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Transit Time Effect
ICHHIT AMIN
SAHIL ALMEIDA
CARROL ALMEIDA
RAMAKANT BANE
SAVITA BADHA
CONTENTS
• INTRODUCTION.
• EFFECTS OF TRANSIT TIME AT VARIOUS
FREQUENCY.
• TRANSIT TIME EFFECT ON NOISE.
• CONCLUSION.
• REFERENCE.
INTRODUCTION


TRANSIT TIME is basically time taken for
movement or transition of electron from
one electrode to another is known transit
time.
And the effect which is caused due to
transit time is known as TRANSIT TIME
EFFECT.
 TRANSIT TIME EFFECT ,it is seen that it
occurs not only in triode tubes but also in
transistor and many other devices which
depends on short time between electrodes.
TRANSIT TIME EFFECT ON TUBES
AT VARIOUS FREQUENCIES
•At low frequencies , it is possible that electrons
leaves the cathode and arrive at anode of tube
Instantaneously.
•But certainly it does not occurs at microwave
Frequency.
•Transit time occurs at appreciable fraction of
RF cycle.
•Due to this many effects results from this situation.
THE EFFECTS ARE AS FOLLOWS
 As soon as the grid and anode signals are 180
degree out of phase , it causes design problem
especially in feedback oscillator.
 Another important effect is that the grid begins
to take more power from the driving source.
 The power is absorbed (dissipated) even when
the grid is negatively biased.
Effects at ultrahigh frequencies
• The area between the grid and cathode
becomes highly negatively charged, as
shown in fig D. This negative charge is
surrounded by an electrostatic field. The
electrostatic field cuts the grid and repels
electrons that are present in the grid.
• As electrons are forced from the grid, the
grid tries to go positive. Unfortunately, this
tendency toward a positive charge attracts
electrons from the mass charge. Thus, as
an electron is forced from the grid; it is
replaced by another from the massed
charge.
• Electrons forced from the grid represent
grid current (Ig), as shown in fig E. The grid
current flows from the grid through Rg, to
the cathode, from the cathode, to the
massed charged, and back again to the
grid.
• The movement of current in this manner ,
in effect, a path for current flow from the
cathode to the grid.
• Because of the current flows between the
cathode and grid, the resistance (rgk)
between these elements is lowered to the
point of a short circuit (low resistance).
• The grid, in effect, is short circuited to the
cathode and ceases to function and this in
turn lowers tube efficiency.
It can be improved by:► Transit
time may be decreased by reducing the
spacing between electrodes.
► Or by increasing the electrode voltages which in turn
increases electron velocity through the tube.
► The problem with the last solution is that the tube
does not give an infinite resistance to current flow.
► If the operating voltage is raised to an operating
potential that is too high, arcing occurs between the
cathode and the plate and most likely will destroy the
tube. This also should be taken under consideration.
► And
for all these reasons transit time are
reduced in UHF tubes by placing the tube
elements very close together .
Transit time effect on noise in transistors
In transistors, if the time taken by the
electron to travel from emitter to the
collector becomes comparable to period of
signal being amplified at VHF range, transit
time effects takes place.
 The input noise of transistor increases.
 Therefore on these frequencies the noise
goes on increasing with increase in
frequency which causes high disturbance.

CONCLUSION

THUS WE CONCLUDE THAT IF ANY TUBE
OR TRANSISTOR FAILS AT HIGH
FREQUENCIES, TRANSIT TIME IS
‘KILLER’ IN ONE WAY OR OTHER.
REFERENCE

ELECTRONIC COMMUNICATION SYSTEM
-BY GEORGE KENNEDY.

GUIDED BY PROF. MAHESH MUNDE.

GOOGLE SEARCH.
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