 the output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current (ac) of a particular frequency as a result of resistance, inductance and capacitance. it is the thÃ©venin equivalent impedance looking back into the output terminals. in the case of a nonlinear device, such as a transistor, the term "output impedance" usually refers to the effect upon a small-amplitude signal, and will vary with the bias point of the transistor, that is, with the direct current (dc) and voltage applied to the device. the small-signal impedance at dc (frequency of 0) is the same as the resistive component of the impedance and is termed output resistance. no real device (battery, generator, amplifier) is a perfect source; each has 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 an ideal voltage source, or in parallel with an ideal current source (see: thÃ©venin'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, having a preamplifier with a 100-ohm output impedance means the output voltage signal appears to be in series with a 100-ohm 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 one half 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 == image:source and load circuit z.png the real output impedance (zsource) of a power amplifier is usually less than 0.1&nbsp;Î©, but this is rarely specified. instead it is "hidden" within the damping factor parameter, which is: : df = \frac{z_\mathrm{load}}{z_\mathrm{source}} solving for zsource, : z_\mathrm{source} = \frac{z_\mathrm{load}}{df} gives the small source impedance (output impedance) of the power amplifier. this can be calculated from the zload of the loudspeaker (t
Electronics