Kildeimpedans

Kildeimpedans, or source impedance, is the impedance that a signal source presents to the rest of a circuit. It is commonly modeled as the Thevenin impedance of the source, denoted Zs, and is in general the combination of the source’s internal resistance and any reactive elements contributed by the source and its connections.

In linear circuit analysis, a real source is often represented as an ideal voltage source Vs in series with Zs. When a load with impedance ZL is connected across this network, the load voltage is determined by a voltage divider: Vload = Vs · ZL / (Zs + ZL). The current drawn is I = Vs / (Zs + ZL). Since Zs can be frequency dependent, both the amplitude and phase of the load signal can vary with frequency.

The source impedance influences several aspects of circuit behavior. It affects voltage division, loading of the source, bandwidth, and stability when the source drives reactive or sensitive loads. For complex impedances, maximum power transfer occurs when Zs is the complex conjugate of ZL; for purely resistive sources and loads, this reduces to Zs equal to ZL. In practice, designers often aim to minimize loading effects and preserve bandwidth rather than maximize power transfer.

Applications and typical values vary by domain. In RF and microwave systems, a standard source and transmission line impedance of 50 ohms is common. In audio and instrumentation, sources tend to present relatively low or well-controlled impedances to ensure adequate drive capability and predictable loading across connected stages.

Measurement and modeling often use Voc and Isc (open-circuit voltage and short-circuit current) to determine Zs, via the relation Zs = Voc / Isc. The Thevenin or Norton formalisms are standard tools for simplifying circuit analysis involving source impedance.