Output impedance

The output impedance is often used to model the source's response to current flow.

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 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.

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 real output impedance (ZS) of a power amplifier is usually less than 0.1 Ω, but this is rarely specified.

In this case, ZL>> ZS, (in practice:) DF > 10 In video, RF, and other systems, impedances of inputs and outputs are the same.

Internal resistance is a concept that helps model the electrical consequences of the complex chemical reactions inside a battery.

Circuit to the left of central set of open circles models the source circuit, while circuit to the right models the connected circuit. Z S is output impedance as seen by the load, and Z L is input impedance as seen by the source.