The circuit diagram
for studying the characteristic
curves of a vacuum diode
filament current If
constant, change in plate current (Ip),
is observed when plate voltage (Vp)
is steadily increased. (Plate characteristics)
(i) is repeated for different constant
values of If. i.e.If1, If2,
The graphs representing these observations
are shown below. These curves are known as static characteristics of
Study of Characteristics (Conclusions)
A fixed number of electrons are emitted per second by the cathode, when a
constant current If is passed through the filament.
For a small plate voltage,
a few electrons reach the plate; Ip
remaining electrons gather between the cathode and the plate and
form a cloud known as space charge.
In a vacuum diode, the heated cathode emits electrons. These
electrons are attracted by the plate when it is at a positive
potential with respect to the cathode. If the plate is at zero
potential or if its potential is slightly positive, the electrons
are not attracted by the plate as soon as they are emitted. In other
words, the rate at which the electrons are attracted by the plate is
less than the rate at which they are emitted by the cathode. The
excess number of electrons remain in the
space between the cathode and the plate, forming a cloud of
electrons. This electron cloud is called the space charge.
This negatively charged electron cloud repels the electrons emitted
by the cathode and prevents them from reaching the plate. Therefore,
the plate current decreases still further.
- The number of electrons emitted from the cathode depends
upon the temperature of the cathode.
- Temperature of the cathode depends on the filament
- Higher the temperature, more is
the number of electrons emitted.
- The potential of a plate with respect to cathode does not
affect the rate of emission, but the number of electrons reaching
the plate depends upon the potential.
- Plate current flows in the diode only when the plate is
made positive w.r.t. cathode. No current
can flow when the plate is negative w.r.t.
- Electron current flow within a diode takes place only from
cathode to the plate. This is known as unidirectional conduction.
When plate voltage is increased,
number of electrons reaching the plate increases.
a stage is reached where
rate of emission of electrons by the cathode = rate of collection of
electrons by the plate.
the plate current reaches its maximum value known as the saturation
Diode is a
"Valve": If plate of the diode is given a negative voltage
with respect to the cathode, it repels electrons emitted by the cathode, hence the plate current is zero. Thus,
current flow through a diode is unidirectional. Diode becomes
conducting element only when plate is at positive potential. Hence, the
diode is called a valve.