Electrical Engineering ⇒ Topic : Load Line

Murli
 
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  
 
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