A suppressor grid is a wire screen (grid) used in a

thermionic valve A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a ...

(

vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...

) to suppress

secondary emission In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the em ...

. It is also called the ''antidynatron grid'', as it reduces or prevents dynatron oscillations. It is located between the

screen grid A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). T ...

and the

plate electrode A plate, usually called anode in Britain, is a type of electrode that forms part of a vacuum tube. It is usually made of sheet metal, connected to a wire which passes through the glass envelope of the tube to a terminal in the base of the tu ...

(

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...

). The suppressor grid is used in the

pentode A pentode is an electronic device having five electrodes. The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode (called a ''trip ...

vacuum tube, so called because it has five concentric electrodes:

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

control grid The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consist ...

, suppressor grid, and plate, and also in other tubes with more grids, such as the hexode. The suppressor grid and pentode tube were invented in 1926 by Gilles Holst and

Bernard D. H. Tellegen Bernard D.H. Tellegen (24 June 1900 – 30 August 1990) was a Dutch electrical engineer and inventor of the pentode and the gyrator. He is also known for a theorem in circuit theory, Tellegen's theorem. He obtained a master's degree in electric ...

at Phillips Electronics. In a vacuum tube

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

s emitted by the heated cathode are attracted to the positively charged plate and pass through the grids to the plate. When they strike the plate they knock other electrons out of the metal surface. This is called ''

''. In the four-electrode vacuum tube, the

tetrode A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). ...

, the second grid, the

, is operated at a positive voltage close to the plate voltage. During portions of the cycle when the plate voltage is below the screen grid voltage,

secondary electron Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation (the ''primary'' radiation). This radiation can be in the form of ions, electrons, or photons with suffici ...

s from the plate are attracted to the screen grid and return to the cathode through the screen grid power supply. This flow of electrons away from the plate causes a reduction of plate current when the plate voltage increases, in other words the plate has a

negative resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordina ...

with respect to the cathode. This can cause distortion in the plate waveform and parasitic oscillations called dynatron oscillations in an amplifier. In the pentode, to prevent the secondary electrons from reaching the screen grid, a suppressor grid, a coarse screen of wires, is interposed between the screen grid and plate. It is biased at the cathode voltage, often connected to the cathode inside the glass tube. The negative potential of the suppressor with respect to the plate repels the secondary electrons back to the plate. Since it is at the same potential as the cathode, the primary electrons from the cathode have no problem passing through the suppressor grid to the plate. In addition to preventing the distortion of plate current, the suppressor grid also increases the electrostatic shielding between the cathode and plate, causing the plate current to be almost independent of plate voltage. This increases the plate output resistance, and the

amplification factor In general an amplification factor is the numerical multiplicative factor by which some quantity is increased. * In structural engineering the amplification factor is the ratio of second order to first order deflections. * In electronics the ampl ...

of the tube. Pentodes can have amplification factors of 1000 or more.

References

{{Thermionic valves Vacuum tubes Electrodes