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What is a TRIODE and how does it work?
The importance of the Grid

A TRIODE is an electronic amplifying device belonging to the vacuum tube family.
In electronics, a VACUUM TUBE is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

There are many types of vacuum tubes depending on their use. They are essentially used for a number of fundamental electronic functions such as SIGNAL AMPLIFICATION and CURRENT RECTIFICATION.
JAES, in its catalog, offers a wide range of triodes from different manufacturers.

In the Anglo-Saxon Countries the triode is simply called “valve”, since it allows to vary the current that passes through it. Very often to explain the basic principle of its operation, the triode is compared to a tap that control the release of a liquid or gas.

The triode was born as a natural evolution of the diode.
As we already explained in our previous video, the function of a diode is to allow the flow of electric current in one direction and virtually block it in the opposite direction. The diode is in fact mainly composed of two electrodes called ANODE and CATHODE. The electron flow conventionally goes from the chatode to the anode following this direction. Opposite polarization of the diode will oppose a virtually infinite resistance. The diode blocks the current reducing it to zero.

Unlike the diode, the TRIODE (as the name suggests) is provided with a third electrode placed between the CATHODE and the ANODE called: CONTROL GRID. All these elements are placed inside a tube in which a vacuum has been created, which means that all the air and other gases have been removed.

For a triode to actually work, it is necessary to create a STRONG POTENTIAL DIFFERENCE between its anode and cathode. For this reason the CATHODE is heated by a separate current flowing through a thin metal filament, in order to emit electrons via thermionic emission.
In this type of triode the GRID usually consists of a cylindrical screen or helix of fine wire surrounding but never touching the cathode. The grid acts like a gate for the electrons, controlling the current flow and thus allowing signal amplification.
Around the grid is placed the ANODE which receives the electrons emitted by the cathode.

This is a fairly simplified representation of the triode. Which means that only the basic elements of the tube are shown, as we can also notice in its symbol, where the cathode and the anode are separated by the grid, which is represented by this dashed line.

As in the diode, the anode collects the electrons emitted by the cathode, which are not hindered by the grid, since they can easily pass through its coils.
However, the grid can significantly influence the movement of the electrons towards the anode, if there’s an electric potential difference between the grid and the cathode. Basically, when there’s no voltage on the grid, the current between the cathode and anode will be at its maximum. On the other hand, a less negative, or positive voltage on the grid will allow more electrons through. Let’s see how:

We can observe how the grid is able to vary the current flow by drawing the characteristic curves of the tube:

Now we are going to apply DIFFERENT VOLTAGE VALUES between the anode and cathode to find out the corresponding CURRENT. The applied voltage is called ANODE VOLTAGE, while the current flowing through the tube is called ANODIC CURRENT.

To perform the measurements we will use this circuit (image above). As we can see here the grid and the cathode are connected, so they have the same voltage. Thanks to this circuit we will see how the triode behaves when the voltage between the grid and the cathode (which is called GRID VOLTAGE and is indicated with the abbreviation Vg) is equal to zero.
When the grid and cathode have the same electric potential, then the triode can be compared to a diode, that’s because the grid does not affect the anode current. We obtain a characteristic curve (image above) which is similar to the curve of a diode.

It is necessary to bring the grid to a different electric potential than the cathode, in order to observe its influence on the anodic current. So let’s try to connect a battery between these two electrodes.

The voltage of this battery is 2 V. We connect its positive pole to the cathode and its negative pole to the grid. Now the electric potential of the grid is 2 V less than the cathode electric potential. So the grid voltage is equal to -2 V.

In this scheme (3B image below) we can notice that the curve obtained with a negative grid voltage lies on the right of the curve obtained with a grid voltage equal to zero. We can now assume that: when we apply a certain anode voltage to the triode, the resulting anodic current is as small as the grid voltage is more negative.

In this scheme (3b) we can see that when the triode operates under point A conditions, in which the ANODE VOLTAGE is 100 V and the GRID VOLTAGE is 0 V, the resulting ANODIC CURRENT will be 12 milliamps. When instead the triode operates under point B conditions, in which the ANODE VOLTAGE is always 100 V but the GRID VOLTAGE is now -2 V, the ANODIC CURRENT will be just 6 milliamps.

This is a very interesting example that allows us to understand that the ANODIC CURRENT in a triode depends not only on the ANODE VOLTAGE (as in the case of the diode) but it also depends on the GRID VOLTAGE. In fact, it is possible to CHANGE THE ANODIC CURRENT by only changing the GRID VOLTAGE and leaving the ANODE VOLTAGE UNALTERED.

That’s because the electrons emitted by the cathode, besides being attracted by the anode, are now experiencing a repulsive force of the negatively charged grid: for this reason, only the fastest electrons emitted by the cathode can pass through the grid and reach the anode, forming the so call ANODIC CURRENT.

Actually, in the triode (as in the diode), an electron cloud is formed around the cathode. This negatively charged electron cloud, along with the grid, prevents the electrons to move towards the anode.
But while in the diode we cannot control the repulsive action that the electron cloud is exerting on the electrons, in the triode, we can just simply change the grid voltage.

Looking at the scheme (3b) we can observe a very important fact:
As we’ve already seen, we can reduce the anodic current from 12 milliamps to 6 milliamps, by just bringing the grid voltage from 0V to -2V and thus leaving the 100 V anode voltage unchanged.

However, the anodic current can be reduced in the opposite way, by changing the anode voltage and leaving the 0 V grid voltage unchanged.

In this case the triode shall operate under point C conditions (3b), in which the ANODIC CURRENT is 6 milliamps and the GRID VOLTAGE is 0 V. In this scheme (3b) we see that this can be achieved by reducing the anode voltage from 100 V to 60 V. Quite a big variation, don’t you think?

From all these considerations we can assume that IF WE WANT TO CHANGE THE GRID VOLTAGE to reduce the anodic current from 12 milliamps to 6 milliamps, a simple variation from 0V to -2V is required.
On the other hand, IF WE WANT TO CHANGE THE ANODIC VOLTAGE and achieve the same result, a variation from 100 V to 60 V is required. This means a 40 V variation, which is twenty times greater than the previous one!

Thanks to the small distance between the grid and the cathode, the grid has a more effective action on the anodic current compared to the anode, which is further away from the cathode.

We can conclude by saying that the triode grid is able to control the anodic current by changing its own voltage.