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.
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
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.
►A►A►A►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. 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. The input noise of transistor increases. Therefore on these frequencies the noise goes on increasing with increase in frequency which causes high disturbance. 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. 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 ELECTRONIC COMMUNICATION SYSTEM -BY GEORGE KENNEDY. GUIDED BY PROF. MAHESH MUNDE. GUIDED BY PROF. MAHESH MUNDE. GOOGLE SEARCH. GOOGLE SEARCH.