The function of each electrode in the pentode

The function of each electrode in the pentode

Anyone who has a basic knowledge of radio knows the functions of the electrodes of a triode. The functions of the electrodes of a pentode mentioned here are actually mainly the functions of the two newly added electrodes. Compared with triodes, the two newly added electrodes are the screen grid and the suppression grid. These two electrodes are named after the functions they play. Figure 3 is a diagram of the electrode distribution inside a pentode, which is usually called a pin diagram.

Figure 3 Electrode distribution inside the pentode

Why did we develop tetrodes, pentodes and beam power tubes when we already had triodes? This is because some characteristics of the triode are limited by its structure. For example, the distributed capacitance between the poles of the triode is too large, which makes the triode unsuitable for working at higher frequencies. In particular, the capacitance c between the screen and the gate of the triode is the most obvious. The higher the operating frequency, the smaller the capacitive reactance of the capacitance between the screen and the gate of the triode, thus limiting the application of the triode in the high-frequency range.

In the early days, in order to improve the characteristics of the triode, people added a screen grid between the control grid and the screen of the triode, which is the tetrode. After adding the screen grid, the adverse effects of the triode caused by the screen grid capacitance are greatly reduced due to the shielding effect of the screen grid, so the upper limit of the operating frequency of the electron tube is improved. Since a screen grid is added between the anode and cathode of the electron tube, the influence of the anode voltage on the cathode is reduced. At this time, when the screen voltage changes, the change in positive current is very small, which is equivalent to an increase in the internal resistance of the anode of the electron tube, and the amplification factor of the electron tube is also significantly increased. However, the secondary emission effect caused by the addition of the screen grid of the tetrode reduces the positive current of the electron tube.

In order to overcome the secondary emission effect of the tetrode, another grid is added between the screen grid and the anode of the tetrode, which is called the suppression grid. This is the pentode. The suppression grid is usually at zero potential (this is also the reason why the suppression grid is often connected to the cathode in the standard connection of the pentode). Since its potential is lower than the potential of the anode and the screen grid, the electric field lines from the anode to the suppression grid are from the anode to the suppression grid, which is used to make the secondary emission electrons return to the anode and cannot reach the screen grid, so it is called the suppression grid.

At the same time, since the pentode has an additional shielding effect of the suppression grid compared to the tetrode, the distributed capacitance of the screen grid of the pentode becomes smaller, and its anode internal resistance and amplification factor become larger. Since the voltage of the suppression grid is usually zero, no electrons will hit the suppression grid of the electron tube, that is, the current of the suppression grid is equal to zero.