Input impedance
Encyclopedia
The input impedance
of an electrical network
is the equivalent impedance
"seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law
. The input impedance is the Thévenin's equivalent
circuit of the electrical network, modeled by an RL (resistor-inductor) or an RC (resistor-capacitor) combination, with equivalent values that would result in the same response as that of the network. It is also called Z11 in terms of Z-Parameters. Generally speaking, the exact definition depends on the particular field of study.
, amplifiers have an input impedance several orders of magnitude higher than the output impedance
of the source device connected to that input. This concept is also called voltage bridging or impedance bridging
. In this case,
In general, this configuration will be more resistant to noise (particularly power line hum). Also the loading effects on the driving amplifier stage are reduced. In certain circuits a voltage follower
stage is used to match the source and load impedance, which results in maximum power transfer.
, and the load impedance all have to be equal (or "matched") to reduce signal reflections, which result in distortion and potential damage to the driving circuitry. This is known as a matched connection, and the process of correcting an impedance mismatch is called impedance matching
. Typical values are 50ohm and 75ohm. In analog video circuits these reflections can cause "ghosting", where the time-delayed echo of the principle image appears as a weak and displaced image (typically to the right of the principal image). In high-speed digital systems, such as HD video, reflections result in interferences and potentially corrupt signal.
carrying high power
, matching the impedances is important for at least two reasons:
In the case of purely resistive impedances (no reactive components), the two types of impedance matching are identical.
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of an electrical network
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
is the equivalent impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
"seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
. The input impedance is the Thévenin's equivalent
Thévenin's theorem
In circuit theory, Thévenin's theorem for linear electrical networks states that any combination of voltage sources, current sources, and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem...
circuit of the electrical network, modeled by an RL (resistor-inductor) or an RC (resistor-capacitor) combination, with equivalent values that would result in the same response as that of the network. It is also called Z11 in terms of Z-Parameters. Generally speaking, the exact definition depends on the particular field of study.
Audio systems
Generally in audio and hi-fi systemsHigh fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
, amplifiers have an input impedance several orders of magnitude higher than the output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
of the source device connected to that input. This concept is also called voltage bridging or impedance bridging
Impedance bridging
In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one which maximizes transfer of a voltage signal to the load...
. In this case,
- Zload, or the input of the driven stage >> Zsource, or the output of the driving stage.
In general, this configuration will be more resistant to noise (particularly power line hum). Also the loading effects on the driving amplifier stage are reduced. In certain circuits a voltage follower
Buffer amplifier
A buffer amplifier is one that provides electrical impedance transformation from one circuit to another...
stage is used to match the source and load impedance, which results in maximum power transfer.
Video and high frequency (RF) systems
In RF systems, the input impedance of inputs, the characteristic impedance of the transmission lineTransmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
, and the load impedance all have to be equal (or "matched") to reduce signal reflections, which result in distortion and potential damage to the driving circuitry. This is known as a matched connection, and the process of correcting an impedance mismatch is called impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
. Typical values are 50ohm and 75ohm. In analog video circuits these reflections can cause "ghosting", where the time-delayed echo of the principle image appears as a weak and displaced image (typically to the right of the principal image). In high-speed digital systems, such as HD video, reflections result in interferences and potentially corrupt signal.
- Zload = Zline = Zsource
Radio frequency power systems
In circuitsElectrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
carrying high power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
, matching the impedances is important for at least two reasons:
- The maximum powerMaximum power theoremIn electrical engineering, the maximum power transfer theorem states that, to obtain maximum external power from a source with a finite internal resistance, the resistance of the load must be equal to the resistance of the source as viewed from the output terminals...
at maximum efficiency will be transferred when the impedances are complex conjugateComplex conjugateIn mathematics, complex conjugates are a pair of complex numbers, both having the same real part, but with imaginary parts of equal magnitude and opposite signs...
matched throughout the power chain, from the transmitterTransmitterIn electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
output, through the transmission lineTransmission lineIn communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
(a balanced pair, a coaxial cable, or a waveguide), to the antennaAntenna (radio)An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
system, which consists of an impedance matching device and the radiating element(s). For maximum power, Zload = Zsource* (where * indicates the complex conjugate) - Failure to match impedances will create standing waves on the transmission line due to reflections. These will be periodic regions of higher than normal voltage. If this voltage exceeds the dielectric breakdown strength of the insulating material of the line then an arcElectric arcAn electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
will occur. This in turn can cause a reactive pulse of high voltage that can destroy the transmitter's final output stage. For reflectionless matching Zload = Zsource (no complex conjugate).
In the case of purely resistive impedances (no reactive components), the two types of impedance matching are identical.
See also
- ImpedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
- Damping factorDamping factorIn audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
- Voltage divider
- Dummy loadDummy loadA dummy load is a device used to simulate an electrical load, usually for testing purposes.-Radio:In radio this device is also known as a dummy antenna or a radio frequency termination. It is a device used in place of an antenna to aid in testing a radio transmitter...
External links
- Calculation of the damping factor and the damping of impedance bridging
- Interconnection of two audio units - Input impedance and output impedance
- Impedance and Reactance
- Input Impedance Measurement
Sources
- The Art of Electronics, Winfield Hill, Paul Horowitz, Cambridge University Press, ISBN 0-521-37095-7
- "Aortic input impedance in normal man: relationship to pressure wave forms", JP Murgo, N Westerhof, JP Giolma, SA Altobelli pdf
- An excellent introduction to the importance of impedance and impedance matching can be found in A practical introduction to electronic circuits, M H Jones, Cambridge University Press, ISBN 0-521-31312-0