impedance is not loaded by the high impedance reaction of the wheel. There is no intention to depend on the man's strength. The power is provided by the petrol. But what if there is no petrol? As in e.g. in a bicycle now it is important to get *power* from the man to the machine. When the road speed is low, a gear selection is required to match the maximum output of man to machine i.e. the impedances are "matched" but as he road speed increases so the peddle speed reaches a maximum that the man can manage and no power is transferred (the impedances have become mis-matched) so a new gear is selected to match the impedances again, to reduce the pedal speed to the "power zone" of the man and so on, up through the gears.

Under certain circumstances it is necessary to match impedances to get the maximum power from input to output. Different requirements might need maximum voltage transfer (low driving high) while others require maximum current transfer ( high driving low ).

Cheers Robin

To see the answer to your question, imagine you are measuring the voltage in some part of an electronic circuit (maybe a battery) with a voltmeter. Now suppose the input impedance of your meter is one ohm. That would almost always be considered a low impedance (but since low and high are relative terms, compared to an impedance of 0.001 ohms, one ohm is high). You'd have a hard time locating a voltmeter with a one ohm input impedance.

Not many circuits will continue to function with a one ohm impedance across them, and some might even be destroyed.

Now imagine a voltmeter with a 10 megohm input impedance. It would be much less likely to interfere with the normal operation of a circuit. Some voltmeters have an input impedance as much as a million times higher than 10 megohms for precisely that reason.

I encourage you to continue your study of electronics, and in particular, the notion of impedance. You will find some interesting information dealing with maximum power transfer.

Chuck

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