electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current and potentially high voltage, whereas low impedance circuits are the opposite (low voltage and potentially high current). Numerical definitions of "high impedance" vary by application. High impedance inputs are preferred on measuring instruments such as

voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. ...

s or

oscilloscope An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...

s. In audio systems, a high-impedance input may be required for use with devices such as crystal microphones or other devices with high internal impedance.

Analog electronics

analog circuits Analogue electronics ( en-US, analog electronics) are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term "analogue" describes the proportional relat ...

a high impedance node is one that does not have any low impedance paths to any other nodes ''in the frequency range being considered''. Since the terms low and high depend on context to some extent, it is possible in principle for some high impedance nodes to be described as low impedance in one context, and high impedance in another; so the node (perhaps a signal source or amplifier input) has ''relatively low currents'' for the voltages involved. High impedance nodes have higher thermal noise voltages and are more prone to capacitive and inductive noise pick up. When testing, they are often difficult to probe as the impedance of an

multimeter A multimeter is a measuring instrument that can measure multiple electrical properties. A typical multimeter can measure voltage, resistance, and current, in which case it is also known as a volt-ohm-milliammeter (VOM), as the unit is equipped w ...

can heavily affect the signal or voltage on the node. High impedance signal outputs are characteristic of some

transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and con ...

s (such as crystal pickups); they require a very high impedance load from the

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...

to which they are connected.

Vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...

amplifiers, and

field effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contro ...

s more easily supply high-impedance inputs than

bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...

-based amplifiers, although

current buffer A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents (or voltages, for a cu ...

circuits or step-down

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s can match a high-impedance input source to a low impedance amplifier.

Digital electronics

digital circuit In theoretical computer science, a circuit is a model of computation in which input values proceed through a sequence of gates, each of which computes a function. Circuits of this kind provide a generalization of Boolean circuits and a mathemati ...

s, a high impedance (also known as hi-Z, tri-stated, or floating) output is not being driven to any defined logic level by the output circuit. The signal is neither driven to a logical high nor low level; this third condition leads to the description "tri-stated". Such a signal can be seen as an open circuit (or "floating" wire) because connecting it to a low impedance circuit will not affect that circuit; it will instead itself be pulled to the same

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

as the actively driven output. The combined input/output pins found on many ICs are actually tri-state capable outputs which have been internally connected to inputs (resulting in three-state logic or four-valued logic). This is the basis for bus-systems in

computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...

s, among many other uses. The high-impedance state of a given node in a circuit cannot be verified by a voltage measurement alone. A

pull-up resistor In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. It is typically used in combination with components such as switches and transistors, which physically int ...

(or

pull-down resistor In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. It is typically used in combination with components such as switches and transistors, which physically int ...

) can be used as a medium-impedance source to try to pull the wire to a high (or low) voltage level. If the node is not in a high-impedance state, extra current from the resistor will not significantly affect its voltage level.

References

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