Month: August 2016

Klystron is the simplest vacuum tube that can be used for amplification or generation of microwave signal. The operation of klystron depends upon velocity modulation which leads to density modulation.
Principle- high velocity electron beam is formed, focused and sent down along a glass tube to a collector electrode, which is at a high positive potential with respect to the cathode.

The input and output are taken from the tube via resonant cavity. The region between buncher cavity and catcher cavity is called drift space. The first electrode controls the number of electrons in the electron beam and focuses the beam. The velocity of the electrons in the beam is determined by the beam accelerating potential. When electrons travel through the drift space, they are subjected to RF potential at a frequency determinate by the cavity resonant frequency. The amplitude of this RF potential between the grids is determines by the amplitude of the input signal in case of an amplifier or by the amplitude of the feedback signal from the second cavity if used as an oscillator.

In velocity modulation, three cases are taken. Firstly, there is no applied voltage and as such electrons passing through the gap A are unaffected and will reach the collector with the same constant velocities.
At point B on the input RF cycle, the alternating voltage is zero and the electrons which passes through the gap A is unaffected by the RF signal (Reference electron, eR).
Secondly, consider C point passing the gap slightly later than the eR, this late electron, eL is subjected to positive RF voltage so that eL is accelerated and catches eR.
Thirdly, consider point A of RF cycle, an electron passing the gap slightly before the eR, called early electron ee and this early electron is subjected to negative RF voltage so ee is retarded and as such eR catches ee. When the electron passes the buncher gap their velocity will be changed according to the input RF signal. This process is known as velocity modulation. The electrons bunch together as they travel in the drift space. The pulsating stream of electrons passes through the gap and excites oscillations in the output cavity. The density of electrons passing the gap B varies cyclically with time. This means the electron beam contains an ac current and variation in current density enables the klystron to have a significant gain and hence drift space converts the velocity modulation into current modulation.

Immunology is the study of immune system and diseases related to immune system simply starting from allergy to very much complexed and dreadful diseases like AIDS, Hepatitis etc. There are various techniques to diagnose the diseases via identifying the presence of antibody or antigen in the serum of the patient. This forms the basis of the above said technique. Whole immune system is based on the interaction between antigen (foreign, non self material) and antibody (gamma globulins formed to combat Infection against that particular antigen). Interaction can be in form of agglutination (blood grouping) or precipitation. This interaction is liable for body’s defense against viral and bacterial infection. The antibody binds with more than one antigen depending upon its specificity or affinity for that antigen.
This technique is named after Orjan Ouchterloy, Swedish Physician in 1948. In this technique, both antigens and antibody are allowed to diffuse on the gel plate prepared by solidification of agarose gel on the plate and wells made onto this gel via gel puncture. Respective wells are filled with antigen and antibody by using micropipette and incubated in moist chamber for 24 hrs. a proper precipitation occurs when the amount of both antigens and antibody are at optimal concentration known as Equivalance point or Equilance zone. After 24 hrs, proper precipitin lines can be observed as a visual signature of antigen and antibody interaction. Three types of patterns are formed viz; pattern of full identity, pattern of partial identity and pattern of non identity. A full continuous line represents pattern of identity stating the immunologically similar nature of antibody for its respective antigen. Continuous line with formation of spur represents pattern of partial identity concluding antibody affinity for one particular antigen. Pattern of non identity is characterized by two crossing lines not meeting with each other concluding that antigens are immunologically not similar for their respective antigens.
Thus it can be concluded that by immunodiffusion assays immunological disesase (HIV, HEPATITIS) and presence of antigens and anibody in the serum.

(i) Intrinsic Semiconductor:-
• Semiconductor in their extremely pure form are called intrinsic semiconductors . eg. Ge & Si
Basically insulators
• The energy gap between the conduction and valence band is so small that the e- may be excited by thermal energy from valence band to conduction band.
• These excited e- present in the conduction band and the remaining unpaired e- in the valence band contribute to the conductivity.
• The conductivity of intrinsic semiconductors is due to the excited e-.
• No. of excited e- increase with increase in temperature.
• Conductivity of semiconductors increase in increase in temperature.
(ii) Extrinsic Semiconductor:-
• Substance which are normally insulators can be made semiconductors when small amount of impurities are added to them. Substances are called extrinsic semiconductors.
(a) n-Type Extrinsic Semiconductors:-
• Group 14 elements Silicon Crystals contains impurity of arsenic (4s2, 4p3) having more e- than Silicon in its valence shell. During the formation of bonds with surrounding Silicon atoms the Arsenic atoms use four of their valence e-.
• The fifth e- of each Arsenic atom is free. These extra e- occupy donor level which is just below the empty conduction band of Silicon Crystal.
• These e- from donor level can easily be excited to the empty conduction band.

(b) p- Type Extrinsic Semiconductors :-
• When the Silicon Crystal contains impurity of Boron or Aluminium (s2p2) having lesser e- than Silicon in its valence shell . We get p-Type Extrinsic Semiconductors e- from valence band of Silicon go to vacant orbital of Boron occupying the acceptor level which lies just above the valence band of Silicon resulting in the increase in conductivity of Silicon.