TRIODE TUBE

Basically De Forest’s Audion vacuum tube came to be known as the triode tube "tri means three "because it had three elements: filament, (anode) plate, and grid (just as the “di” in the name diode (di means two) refers to two elements, (anode) plate and filament). Later developments in diode tube due to space charge problem the technology led to the refinement of the electron emitter: instead of using the filament directly as the emissive element, element, like metal strip called the cathode could be heated by the filament.

This modification was necessary in order to avoid some undesired effects of an incandescent filament as the electron emitter. First, a filament experiences the voltage drop along its length, as current overcomes the resistance of the filament material and dissipated thermal energy. It means that the voltage potential between different points along the length of the filament wire and another element in the vacuum tube won't ​be constant. For this and other similar reasons, AC (alternating current) used as a power source for the heating filament wire would tend to introduce unwanted AC “noise” and other signal in the rest of the vacuum tube circuit. Furthermore, the surface area of a thin heating filament was limited at best, and limited surface area on the emitting-electron element tends to place a corresponding limit on the vacuum tube’s current-carrying capacity.

The cathode was a thin metal cylindrically fitted snugly over the twisted wire of the heating filament. The cathode cylinder would be heated by the heating filament wire enough to emit freely electrons, without the undesirable side effects of actually carrying the heating current as the filament wire had to. The vacuum tube symbol for a triode with an indirectly-heated cathode in the following figure (same like this).

 

that why the heating filament is necessary for all but the few types of vacuum tubes, it is mostly omitted in the symbol for simplicity, or it may be included in the drawing but with no power connections drawn like it:


there is simple triode circuit is shown to illustrate its basic operation as an amplifier:

The low-voltage AC (alternating current) signal connected with grid and cathode alternately suppresses, then enhances the electron flow between the cathode and anode (plate). This reason a change in voltage on the output of the tube circuit (between anode and cathode). The AC (alternating current) current and voltage magnitudes on the vacuum tube’s grid are generally quite small compared to the variation of the current and voltage in the anode (plate) circuit. therefore, the triode tube functions as an amplifier of the input AC signal (taking high-voltage, high-current DC power supplied from the large (direct current) DC source on the right and “throttling” it by means of the tube’s controlled conductivity).

In the triode tube, the amount of current from cathode to anode (plate) (the “controlled” current is a function both of cathode-to- grid voltage (the controlling signal) and the anode plate-to-cathode voltage (the electromotive force available to push electrons through the vacuum). Unfortunately, neither of these independent variables having a purely linear effect on the amount of current through the device (usually referred to simple as the “anode current”). That is, the triode current doesn't necessarily respond in a directly, proportional manner to the voltages applied. In this particular amplifier circuit, the nonlinearities are compounded, as anode voltage (with respect to cathode) changes along with the grid element voltage (also with respect to cathode) as anode (plate) current is throttled by the vacuum tube. The result will be an output voltage waveform that doesn’t precisely resemble the waveform of the input voltage. it means that, the quirkiness of the vacuum triode tube and the dynamics of this particular circuit will distort the wave shape. If we really wanted to get complex about how we stated this, we can say that the vacuum tube introduces harmonics by failing to exactly reproduce the input waveform.

 

Another problem with vacuum triode tube behavior is that of stray capacitance. Remember that any time we have two conductive surfaces separated by an insulating medium, a capacitor will be formed. Any voltage between those two conductive surfaces will generate an electric field within that insulating region, potentially storing energy and introducing reactance into a circuit. Such is the reason with the vacuum triode tube, most problematically between the grid and the anode plate. It is as if there were tiny capacitors connected between the pairs of elements in the vacuum tube:

Now, this stray capacitance is quite miner, and the reactive impedances usually high. mostly, that is, unless radio frequencies are being dealt with. As we saw with De Forest’s Audion tube, radio was probably the prime application for this advance technology, so these “tiny” capacitances became more than just a potential problem. Another modification in vacuum tube technology was necessary to overcome the limitations of the triode tube.

Limitations of a Triode:

We know that three electrodes having an equal distance are found inside the triode vacuum tube and any two of these electrodes assume the shape of a simple capacitor (According to fundamental electronic and electricity theory, the metal plates function like a capacitor due to the existence of electro static field between each two charged metal plates. It has furthermore to be remembered that several electrostatic fields exist amongst the charged electrodes of a triode and if a dielectric is put between these two charged metals plate or piece, a capacitance occurs between both of these plates. thus, the following three (3) capacitances were found in a triode tube, which are known as inter electrode capacitances

1. Capacitance between grid and cathode (CGK)

2. Capacitance between grid and plate (CGP)

3. Capacitance between plate and cathode (CPK)

Inter electrode capacitance depend on the size of metallic plates, distances between these plates and the type of dielectric. However, the value of these capacitances is small 2-10 (µµf) micro-micro farads.

The values of the capacitances on low frequency is small while the value of reactance is quite large (because XC=1/2 fc). thus, if the number of RF (Radio frequencies) is having very high, reactance so gets low level or smaller. The unpleasant effect likes instability and oscillation occur as a resultant of this. (In particular the mutual capacitance of an anode and grid (CGP) has the capacity of an energy feedback from output or anode circuit to grid or input circuit, due to which unnecessary impacts take place). Therefore, it is not beneficial for use triode on so other many frequencies. The inter electrodes capacitances in similar radar or other circuits, due to the utilization of ultra-high frequency (u.h.f) is highly objectionable. thus, in order to improve characteristics and enhance the efficiency of a triode tube, or for reducing inter electrodes capacitances of a triode tube multi electrodes tubes are manufactured. For this end, a screen grid is included between a triodes tubes control grid and its plate, due to which inter electrodes capacitances minimizes to a great extent. Screen grid is provided with a positive supply High voltage (HT) and its voltages are also less as compared to plate voltages.