Voltage source

A mathematical model of an amplifier will include dependent voltage sources whose magnitude is governed by some fixed relation to an input signal, for example.

The internal resistance of an ideal voltage source is zero; it is able to supply or absorb any amount of current.

No real voltage source is ideal; all have a non-zero effective internal resistance, and none can supply unlimited current.

However, the internal resistance of a real voltage source is effectively modeled in linear circuit analysis by combining a non-zero resistance in series with an ideal voltage source (a Thévenin equivalent circuit).

An ideal voltage source provides no energy when it is loaded by an open circuit (i.e. an infinite impedance), but approaches infinite energy and current when the load resistance approaches zero (a short circuit).

A real-world voltage source has a very low, but non-zero internal resistance and output impedance, often much less than 1 ohm.

In the case of transistor current sources, impedance of a few megohms (at low frequencies) is typical.