Thevenin's Theorem states that any circuit, no matter how complex, acts as if it were a single voltage source and a single impedance in series with that voltage source (for now, an impedance is a resistor, it impedes the flow of electrical current). The Thevenin equivalent circuit can be determined by dividing the open circuit voltage by the closed circuit current. This can be illustrated by looking at the example circuit in Series-Parallel Circuits above. By choosing two points on the circuit (denoted by the circles in the diagrams below) you can determine the Thevenin equivalent circuit as seen at those points, by measuring the open circuit voltage and the closed circuit current at those points.

Any component or circuit has a Thevenin equivalent circuit. Particularly, any component or circuit that produces voltage, regardless of the actual complexity, will act as if it were a single voltage source in series with a single resistor (as per Thevenin's theorem). This is true for batteries, dynamic microphones (which are essentially sound-powered AC generators), radio-receiving antennas or anything else that converts non-electrical energy to electrical energy. Likewise, any circuit that delivers a voltage or current to another circuit can be reduced to a Thevenin equivalent circuit. The Thevenin equivalent resistance (properly called Thevenin equivalent impedance) has different names for different situations. For example, the Thevenin equivalent impedance of a battery is called its "internal resistance". However, for most circuits and components the Thevenin equivalent impedance is called "output impedance." ^[2]

An electronic circuit consists of electronic components or subcircuits connected together. A typical arrangement is a public address system. Here you have a microphone that connects to a preamplifier, which connects to a power amplifier, which connects to a loudspeaker. Energy is passed from one stage to the next until the ultimate function of the circuit is performed.

At the beginning of the public address system, the microphone connects to a preamplifier. The input of the preamplifier exhibits some opposition to electrical current flow (impedance) as does any circuit. The impedance seen at the input of a circuit is called input impedance. Just as the output impedance of the microphone can be represented by a resistor, the input impedance of the preamplifier can also be represented by a resistor. Therefore, the part of the system that consists of the microphone and the input of the preamplifier can be represented with the Thevenin equivalent circuit of the microphone connected to a resistor that represents the input impedance of the amplifier.