Travelling Wave Tube: =V (Pitch/2πr)
Travelling Wave Tube: =V (Pitch/2πr)
Travelling Wave Tube: =V (Pitch/2πr)
Travelling wave tubes are broadband microwave devices which have no cavity resonators like
Klystrons. Amplification is done through the prolonged interaction between an electron beam and
Radio Frequency (RF) field.
Construction
Travelling wave tube is a cylindrical structure which contains an electron gun from a cathode tube. It
has anode plates, helix and a collector. RF input is sent to one end of the helix and the output is
drawn from the other end of the helix.
An electron gun focusses an electron beam with the velocity of light. A magnetic field guides the
beam to focus, without scattering. The RF field also propagates with the velocity of light which is
retarded by a helix. Helix acts as a slow wave structure. Applied RF field propagated in helix,
produces an electric field at the center of the helix.
The resultant electric field due to applied RF signal, travels with the velocity of light multiplied by the
ratio of helix pitch to helix circumference. The velocity of electron beam, travelling through the helix,
induces energy to the RF waves on the helix.
The following figure explains the constructional features of a travelling wave tube.
Thus, the amplified output is obtained at the output of TWT. The axial phase velocity Vp
is represented as
Vp=Vc(Pitch/2πr)
Where r is the radius of the helix. As the helix provides least change in Vp
phase velocity, it is preferred over other slow wave structures for TWT. In TWT, the electron gun
focuses the electron beam, in the gap between the anode plates, to the helix, which is then collected
at the collector.
It is assumed that oscillations already exist in the tube and they are sustained by its operation. The
electrons while passing through the anode cavity, gain some velocity.
Operation of Reflex Klystron
The operation of Reflex Klystron is understood by some assumptions. The electron beam is
accelerated towards the anode cavity.
Let us assume that a reference electron er crosses the anode cavity but has no extra velocity and it
repels back after reaching the Repeller electrode, with the same velocity. Another electron, let's say
ee which has started earlier than this reference electron, reaches the Repeller first, but returns
slowly, reaching at the same time as the reference electron.
We have another electron, the late electron el, which starts later than both er and ee, however, it
moves with greater velocity while returning back, reaching at the same time as er and ee.
Now, these three electrons, namely er, ee and el reach the gap at the same time, forming an electron
bunch. This travel time is called as transit time, which should have an optimum value. The following
figure illustrates this.
The anode cavity accelerates the electrons while going and gains their energy by retarding them
during the return journey. When the gap voltage is at maximum positive, this lets the maximum
negative electrons to retard.
The optimum transit time is represented as
2. Operation:
Schottky diode is often referred as “majority carrier diode”..
When materials are joined, electrons in n-type silicon immediately flow into metal because the
electrons in semi conductor are at higher energy level than metal and hence electron flow is
established. The flow of electrons stops when Fermi level of two materials are at same level.
Due to flow of electrons into metal from semiconductor, metal develops negative charge while
semiconductor develops positive charge this give rise to depletion region at the boundary of two
materials and corresponding voltage is known as built-in potential.
On the application of a forward bias greater than the built-in-potential the current starts to flow
through the diode.
The width of the barrier in schottky is less than that of normal PN junction diode in both forward and
reverse bias region. Thus when we apply same voltage in forward bias and reverse bias to schottky
and pn junction diode we get higher current from schottky diode. The effect is desirable in forward
bias but undesirable in reverse bias.
Tunnel Diode
Tunnel diode is a thin junction diode which under low forward bias conditions exhibits negative
resistance useful for oscillation or amplification.
The junction capacitance of the tunnel diode is highly dependent on the bias voltage and
temperature.
A very small tin dot about 50μm in diameter is soldered or alloyed to a heavily doped pellet of n-
type Ge, GaSb or GaAs.
The pellet is then soldered to a kovar pedestal, used for heat dissipation, which forms the anode
contact.
The cathode contact is also kovar being connected to the tin dot via a mesh screen used to reduce
inductance.
The diode has a ceramic body and hermetically sealing lid on top.
In tunnel diode semiconductor material are very heavily doped, as much as 1000 times more than in
ordinary diodes.
This heavy doping result in a junction which has a depletion layer that is so thin (0.01μm) as to
prevent tunneling to occur.
In addition, the thinness of the junction allows microwave operation of the diode because it
considerably shortens the time taken by the carriers to cross the junction.
A current-voltage characteristics for a typical Germanium tunnel diode is shown in figure.
Forward current rises sharply as voltage is applied. At point A, peak voltage occurs.
As forward bias is increased past this point, the forward current drops and continues to drop until
point B is reached, this is the valley voltage.
At point B current starts to increase once again and does so very rapidly as bias is increases further.
Diode exhibits dynamic negative resistance between A and B therefore, useful for oscillator
applications.
Varactor Diode
Varactor or varicap(variable capacitance) is diode operated in reverse bias region.
It act as variable capacitor whose capacitance change with respect to voltage applied across its
terminal.
We can generalize this variable capacitor as parallel capacitor.
1. Construction:
The junction in the varactor diode is formed by joining two semiconductors one p-type material and
other n-type material.
At the base we have heatsink to moderate the temperature of the diode and for cooling purpose.
Glass passivation is provided to create a shell that protects from corrosion.
Metal contact is provided for the connection with battery.