INTRODUCTION TO ELECTRON TUBES

grid-leak biasing, there is one thing you should bear in mind. Because the bias is derived from thepositive input signal through capacitive action, the input signal must go through several positivealternations before the final operating bias voltage is achieved. We will explain why this is so in thefollowing discussion.View A of figure 1-25 shows the circuit under quiescent conditions. You will notice that the circuitis similar to the one we used to explain the action of a triode. The only additions are the grid resistor, R gcoupling capacitor, C c , and resistance rgk. Resistance rgk doesn't exist as a physical component, but it isused to represent the internal tube resistance between the triode's cathode and grid. Electrically, rgk isquite small, about 500 ohms. Under quiescent conditions, some conduction occurs through the tube. Someelectrons will strike the wires of the grid, and a small amount of GRID CURRENT will flow through R gto ground. This will cause the right-hand plate of C c to go slightly negative. This slight negative chargewill, in turn, keep the grid of the tube slightly negative. This limits the number of electrons that strike thegrid wires.Figure 1-25.—Shunt grid-leak biasing.In view B of the figure, the first positive alternation of a series of ac alternations, E in is applied to thecircuit. The positive-going voltage causes the left-hand plate of C c to go positive. The left-hand plate mustlose electrons to go positive. These electrons leave the left-hand plate of C c and travel to the input sourcewhere they will be coupled to ground. From ground, current flows through R g causing a negative (bottom)to positive (top) voltage drop across R g . In effect, the ac signal has been coupled across the capacitor.Because of this, capacitors are said to pass the ac signal while blocking dc. (In reality, the ac signal iscoupled around the capacitor.) In view C of the figure, the positive-going voltage at the top of R g will becoupled to the grid causing the grid to go positive. The positively charged grid will attract electrons fromthe electron stream in the tube. Grid current will flow from the grid to the right-hand plate of C c . This willcause the right-hand plate to go negative. (Electrostatic repulsion from the right-hand plate of C c willforce electrons from the left-hand plate of C c , causing it to go positive.) The electrons will flow throughthe signal source, to ground, from ground to the cathode, from the cathode to the grid, and finally to the1-34