Electronic – How to a signal itself have a high impedance

impedancepiezoelectricitysensor

Observation: Recently I worked with a piezoelectric sensor and read that the sensor itself has a high impedance. Because of this, an op amp of high impedance was required to amplify the signal.

Question: How is it possible for a signal to have high impedance? How can the signal itself have resistance / reactance (I understand there will be very low amounts of resistance in the wire that the signal travels across.) Shouldnt the signals resistance / reactance be determined by which circuit is connected to?

Disclaimer:
My background is in computer science and my knowledge of electronics is pocket sized. Sorry in advance if this question is based of misconceptions on my part.

Best Answer

In electronics a high impedance signal would refer to a signal from a high impedance source (and low power). I've only observed this expression applied to low voltage applications. It would not be applied to a high voltage source which is generally also high impedance.

In your observations you indicate that the source has a high impedance and that it is therefore required that the load also has high impedance - so you're not talking about a high impedance signal!

Officially a signal does not have an impedance, but we can come very close to it when the signal is in a transmission line. At that time it must have the transmission line's impedance. That is: the voltage to current ratio in the transmission line is determined by the transmission line - nothing else. Reflections at the terminals of the transmission line balance it out. The wires with low amounts of resistance are in fact the transmission lines where I would technically accept to talk about the impedance of a signal, but that would not be what somebody means by the impedance of a signal.

A. Can a signal have high impedance?

Think of high impedance signal as a signal coming from a source with a high internal impedance and a very small power level. Avoid using it as an expression as it is technically incorrect.

B. Is the signals resistance/reactance determined by the circuit it is connected to?

When refering to the signal source impedance, that impedance is "by definition" determined by the source, not by the circuit it is connected to. The circuit it is connected to will contribute to the resulting current flow(s) and voltages.

In my opininion it is more correct to state that the signal impedance is determined by the transmission line through which it flows, not by the circuit it is connected to. But that is not the "conventional meaning" of "signal impedance" in engineering conversations. It would only be understood as such in in-depth exchanges where such definitions would be avoided.

C. Extra info

When a signal source is high impedance with low power, it is best to amplify the power with a circuit that has a matching load. The load should be the "complement" of the source. If the source would be 1 MOhm (purely resistive), then the amplifying circuit ideally has an input impedance of 1MOhm to transfer as much power as possible. Other factors come into play. In most cases though, the input impedance will be chosen to be higher than 10 MOhm as a lot of engineers will choose to build a voltage amplifier rather than a power amplifier.