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Electronics I Laboratory Manual

The load line is a graphical method used to solve standard simple circuits.  The method can be applied to any device, but in particular it is normally used when a non–linear device[2]is part of the circuit.

Let’s look at the general concept. In many circumstances we encounter a general circuit like the one shown in Fig. 3.1 composed of a power supply – called Vs in this case –, a resistor – RL –, and a device – the box shown in the figure – which can, for instance, be a diode, another resistor, or any other two–terminal device. The resistor is called the load and the device supplies voltage and current to the load.

The load normally represents another circuit.

The unknown parameters of the circuit are the current, I, which is common to all elements, the voltage across the device, VD, and the voltage across the load, VL.  Because the load is a resistor its voltage can be expressed in terms of Ohm’s Law, or

 VL     =     IRL (3.1)

In order to find the values of these parameters we should solve the circuit equations.  The voltage equation around the loop (the Kirchoff’s voltage equation) can be expressed as follows:

 Vs     =     VD      +      VL (3.2)

After substituting Eq. 3.1 in the above equation, we get

 Vs     =     VD     +     IRL (3.3)

The device voltage can be obtained by rearranging this equation. Thus,

 VD     =     Vs     –     IRL (3.4)

In real applications we normally know the values of the supply voltage (Vs) and of the load (RL).  Therefore, we have two unknowns (I and VD) in Eq. 3.3 (or in Eq. 3.4).  To find these two values – as the reader knows – we need to have two equations in terms of I and VD

[2] See the definition of non-linear devices on page 10

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