Bihar BoardEnglish MediumSTD 12 SciencePhysicsSEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND SIMPLE CIRCUITS2 Marks
Question
Write a short note on 'Valves' in electronics.
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Answer
→Before the discovery of transistors in 1948, valves (vacuum tubes) were widely used in electronic circuits. →Types of valves are as follows: (i) Vacuum diode : which has two electrodes Anode (often called plate), and cathode. (ii) Vacuum triode : which has three electrodes - cathode, plate and grid. (iii) Vacuum tetrode : which has anode, cathode and 2 grids, so total four electrodes. (iv) Vacuum pentode : which has 5 electrodes. →In a vacuum tube, the electrons are supplied by a heated cathode (current is passed through the filament kept very close to the cathode and by heating the cathode, electrons are emitted by Thermionic emission process.) →Controlled flow of electrons in vacuum is obtained by varying the voltage between its different electrodes. Vacuum is required in the inter-electrode region (/space). Otherwise the moving electrons may lose their energy on collision with the air molecules in their path. →In these devices, the electrons can flow only from cathode to the anode (i.e. only in one direction). Therefore such devices are generally referred to as valves. →These vacuum tube devices are bulky, consume high power, operate generally at high voltages ( $\sim 100 V$ ) and have limited life and low reliability.
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