an electron tube containin three electrodesi is known as triode.in vacuum diode consists of two electrodes namely cathode and anode or plate. we have cloud of negatively charged electrons near the cathode called space charge. in triode i have extra electrode named grid and this grid eliminates space charge around cathode. how it happens? because grid is given negative voltage known as bias . if this bias is negative how the grid removes space charge which is also negative charge.
my question is how triode removes the space charge?
if it is by bias how it happen beause both bias and space charge is negative?
folks please help
Did u hear the term 'Thermoionic emission'?
When a metal piece is kept in a vacuum and is heated to incandescence, the electrons escape away from the surface atoms and surround the metal object in a cloud of free electrons. This is called thermionic emission or Edison effect.This was discovered by Thomas Alva Edison.It was the basic of the electric bulb and diodes,triodes etc.
In a common thermoionic tube cathode & anode are enclosed within an evacuated bulb and leads to the respective electrodes are brought out through the base of the bulb.Cathode is a hollow cylinder (usually) and is heated by a fine resistance wire within it.Electrons escape from cathode forming space charge and are attracted to the plate or anode. If the potential diff (pd) is small only a few escaped electrons reach the plate. Majority of electrons penetrate a small distance to the cloud of space charge and return to cathode.As plate potential increase more and more electrons are drawn to it.When further increase of pd doesn't increase plate current then this state is called saturated.BTW minimum energy required to escape electrons is called work potential.Now, in a diode emission occurs at cathode only so its flow is in a single direction, from cathode to anode.Usually circuit application of diode use this one-directional flow.eg. rectifier to convert a.c to d.c.
Problem comes with the introduction of grid.Tht was some scientist called Lee(Sorry, I forgot the whole name) who innovated the electron flow by inserting a third electrode called grid between cathode and anode.It is usually open either screen or coil of naked wire. When grid is negative the resulting field increase the space charge near the cathode and decrease the electron flow through the grid to the plate. As grid is nearer to anode the current flow is effected by change in grid voltage and not anode voltage (plate voltage) like the earlier case of diode."A relatively small variation in voltage on the grid causes a large variation in anode current"(Yeah!This is a quotation of my Physics teacher).The saturation of space charge is reduced by repulsion effect. It can thus function as amplifier and called triode.This is very useful in signal processing.Mobile phone is an example using amplifier for speaker, radio transmission and signal processing.
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