3.Triode
In 1906 Dr Lee De Forest (1873-1961), an
American scientist place the third electrode between cathode and plate
of a vacuum diode. The resulting devices was called as triode.
Simplified stucture and symbol of triode
seen on Figure 13. As implies by its name triode have three electrodes,
cathode, grid and plate.
Cathode stands on the centre surrounded
by grid which is in the form of wire mesh. There is spaces between that
wire mesh, plate cover both cathode and grid. Heated electron
from cathode move to plate through the gap between those wires mesh.
In the real application of triode, grid
controls the electron flow inside triode.
To get a clear idea about how control grid
work let us see Figure 14
When grid stands on negative
voltage (Figure 14.A) or less than zero, the negative charge of grid will
push back heated electron from cathode to emit and reach plate, so under
this condition there is no electron emit from cathode and reach plate.
Regarding to this situation there is a certain grid voltage that is to
totally unabled heated electrons from cathode to emit and this grid voltage
known as "grid cutt voltage"
When grid stands on zero
volt (Figure 14.B) the heated electron from cathode start to emit and reach
the plate. That's because zero voltage is still positive compare to negative
charge of electron. Zero voltage is enough to attract heated electron to
emit from cathode.
When grid stand on positive
voltage (Figure 14.C) positive charge from grid will attract strongly heated
electron from cathode to emit thorugh gap between wire mesh and reach plate.
As grid positive voltage increase up, the amound of emitted electron will
also increase up.
Note : From now I will use Ec to represent grid voltage, Ib for plate current and EB for plate voltage.
* Triode Characteristic
There are two important characteristic of
triode, Plate Characteristic and Mutual Characteristic.
Test circuit to achieve those characteristic
seen on figure 15 and resulting characterisctic seen on Figure 16 and Figure
17.
On Figure 15 you'll find that there are
two different variable voltage supply for grid and plate.
Grid voltage, Plate voltage and plate current
measured by different meter. By this way both plate and mutual characteristic
as seen on Figure 16 and 17 will achieve.
Plate Characteristic shows the coorelation
bettwen plate voltage and the resulting plate current at the same grid
voltage and Mutual Characteristic shows the coorelations between grid voltage
and the resulting plate current at the same plate voltage. On the real
application both Mutual and Plate characteristic are widely used to design
tube electronic circuits.
* vacuum Tube Constant
Tubes have there constant which are widely
used in tube electronic design.
- Amplification factor (mu)
Mu indicates how sensitive grid voltage
to control the changing of plate current.
Mu defined as the ratio of small
changing in plate voltage to small changing in grid voltage at the same
plate current.
mu = d(Ep)/d(Ec) ----> for d(Ib) = 0
- Transconductance (gm)
Transkonductance indicates
how sensitive grid voltage to influen plate current.
Transconductance defined as
the ratio of small changing in plate current to small changing in plate
current.
gm = d(Ib)/d(Ec)
----> for d(Eb)= 0
- Plate Resistance (rp)
Plate resistance is the ratio
of small changing in plate voltage to small changing in plate current.
rp = d(Eb)/d(Ib)
----> for d(Ec)= 0
- Coorelation between mu gm and rp
mu = d(Ep)/d(Ec) ...........................................................Eq 1
Multiply right side of the Eq 1 with 1 or d(Ib)/d(Ib) to make it become
mu = d(Ep)/d(Ec) X d(Ib)/d(Ib)................................Eq 2
Then modify Eq. 2 become
mu = d(Ib)/d(Ec) X d(Ep)/d(Ib).................................Eq 3
where d(Ib)/d(Ec) = gm and d(Ep)/d(Ib) = rp
Finally we can write Eq 3 as
mu = gm x rp .................................
Eq 4
* The Disadvantages
of Triode
When the first time triode
developed by Dr. Lee De Forest it gave many idea for further developement
that never imagine before, but triode still have two disadvantages.
a) Amplification Factor (mu)
Triode's mu still
consider as not high enough for many electronic applications
The highest mu
I've ever found is 100, from 12AX7
b) Interelectrode capacitance between all
the triode electrode make it become unsufficient for high
frequency application.
Those interelectrode capacitance are Cgk ( between grid dan katoda) , Cgp
(between grid and plate),
and Cpk (between plate and cathode). Generally value of the
interelectrode capacitance
are in the range of 2 - 12 picofarad.
Especially for Cgp, on
high frequency it will act as Miller Feedback capacitance that limit the
high
frequency perfomance
of the tube.
Note About Triode Disadvantages
All the disadvantages
I describe above relate to general application of triode, but in the case
of Tube Audio Application, me and also many others tube lovers believe
that triode is better compare to other tube. Triode perform sweet
sound compare to the others tube, to support my opinion about triode perfomance
I ask you to see the trend of Tube Audio nowdays that many
tube manufacture produce amplifier using triode, especially power amplifier
using 300B direct heated triode.
You can forget about Triode disadvantages
for high frequency application because Audio amplifier only work up to
maximum 20 kHz.