4. Tetrode
As implies by its name, tetrode consist
of 4 electrode, cathode, control grid, plate and screen grid.
The simplify structure and symbol of tetrode
seen of figure 19.
Tetrode was developed to elliminate the
disadvantages of triode in case of interelectrode capacitance, especially
the capacitance between grid and plate (Cgp) that cause inefficiency in
triode working perfomance around high frequency region. To overcome that
matter the fourth electrode called as screen grid inserted into triode.
As seen on Figure 20 now Cgp that appears in triode become the series combination
between C1 and C2 which is totally smaller in value compare to Cgp itself.
Beside decreasing interelectrode capacitance
screen grid also acts as the electrostatic shield between plate and control
grid. As the result the change in plate voltage has only little effect
on the value of plate current as seen on tetrode plate characteristic (Figure
22).
The heated electrons from cathode will emitt
and move to the plate through the gap between wire mesh of screen grid.
Some of these electrons will attracted by screen and become screen grid
current, but the amount of screen grid current is small compare to the
total plate current.
* Tetroda Characteristic
Test circuit to achieve tetrode characteristic
seen on figure No 21 bellow.
As seen on Figure 21, there are three different
voltage source: control grid supply (Ec), Screen Grid supply (Esg) and
Plate supply (Eb) also meters to measure them.
The experiment done by increasing plate
voltage from 0 V to certain maximum acceptable voltage at every certain
grid voltage. Then as the result certain value of plate current will indicated
by amperemeter. The combination between grid voltage, plate voltage and
plate current are used to graph the plate characteristic seen on
Figure 22.
The following points may be noted from tetrode
characteristic :
Portion a-b
In this portion, the increament of plate
voltage followed by plate current.
Portion b-c
Plate current decrease as the plate voltage
increase, it cause by the secondary emission.
The emitted electron move from cathode to
plate through control grid and then arrive on plate. After those electrons
arrive on plate secondary electron will occur because the emitted electron
strike free electron within plate. By the presence of screen grid with
higher voltage than plate voltage, the secondary electron will attracted
by screen grid to be screen grid current. Thus the total plate current
will decrease.
Actually the secondary emission also occurs
on triode, but because triode does not have screen grid so the
secondary
electron will be pulled back to plate.
Portion c-d
In this region plate voltage already pass
over screen grid voltage so the plate have more power to pull back secondary
electron. The increment of plate voltage followed by plate current.
Portion d-e
In this region plate curve has become flat,
plate voltage has only little effect on plate current.
This happen because now screen grid acts
as electrostatic shield for control grid against electric field produce
by plate.
* Tetroda Constant
- Mu
Tetroda has higher mu than
triode usually more than 100.
- rp
Tetroda has high plate resistance
this is also effect of screen grid which acts as
the electrostatic
shield for control grid
- gm
From the Equation 14 we found the
relationship mu = gm x rp
Because of the higher value in rp
and mu so gm does not change and it still about
the same with the gm
of triode
* Disadvantage of tetroda
When AC signal apply
into tetrode, plate voltage will move up and down as the AC signal always
move up and down. There will be a situation when plate voltage move accros
portion b-c of the curve and the tetrode output signal will be distorted.
This effect called as " Dynatron
Effect".