 This is Dr. Rupali Sherke working as an associate professor at Wolchen Institute of Technology Sholapur. In this video lecture I am going to discuss on the High Frequency Limitations on the Conventional Tubes. Learning outcomes at the end of this session students are able to describe high frequency limitations of the conventional tubes. These are the contents which will be covered in this video. Now, before discussing with the limitations of the high frequency, let us first see what are the conventional tubes. Just recall what are the conventional tubes. A conventional tubes are nothing but a tubes which are operating at a frequency range in between 300 MHz to 3000 MHz frequency. Now these conventional tubes are nothing but a vacuum tubes. We are familiar with that vacuum tubes like triode, tetrodepentode. Nowadays we are not using these devices but still there are some devices which are using the principle of these devices like CRT tube or a television tube. Now this vacuum tube is nothing but it consists of a gas or some liquid in them. So we are calling it as a vacuum tube and this vacuum tube consists of a electrons which are flowing from cathode to anode. There is a electron heating gun across the cathode. The electrons are emitted from the cathodes and move towards the anode. Cathode is a negative potential and anode is at the positive potential. In some cases cathode, anode is also called as a plate. Now to control the flow of electrons there are one or more grid inserted in this. This is the equivalent circuit of the vacuum tube. Now these grids are used for the controlling actions. So these devices are called as a voltage control devices and they are usually used for the very high voltage and to generate the high power also. Now when you make use of these devices they are at a high frequency there comes a limitations. There comes which are known as a high frequency limitations because conventional tubes are useful only at a low frequencies that is we are already discussed the range between the 300 megahertz to 3000 megahertz. When you make use at the micro frequency that is in a gigahertz frequency range then there comes a limitations on them. That limitations are inter electrode capacitance effect, lead induction effect, transit time effect, gain bandwidth limitation effect due to the RF losses and effect due to the radiation losses. Let us see this in the detail. The first inter electrode capacitor which is abbreviated as IEC effect. Now you can see that the if in the equivalent circuit of this vacuum tube that is a cathode and anode there are the plates which are been or capacitors which are been at the connecting between the two plates. They are represent this equivalent circuit in which we represent the two plates are been connected through the capacitors and this capacitors are been abbreviated with the C between C k, G k which is between the grid and the cathode and the capacitor which is connected between the plate and cathode is a C pk and the plate which is the capacitor which is been connected between the grid and the plate is a C capacitor C pg. Now when you see that the behavior of this capacitors are measured by the reactants, reactants are nothing but a xc which is given by xc is equal to 1 upon 2 pi fc. Now when you see this equation this reactants where this reactants takes place, this takes place at the lead, lead of the plates, leads of the plates. So when you see this relation the frequency, as the frequency increases, as the frequency increases its reactants decreases because it is inversely proportional, it is inversely proportional to the change in the, now when you go for the high frequency its reactants goes on decreasing. As this reactants goes on decreasing this capacitor will not be this, the reactants goes on this capacitors will act like a short circuit in this case. So as it acts in the short circuit the output are going to be reduced. So to overcome this there is one solution to increase the size of the capacitor because as the size of the capacitor increases the reactants can increase but increasing the size of the capacitor is physically not possible. Now the second effect is a lead inductance that is a Li effect. Now this effect is also been observed at the lead positions because the inductors which are been represented they are represented at the, from between the lead and the plates. Now this inductors, these inductors are responsible for getting the outputs. Now when the output at the lead, when you measure the outputs we are measuring at the lead points. Now due to this inductors, due to this inductors the reactants of this inductor increases as the frequency increases. Now you can see by this relation xl is equal to 2 pi fc. Now as the frequency increases this reactants also increases. Now instead of the getting the output at the lead the maximum output is received at the inductor only. So as the frequency increases the output will be across the lead only. It will not be at, so automatically the output decreases. Now this is a, this due to this effect again at a high frequency there comes a limitations over here. Now the third effect is called as a transit time effect. This transit time effect, what is a transit time now before explaining. Now let us consider a low frequency signal. This is a low frequency signal now and this is a high frequency signal. Now you can compare these two waves. The first is a low frequency and second is a high frequency. Now the time taken, a time taken from the electron to move from anode, cathode to anode is called as a transit time that is measured by the term. Transit time is a time taken from, from for the electron to travel from cathode to anode. This transit time is given by tau, it depends on the T by V0 where V0 is a velocity, D is a distance between the cathode and the anode and it is mined by my tau. You can see in the waveform this much is suppose we are distance we are measuring. Now this distance, this tau time for the high frequency you can see that there are two cycles which are being covered. Now this is for low frequency and this is for the high frequency. And the high frequency when you consider for the high frequency then the cycle of passing of the waveform is double compared to the single wave. So what happens there will be a phase shift in the signal. Let us see that how this can be controlled. Now the electric static energy which is nothing but applied potential energy we can called as potential energy which is given by E V. V is nothing but applied voltage and the kinetic energy which is generated due to the potential energy is one half m V square where V0 is the velocity of the electron. At the equivalent or equilibrium position the kinetic potential energy is compared with the kinetic energy. If when you equate this equation then the equation for the velocity we are getting as a under root 2 E V by m, m is a mass of the electron. Now when you substitute the V0 at the time duration transit time equation it is d upon under root 2 E V by m. Now once you know the mass of the electron you know the electron because this are going to a constant value and d then and the applied voltage the transit time can be controlled. But at a high frequency when it is been observed at a low frequency this transit time is negligible. So the plate current and the grid voltage are in phase but at a high frequency they does not remain in the phase but they change by some angle theta which is known as a transit time effect. This phase change may cause the measurement of the in the measuring of the gain. The next effect is a gain bandwidth limitation. Now the gain bandwidth limitation is given by the product which is given by A max into bandwidth. What is this A max? A max is nothing but a gain which is multiplied with the bandwidth. The bandwidth the this product this product is always equal to the C. Now as you know the relation as bandwidth increases the bandwidth increases at the frequency as a frequency increases bandwidth increases. So this constant value may increase this may create an issue. Now this next effect is the effect due to the RA losses. As you know that the RA losses are specified as a R square I square R by R I square R which are nothing but a power equation which is affected on the power. Now at a high frequency if you at a high frequency when the current has a tendency to conflict itself towards the smaller cross section area of the conducting towards the surface. So as the conductivity of this is given by under root 2 by mu omega mu sigma. Now this conductivity is inversely proportional to the root of the frequency and it is always directly proportional to the A effect. A effect is nothing but a effective area of the conductor. Now when you see this and the next is a radiation losses as you know that as the frequency is inversely proportional to the wavelength as the frequency increases its wavelength increases. As the wavelength the sorry wavelength decreases as the frequency increases wavelength decreases. If the wavelength of the signal is less than the surface then it start radiating. It will emit the radiation this radiation losses causes as the frequency increase because the wavelength is decreasing. These are the few limitations which are been discussed these are the references for the