Electrical Engineering ⇒ Topic : Load Line
Figure (1) shows the circuit model of a triode valve in which RL is the load resistance present in the plate circuit. The plate supply voltage is Ep and ip is the current flowing in the plate circuit for a fixed grid potential. The potential difference between plate and cathode is ep and that of grid and cathode is eg. The potential difference across the load is RL.
Figure (1) Circuit of a triode valve.
If Ep and RL are fixed, Ep/RL, is also fixed and let it be equal to C. Let m = 1/RL 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 (iP) to plate voltage (eP) for a fixed value of load resistance (RL) and plate supply voltage (EP).
The load line cuts the plate current axis at a point A where eP = O. From Eq. (1) ip = EP/RL, denotes the maximum value of the plate current. Similarly, the load line cuts the plate voltage axis at B where ip = O. From Eq. (1), we get ep = Ep when ip = 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 Ep and load impedance RL. 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 Ep due to increase of RL.
Dynamic characteristic is the plot of plate current ip and grid voltage eg for a given load
impedance RL and fixed supply voltage Ep 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 ip and the abscissa is called quiescent voltage Ep. 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 RL.
The dynamic characteristic curve is linear so output waveform is least distorted when triode is being used as an amplifier
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