Load-Line Construction

Author: N.H. Crowhurst

We can find out more about amplification by plotting the curves of plate current and grid voltage with resistance connected in the plate circuit. However, we have to plot a new curve for each value of resistance with which we want to experiment. If, for some reason, we are not satisfied with any of the resistance values that we have already tried, the only thing to do is get the equipment out and plot some more curves.

Obtaining plate current versus grid voltage curves for various values of plate voltage, without plate load resistor.

Obtaining plate current-grid voltage curves with plate coupling resistor Rc in circuit

Setup for taking plate voltage-plate current curves at different grid voltages

Fortunately there is a more direct way of obtaining all these curves. To start with we plot quite a different set of curves. Each of these curves shows all the possible combinations of plate current and voltage that can occur for one fixed potential on the grid. If we can draw, on the same graph, another line we can find all the possible relationships in the plate circuit, corresponding to whatever resistance or other circuit component is connected. Then these curves can be used to find how the circuit - any circuit - will work.

For example, if the supply voltage used is 250 volts, and the resistance used is 20,000 ohms, without any plate current flowing, the plate potential will be the same at both ends of the resistor - 250 volts. If 1 milliampere of plate current flows, the drop across the resistor will be 20 volts, leaving 230 volts at the plate. With a plate current of 2 milliamperes, the drop will be 40 volts, leaving 210 volts at the plate, and so on. With 12.5 milliamperes flowing, the whole 250 volts will drop in the resistor, leaving 0 at the plate. These possibilities are shown by drawing a straight line through all these points.

Construction of a load line