INTRODUCTION TO ELECTRON TUBES

conduction on the negative half-cycle decreases the gain of that half-cycle. The overall effect of allowingcathode biasing to follow the input signal is to decrease the gain of the circuit with ac inputs.This problem can be overcome by installing C k . The purpose of C k is to maintain the cathode biasvoltage at a constant level. In common usage, the action of C k is referred to as "bypassing the ac signal toground."The action of C k will be explained using figure 1-23. View A shows the circuit under quiescentconditions. With some conduction through the tube, the cathode and the tops of R k and C k are at +10volts.Figure 1-23.—Effect of the bypass capacitor.In view B, the positive-going signal is applied to the grid. This causes increased conduction throughthe tube, which attempts to drive the cathode to +20 volts. But notice that the top of C k is still at +10 volts(remember capacitors oppose a change in voltage). The top plate of C k is, in effect, 10 volts negative inrelation to the top of R k . The only way that C k can follow the signal on the top of R k (+20 volts) is tocharge through the tube back to the source, from the source to the lower plate of C k . When C k chargesthrough the tube, it acts as the source of current for the cathode. This causes the cathode to remain at +10volts while the capacitor is charging.View C of the figure shows the same signal. Under these conditions, conduction through R k willdecrease. This will cause a decrease in current flow through R k . Decreased current means decreasedvoltage drop. The top of R k will try to go to +5 volts. C k must now go more negative to follow the top ofR k . To do this, current must flow from C k through R k , to the top plate of C k . This discharging of C k willincrease current flow through R k and increase the voltage drop across R k , forcing the top to go morepositive. Remember, the voltage drop is due to current flow through the resistor. (The resistor could careless if the current is caused by conduction or capacitor action.) Thus, the cathode stays at +10 voltsthroughout the capacitor-charge cycle.There is one point that we should make. C k and R k are in parallel. You learned from previous studythat voltage in a parallel circuit is constant. Thus, it would seem impossible to have the top of R k at onevoltage while the top plate of C k is at another. Remember, in electronics nothing happens instantaneously.There is always some time lag that may be measured in millionths or billionths of seconds. The action ofC k and R k that was just described takes place within this time lag. To clarify the explanation, the voltagesused at the components R k and C k were exaggerated. Long before a 10-volt differential could existbetween the tops of R k and C k , C k will act to eliminate this voltage differential.1-32