# electronics low/high impedance

Discussion in 'Electronic Design' started by c5f8, Mar 13, 2007.

1. ### c5f8Guest

I have a general question. Electronics articles sometimes state that a
circuit or electronic device such as a sensor has either low input
impedance or high input impedance. The articles never state why it's
important to know what the input impedance is.

I've seen both cases where a product is advertised with the low/high
impedance statement and suggesting that there product is somehow
better because of that statement.

Can someone explain when it's beneficial to have low input impedance
and when it's beneficial to have high input impedance?

2. ### Kevin G. RhoadsGuest

Can someone explain when it's beneficial to have low input 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.

3. ### c5f8Guest

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.

4. ### Rich GriseGuest

That's what he just did. Maybe you'd better take a remedial reading course.

Good Luck!
Rich

5. ### RobinGuest

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

6. ### chuckGuest

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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