Output resistance(Ra) is the term used to describe the resistance at the output of a circuit or device. In most cases, the output resistance is a frequency-dependent, complex resistance with magnitude and phase and is referred to as the output impedance.
The output resistance of a component cannot be measured with an ohmmeter. It is determined mathematically using the output current and output voltage (delta u)/(delta i) at the board and represents the internal resistance( Ri) of the board.
To achieve optimal matching between electrical assemblies, the output resistance and the input resistance of the following circuit must be matched: Optimal voltage matching occurs when the output impedance is low resistance and the input impedance of the following circuit is high resistance, optimal current matching occurs when the output impedance is high resistance and the following input impedance is low resistance, and optimal power matching occurs when the internal resistance (Ri) is equal to the output resistance (Ra).