1.2 Current and Voltage Sources – Pulse and Digital Circuits


Normally either ac or dc sources are used as current and voltage sources. A source can be either a voltage source (Thévenin source) or a current source (Norton source). An ideal voltage source should have zero internal resistance so that when current is drawn from the source, there is no voltage drop across the internal resistance of the source and the entire source voltage is available at its output terminals. Similarly, in a current source, no appreciable amount of current should flow through the internal resistance of the generator and the entire source current should flow through the load. For this, the internal resistance of the current source should ideally be infinity.

Figure 1.1(a) shows a practical voltage or Thévenin source and Fig. 1.1(b) a current or Norton source. It is possible to convert a Thévenin source into a Norton source and vice versa. To convert the Thévenin source [represented in Fig. 1.1(a)] into a Norton source [see Fig. 1.1(b)], we calculate the current (I) in the circuit using the relation I = V/Rs, where RS is the internal resistance in shunt with the current source I. Similarly, to convert the Norton source into a Thévenin source as shown in Fig. 1.1(a), we calculate the voltage (V) across RS as V = IRS, where RS is its internal resistance in series with the source V. Consider the single-loop network using a voltage source, as shown in Fig. 1.2. From Fig. 1.2:

FIGURE 1.1(a) Thévenin or voltage source

FIGURE 1.1(b) Norton or current source


The single-loop network shown in Fig. 1.2 is analysed using Ohm’s law. In this circuit, R1 and R2 comprise a potential divider. So, Eq. (1.1) is used to calculate VR2 directly instead of first calculating the current and then the voltage.

However, to analyse a network that has more than one loop, i.e., calculate the current in a given loop or voltage across the given branch, two basic network theorems—Kirchoff’s voltage law and Kirchoff’s current law—are used.

FIGURE 1.2 A single-loop network