The operating modality of an electronic vacuum tube can be easily explained assimilating it to a hydraulic valve. Suppose a bucket, filled with water, is connected to a pipe. Suppose also that the pipe is connected to a hydraulic valve to control the flow of water. Refer to Figure 1, to follow the example. If the position of the bucket is higher than that of the other end of the pipe, the potential energy of the water, contained in the bucket, is higher than that of the end of the pipe. This implies that the water naturally flows from the bucket toward the bottom end of the pipe, producing a current of water.
However, if the valve is closed, as in Figure 1 a), there is no water flow. When the valve is fully open, as in Figure 1 b), the water flows freely. If the position of the valve is dynamically controlled with a signal, as sketched in Figure 1 c), the intensity of the water flow, through the pipe, follows the signal-controlled position of the valve.
2.1.1 Operating point
Suppose an oscillating signal (the input signal) is applied to the valve. The input signal has the effect of shifting the valve in a more open or less open position. The variation of the flow at the end of the pipe (output or amplified signal) follows the position of the valve.
In order to correctly amplify the input signal, the position of the valve, in correspondence of the no-signal state, needs to be carefully chosen. This is the quiescent position of the valve. Suppose that the quiescent position of the valve is set so that, when there is no signal, the valve is fully closed. In this case, the flow variation cannot follow the negative part of the input signal. In fact, when the input signal goes below the quiescent state, since the valve is already fully closed, no further reduction of the flow is possible. Similarly, if the valve is fully open at the quiescent state, it is not possible to follow the positive part of the signal. When the input signal is above the quiescent state, no further increase of the flow is possible, since the valve is already fully open.
The quiescent position of the valve is referred as the operating point. In a few words, it is the position of the valve when no signal is applied to it. Alternatively, we can consider the operating point as the amount of water that flows when no signal is applied to the valve.
In general, when the operating point is too high, as shown in Figure 2 a) the highest parts of the signal is lost. When the operating point is too low, as shown in Figure 2 b) the lowest parts of the signal is lost. In Figure 2 c) the entire dynamic range of the signal is correctly amplified.
We will see that choosing an appropriate operating point, of an electronic vacuum tube, is a very relevant aspect.