Transit Time Effect

1
Transit Time Effect
Presentation on
Presented by -

• ICHHIT AMIN
• SAHIL ALMEIDA
• CARROL ALMEIDA
• RAMAKANT BANE
• SAVITA BADHA

2
CONTENTS

• INTRODUCTION.
• EFFECTS OF TRANSIT TIME AT VARIOUS FREQUENCY.
• TRANSIT TIME EFFECT ON NOISE.
• CONCLUSION.
• REFERENCE.

3
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.

4

• 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.

5

• 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.

TRANSIT TIME EFFECT ON TUBES AT VARIOUS
FREQUENCIES
6
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.

7
(No Transcript)
8
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.

9

• 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).

10

• The grid, in effect, is short circuited to the
  cathode and ceases to function and this in turn
  lowers tube efficiency.

11
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.

12

• And for all these reasons transit time are
  reduced in UHF tubes by placing the tube elements
  very close together .

13
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.

14
CONCLUSION

• THUS WE CONCLUDE THAT IF ANY TUBE OR TRANSISTOR
  FAILS AT HIGH FREQUENCIES, TRANSIT TIME IS
  KILLER IN ONE WAY OR OTHER.

15

• REFERENCE
• ELECTRONIC COMMUNICATION SYSTEM
• -BY GEORGE KENNEDY.
• GUIDED BY PROF. MAHESH MUNDE.
• GOOGLE SEARCH.

16
THATS ALL FOR THE DAY
THANK YOU
QUESTIONS ?????