output impedance

 

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.

The impedance at DC (frequency of 0) is the same as the resistance component of the impedance and is termed output resistance.

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.

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.

For example, a preamplifier with 100Ω output impedance means the output voltage signal appears to be in series with a 100Ω resistor.

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.)

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

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:

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.

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.

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.

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:

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

Internal resistance varies with the age of a battery, but for most commercial batteries the internal resistance on the order of 1 ohm.

See also

• Impedance
• Input impedance
• Damping factor
• Voltage divider

External links

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

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