尝试翻译下面段落:电路原理 Active electrical elements occur in tow basic forms: voltage sources and current sources. In their ideal form, voltage sources generate a constant voltage independent of the current draw from the source. The aforementioned battery and generator are regarded as voltage sources since their voltage is essentially constant with load. On the other hand, current sources produce a current whose magnitude is independent of the load connected to the source. Although current sources are not as familiar in practice, the concept does find wide use in representing an amplifying device, such as the transistor, by means of an equivalent electrical circuit. Symbolic representations of voltage source and current source are shown in Fig. 1-1A-2.