    Next: Dependent Sources Up: Introduction to Electronics Previous: Passive elements   Contents

# Sources Ideal voltage source: Whatever amount of current is drawn from it the voltage at the terminals is always same. Whenever the terminals are short circuited (resistance of between the terminals) the infinite amount of current flows to maintain voltage. This is hypothetical condition, why? Ideal current source: Whatever load or network of elements is connected to source, the current pumped by the source into the load always remains same. Whenever the terminals are open circuits (Terminals are not connected to any thing) the voltage across the terminals becomes to maintain the same amount of current through terminals. This is also hypothetical condition, why?

Can I leave a current source as shown in figure 3.3? In this case, voltage across the terminals will be .

Non Ideal Voltage Source: See the circuit in figure 3.4. A non ideal voltage source is modeled with an internal resistance of source . Thus battery terminal voltage changes with the load current. Under no load, i.e. for zero current, . When a load current flows, . For a new battery, generally, is negligible, and it increases as the battery gets discharged. is a function of electrolyte and terminal materials.

Non Ideal Current Source: See the circuit in figure 3.5. A non ideal current source is modeled by an internal conductance in parallel with the source. .
From the figure, we see that: . Ideal current source has , i.e., .

Non-ideal voltage source and current source analysis: The source is non-ideal, hence v is not constant. If it is linear circuit, and are linearly related. is cause, and is effect. Thus we get:  (3.1) (3.2) This equation is equivalent to fig.3.7. Thus a battery can be represented by 3.8: Similarly, for a nonideal current source (fig.3.9), if it is linear, (3.3) (3.4)

For ideal current source, always. Thus .  In the above, the sources are modelled using ideal voltage (current) sources whose voltage (current) remains constant.

We can also have source whose output can be controlled. These can be used to model certain real life devices (e.g., transistor) We will study transistor later during the course.

Subsections    Next: Dependent Sources Up: Introduction to Electronics Previous: Passive elements   Contents
ynsingh 2007-07-25