The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, inductance and capacitance. Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Inductance (or electric inductance) is a measure of the amount of magnetic flux produced for a given electric current. ... Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. ...

The impedance at DC (frequency of 0) is the same as the resistance component of the impedance and is termed output resistance. Electrical impedance, or simply impedance, is a measure of opposition to a sinusoidal alternating electric current. ...

It is important to realize that no real device (battery, generator, amplifier) is a perfect source; all have an internal impedance, though this may have little practical effect, depending on the circuit and the load. A Pair of AA Energizer Alkaline Cells Symbols representing a single Cell (top) and Battery (bottom), used in circuit diagrams. ... “Dynamo” redirects here. ... Generally, an amplifier is any device that uses a small amount of energy to control a larger amount of energy. ...

Depending on perspective, this impedance can be modeled as being in series with a perfect voltage source, or in parallel with a perfect current source (see: Thevenin's theorem, Norton's theorem, Series and parallel circuits). Both models are equivalent, and one may choose whichever model is most convenient for analysis. It has been suggested that this article or section be merged with Current source. ... It has been suggested that this article or section be merged with Voltage source. ... Thevenins theorem for electrical networks states that any combination of voltage 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 can also be applied to general impedances, not just resistors. ... Nortons theorem for electrical networks states that any collection of voltage sources and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R. For single-frequency AC systems the theorem can also be applied to general impedances, not just... This article or section does not adequately cite its references or sources. ...

For example, a preamplifier with 100Ω output impedance means the output voltage signal appears to be in series with a 100Ω resistor. The ohm (symbol: Ω) is the SI unit of electric resistance. ... Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ...

Measurement

The source resistance of a purely resistive device can be experimentally determined by increasingly loading the device until the voltage across the load (AC or DC) is 1/2 of the open circuit voltage. At this point, the load resistance and internal resistance are equal.

It can more accurately be described by keeping track of the voltage vs current curves for various loads, and calculating the resistance from Ohm's law. (The internal resistance may not be the same for different types of loading or at different frequencies, especially in devices like chemical batteries.) A voltage source, V, drives an electric current, I , through resistor, R, the three quantities obeying Ohms law: V = IR Ohms law states that, in an electrical circuit, the current passing through a conductor from one terminal point on the conductor to another terminal point on the conductor...

The generalized source impedance for a reactive (inductive or capacitive) source device is more complicated to determine, and is usually measured with specialized instruments, rather than taking many measurements by hand.

Audio amplifiers

Image File history File links A simple source and load circuit. ...

The real output impedance (Z_source) of a power amplifier is usually less than 0.1Ω, but this is rarely specified. Instead it is "hidden" within the damping factor parameter, which is: In audio system terminology the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. ...

Solving for Z_source,

gives the small source impedance (output impedance) of the power amplifier. This can be calculated from the Z_load of the loudspeaker (typically 2, 4, or 8 ohms) and the given value of the damping factor.

Generally in audio and hifi, the input impedance of components is several times (technically, more than 10) the output impedance of the device connected to them. This is called impedance bridging or voltage bridging. High Fidelity is also the title of a book by Nick Hornby and a film directed by Stephen Frears, based upon Hornbys book. ... For the amplifier configuration, see bridged amplifier. ...

In this case, Z_load>> Z_source, DF > 10

In video, RF, and other systems, impedances of inputs and outputs are the same. This is called impedance matching or a matched connection. Impedance matching is the practice of attempting to make the output impedance of a source equal to the input impedance of the load to which it is ultimately connected, usually in order to maximize the power transfer and minimize reflections from the load. ...

In this case, Z_source = Z_load, DF = 1/1 = 1

The actual output impedance for most devices is not the same as the rated output impedance. A power amplifier may have a rated impedance of 8 ohms, but the actual output impedance will vary depending on circuit conditions. The rated output impedance is simply that impedance into which the amplifier can deliver its maximum amount of power without failing. An amplifier is a device which changes a small movement into a larger movement. ...

Batteries

Internal resistance is a concept that helps model the electrical consequences of the complex chemical reactions inside a battery. It is impossible to directly measure the internal resistance of a battery, but it can be calculated from current and voltage data measured from a circuit. When a load is applied to a battery the internal resistance can be calculated from the following equations: A Pair of AA Energizer Alkaline Cells Symbols representing a single Cell (top) and Battery (bottom), used in circuit diagrams. ...

or

where

R_B is the internal resistance of the battery

V_S is the battery voltage without a load

V is the battery voltage with a load

R_L is the total resistance of the circuit

I is the total current supplied by the battery

See also

• Impedance
• Input impedance
• Damping factor
• Voltage divider

Electrical impedance, or simply impedance, is a measure of opposition to a sinusoidal alternating electric current. ... The input impedance or sometimes loading impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to its input. ... In audio system terminology the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. ... In electronics, a voltage divider is a simple device designed to create a voltage (Vout) which is proportional to another voltage (Vin). ...

External links

• Calculation of the damping factor and the damping of impedance bridging