Circuit Response
every model has an edge, and the figure carries it
Concept

Ideal voltage source — where it appears

A source whose terminal voltage does not depend on the current drawn from it, which every real one approximates over a range of current and frequency. The range is what matters: a nine-volt battery is one until the current is enough for its half-ohm of internal resistance to be noticed.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

A 9 V source with 500 mΩ inside it. The ideal source is the flat line. The solved terminal voltage leaves it at a rate set entirely by the internal resistance: 1% low at 180 mA, half gone at 9.0 A.

The source that is not a source

An ideal voltage source holds its voltage at any current, which makes it the flattest line in the subject and the most commonly assumed model in it. Its edge is a current, set by one resistance nobody draws — and past that current the model is not approximately right, it is describing a different object.

networks · Source model A five-volt regulator's output impedance, with 1.00 Ω of series resistance. 0.430 mΩ at direct current, 1.95 Ω at 10.0 kHz — a factor of 4.52e+3 — and it has already doubled by 4.81 Hz. The upper curve is the same circuit with its loop opened, and the ratio between them is the loop gain. A regulator is a voltage source below a frequency and the datasheet's milliohms are the value at the bottom of it.

A source below a frequency

A five-volt regulator's output impedance is 0.43 milliohms, which is the number a datasheet quotes. It has doubled by 4.8 hertz, is ten times worse by 27, and reaches 1.95 ohms at ten kilohertz — four and a half thousand times its own specification, and higher there than the same circuit with its feedback loop cut. Two routes to that curve, sharing only the netlist, agree to a part in ten to the thirteenth.

applied · Regulator

Named alongside it

The objects these essays reach for when they reach for this one.

Internal resistanceLoad regulationLoop gainMaximum power transferOutput impedanceReturn ratioSeries-pass regulatorShort circuit currentThevenin equivalent

All concepts