Electrical Engineering ⇒ Topic : Load Line

LOAD LINE

Figure (1) shows the circuit model of a triode valve in which R_L is the load resistance present in the plate circuit. The plate supply voltage is E_p and i_p is the current flowing in the plate circuit for a fixed grid potential. The potential difference between plate and cathode is e_p and that of grid and cathode is e_g. The potential difference across the load is R_L.

                                      Figure (1) Circuit of a triode valve.

Thus ,                                     

                                                                ...........(1)

If E_p and R_L are fixed, E_p/R_L, is also fixed and let it be equal to C. Let m = 1/R_L represents the slope of the line of Eq. (1). Equation (1) can be written in the form,

                                                 y= mx + C                                             ..............(2)

Equation (1) or Eq. (2) represents the load line. Load line is defined as a graph representing the variation of plate current (i_P) to plate voltage (e_P) for a fixed value of load resistance (R_L) and plate supply voltage (E_P).

The load line cuts the plate current axis at a point A where e_P = O. From Eq. (1) i_p = E_P/R_L, denotes the maximum value of the plate current. Similarly, the load line cuts the plate voltage axis at B where i_p = O. From Eq. (1), we get e_p = E_p when i_p = O.

Load line is the line joining the points A and B shown in Figure 2 (a). It does not depend on the tube characteristics. It depends only on the plate supply voltage E_p and load impedance R_L. These two factors are external to the tube

                                     figure (2) Dynannic characteristics of a triode valve.

The slope of the line will decrease for the same plate supply voltage E_p due to increase of R_L.

Dynamic characteristic is the plot of plate current i_p and grid voltage e_g for a given load

impedance R_L and fixed supply voltage E_p which is shown in Figure 2 (b).

To draw this graph, knowledge of Q for the load line and static characteristics of the tube is required. The quiescent operating point Q represents the point where the plate characteristics for a chosen value of grid potential intersect the load line.

The ordinate of Q represents the quiescent current i_p and the abscissa is called quiescent voltage E_p. Quiescent operation is known as the working condition of the tube when no signal is applied to the grid.

From Figure 2 (b), it is clear that the slope of the dynamic characteristics is less than the static characteristics due to load impedance R_L.

The dynamic characteristic curve is linear so output waveform is least distorted when triode is being used as an amplifier