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 (AC) of a particular frequency as a result of resistance, inductance and capacitance. It is the Thévenin 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: 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, having a preamplifier with a 100-ohm output impedance means the output voltage signal appears to be in series with a 100-ohm resistor.

Other articles related to "output impedance, output, output impedances":

Wilson Current Mirror - Circuit Operation - Input and Output Impedances and Frequency Response
... A circuit is a current source only to the extent that its output current is independent of its output voltage ... In the circuits of Figures 1 and 2, the output voltage of importance is the potential from the collector of Q3 to ground ... The measure of that independence is the output impedance of the circuit, the ratio of a change in output voltage to the change in current it causes ...
Wilson Current Mirror - MOSFET Implementation
... the transistor pairs M1-M2 and M3-M4 are exactly matched and the input and output potentials are approximately equal, then in principle there is no static error, the. 5 forces the drain current of M1 to equal the input current and the output configuration assures that the output current equals the drain current of M2 ... effect of lowering the contribution of the mirror transistors to the output current noise because the drain current noise density in a MOSFET is proportional to the transconductance and therefore ...
Tube Sound - Design Comparison - Output Impedance
... In an ideal current or transconductance amplifier the output impedance approaches infinity, while practically all commercial audio amplifiers are voltage amplifiers, and their output impedances have been ... Due to the nature of vacuum tubes and audio transformers, the output impedance of an average tube amplifier is usually considerably higher than of the modern audio amplifiers produced completely ... Thus, most tube amplifiers with their higher output impedance are closer to the idea of a transconductance amplifier than the solid state voltage amplifiers ...
Widlar Current Source - Output Impedance - Current Dependence of Output Resistance
... resistor values is    in ohms, and      is the output resistance due to the Early effect when VCB = 0 V (device parameter VA is the Early voltage). 6 shows that a value of output resistance much larger than rO of the output transistor results only for designs with IC1 >> IC2 ... Figure 3 shows that the circuit output resistance RO is not determined so much by feedback as by the current dependence of the resistance rO of the output ...

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