Load Line Analysis

The output characteristics are determined experimentally and indicate the relation between V_CE and I_C. However, the same information can be obtained in a much simpler way by representing the mathematical relation between I_C and V_CE graphically.

The relationship between V_CE and IC

As discussed before,the relationship between V_CE and IC is linear so that it can be represented by a straight line on the output characteristics. This is know as a load line. The points lying on the load line give the possible values of V_CE and I_C in the
output circuit. As in a transistor circuit both d.c. and a.c. conditions exist, therefore, there are two
types of load lines, namely ; d.c. load line and a.c. load line. The former determines the locus of I_C
and V_CE in the zero signal conditions and the latter shows these values when the signal is apply.

(i) d.c. load line

It is the line on the output characteristics of a transistor circuit which gives
the values of I_C and V_CE corresponding to zero signal or d.c. conditions

Consider the transistor Load Line Analysis amplifier shown in Fig. 1. In the absence of signal, d.c. conditions
prevail in the circuit as shown in Fig. 2(i). Referring to this circuit and applying Kirchhoff’s
voltage law,

V_CE = V_CC − I_C R_C − I_E R_E
or V_CE = V_CC − I_C (R_C + R_E)

As for a given circuit, V_CC and (R_C + R_E) are constant, therefore, it is a first degree *equation and
can be represented by a straight line on the output characteristics. This is know as d.c. load line and
determines the loci of V_CE and I_C points in the zero signal conditions. The d.c. load line can be readily
plotted by locating two end points of the straight line.

The value of VCE will be maximum when IC = 0. Therefore, by putting IC = 0 in exp. (i), we get,
Max. VCE = VCC
This locates the first point B (OB = VCC) of the d.c. load line.

This equation is know as load line equation since it relates the collector-emitter voltage (V_CE) to the
collector current (I_C) flowing through the load.

The value of IC will be maximum when V_CE = 0.
∴ Max. I_C = C_C
C E
V
R _R +
This locates the second point A (OA = V_CC/R_C + R_E) of the d.c. load line. By joining points A and
B, d.c. load line AB is constructed [See Fig. 2 (ii)].

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