Vacuum tubes are of four type vacuum diode, vacuum triode, vacuum tetrode, vacuum pentode. Different vacuum tubes have different V-I characteristic. The reliability of a vacuum tube is mainly depends on its V-I characteristic. V-I characteristic means the graph between the voltage and current passing through a vacuum tube.
V-I characteristic of vacuum diode
Vacuum diode is made up of three parts plate, cathode and filament. V-I characteristic of vacuum diode is mainly depends upon plate and cathode. In vacuum diode cathode is heated up by the heated filament and anode is not connected to the external circuit. Some electrons are emitted from the cathode surface and travel a short distance due to the initial velocity. Then they will form a cloud on the cluster around the cathode. This cloud or cluster of electrons is known as space charge. Space charge is negative and electrons are leaving from the cathode surface. So cathode surface becomes positively charge. As more electrons comes out the surface charges experience a retarding force and due to this retarding they will return to the cathode. The reason for this retarding force will be outgoing electrons experience an attractive force from the positively charge cathode surface and the outgoing electrons experiences force from negatively space charge. If anode is given a positive voltage with respect to cathode then the filament current will decreases. When we increases the plate voltage then electron from the space charge are now attracted by the plate terminal. This result in a proper flow of plate current and from this we will get V-I characteristic.
V-I characteristic of vacuum triode
Vacuum triode has four major parts plate, cathode, filament and control grid. V-I characteristic of a vacuum triode is static characteristic. Vacuum triode has three parameters plate A.C resistance, mutual trans conductance and amplification factor which determines its characteristic. In vacuum triode electrons are travelled from cathode to plate. But due to the flow of electrons the control grid present in vacuum triode will divide the cathode and plate into two capacitors. Due to this inter electrode capacitance effect develop and which decrease the efficiency of vacuum triode. When the electrons emitted from cathode reaches the grid terminal they will store by the capacitor develop by the cathode and plate terminal. But due to the presence of control grid space charge formation will be stopped in vacuum triode.
V-I characteristic of vacuum tetrode
In vacuum tetrode an additional component is added which is called screen grid. The V-I characteristic of the vacuum tetrode are plotted between the plate current and plate voltage for fixed value of both screen grid voltage and control grid voltage. The V-I characteristic of vacuum tetrode will results the following conclusion
1. The plate current rises rapidly as plate voltage is increases.
2. Over a plate voltage ranges from 25 to 75 volt the plate current decreases with the increase in voltage. In this region the tetrode behaves as a negative resistance.
3. If we change the screen grid voltage the the characteristic of the vacuum tetrode will also changes.
4. Above 75 volt characteristic are flatten.
V-I characteristic of vacuum pentode
In vacuum pentode suppressor grid is installed as a additional component to increase its efficiency. V-I characteristic of vacuum pentode is plotted between plate current and plate voltage. As we increases the plate voltage the current will also increases at the positive grid voltage zero volt. It can be use for the amplification purposes. There are no drawbacks of vacuum pentode.