This article will be dedicated to another type of electron tube, the TRIODE, that can be compared to a faucet, since it allows to vary the current flowing through it, precisely like a tap makes it possible to adjust the flow of water.
CONSTITUTION OF TRIODE
The triode is so called because it comprises three electrodes: in addition to the cathode and the anode, the tube is provided with a third electrode disposed between the first two. This electrode is said GRID because in the first triode, realized in 1907 by American Lee De Forest (1873-1961), It was precisely constituted by a metal grid.
In triode currently used (fig. 1-a), the grid is constituted by a thin metal wire wound in a spiral around the cathode, without touching it; around the grill is disposed the anode, which has been drawn sectioned so that in the figure even if it were the grid visible.
Even the grill, as the cathode and the anode, He is part of a dedicated pin for its connection to the external circuit to the tube.
The triode shown in fig. 1-a is greatly simplified, not having been indicated the filament and its legs, nor the supports that maintain the various electrodes in the respective positions: They were shown only the essential elements of the tube which also appear in the graph of Fig sign. 1-b, with which it represents the triode in the circuit diagrams.
As in the diode, the anode collects the electrons emitted from the cathode, which they are not hampered by the grid, because they can easily pass through its coils.
The grid can affect, however, on the movement of electrons that are directed towards the anode if it is located at an electrical potential different from that of the cathode: in this way the grid can serve to vary the current flowing through the triode, as you will see in the next section.
Features of the triode
The effect of the grid on the current flowing through a triode can be put in evidence by tracing the characteristics of the pipe bends.
For this purpose, one proceeds as has been done in the case of the diode, applying a voltage between the anode and the cathode and measuring it for different values of the corresponding current flowing through the tube: Also for the triode, the voltage and current above constitute respectively the anode voltage and the anode current.
To perform the measurements using the same circuit as already used for the diode, whose scheme is shown in fig. 2-a: in the diagram you see that the grid is connected to the cathode, so that you have the same electrical potential of this electrode, to consider first how it behaves the triode when the voltage between these two electrodes (which is said grid voltage and is denoted by Vg ) It is equal to zero.
Under these conditions the triode is equivalent to a diode, because the grid does not make its influence felt on the anode current: you get, indeed, a characteristic curve (fig. 2-b) whose performance is similar to that of the characteristic curve of the diode.
Also in this case the curve was plotted with a first portion to entire line to indicate the values of voltage and current for which is not exceeded the maximum anode dissipation.
To see the effect of the grid on the anode current necessary to bring this electrode to a different potential from that of the cathode: this can be achieved by connecting, eg, a stack between the two electrodes, as seen in Fig. 3-a.
It is used by a stack 2 V, by connecting its positive pole to the cathode and its negative pole to the grid, in this way the grid is to be at a lower potential 2 V to that of the cathode, and then the grid voltage is now equal to - 2 V.
It considers the behavior of the tube with negative grid voltage because in the majority of applications, especially in radio receivers, triodes are used under these conditions.
Applying again to a triode anode voltage and measuring it again for different values of the corresponding anode current, you can also draw in this case the characteristic (fig. 3-b), which it is distinct from that obtained with the grid voltage equal to zero: as seen in Fig. 3-b, the features are marked by scoring in each of the grid voltage with which they were obtained.
The characteristic obtained with negative grid voltage is to the right of that obtained with the grid voltage equal to zero: it follows that, with a certain anode voltage applied to the triode, the anode current is less, the more negative is the grid voltage.
In fig. 3-b display, eg, that when the triode operates in the conditions indicated by the point A, ie with an anode voltage of 100 V and a grid voltage 0 V, the anodic current is of 12 mA; when instead the triode operates in the conditions indicated by the point B, that is to say still with an anode voltage of 100 V but with grid voltage - 2 V, the anodic current is just 6 mA.
This example clearly shows that in a triode the anodic current depends not only on the anode voltage, as in the diode, but also by the grid voltage: indeed, while leaving unchanged the anode voltage, it was possible to vary the anode current by varying the grid voltage.
This is due to the fact that on the electrons emitted by the cathode is exercised now, in addition to the force of attraction by the anode, also a force of repulsion on the part of the negative grid: Consequently, only the fastest among the emitted electrons can pass through the grid and reach the anode, thus constituting the anode current.
In reality, Also in triode, as in the diode, It is formed around the cathode of an electron cloud, with its negative space charge, competes with the grid to hinder the path of the electrons towards the anode.
It must be noted that, while you can not control the action of repulsion that the electron cloud exerts on electrons, This is possible for the grid, because it is enough to vary its voltage.
From the two characteristics of fig. 3-b is also a very important fact.
As seen, It can reduce the anode current by 12 mA and 6 mA bringing the grid voltage from 0V to - 2V and leaving unchanged the value of 100 The anode voltage V.
It, however, can reduce in the same way by varying the anode current and the anode voltage while leaving unchanged the value of 0 V of the grid voltage.
In this case the triode must be brought to operate in the conditions indicated by the point C, to which precisely corresponds to an anodic current of 6 mA and a grid voltage 0 V: in Figure. 3-b you see that this can be achieved by reducing the anode voltage by 100 V a 60 V.
From these considerations it can be deduced that in order to reduce the anode current by 12 mA and 6 mA just a variation of 2 V (da 0V a — 2V) if it acts on the grid voltage, whereas if it acts on the anode voltage should be a variation of well 40 V (gives 100 V a 60 V), twenty times greater than the previous.
This fact is due to the small distance that exists between the grid and the cathode, for which the same grid can act more effectively on the anode current of the anode, which is located farther away from the cathode.
It can therefore be concluded that the grid of a triode is adapted to control the anode current by means of the variations of its tension.
It should also be noted that the anode current control by the grid occurs without expenditure of energy: indeed, since generally the grid is negative with respect to cathode, no electron can move on it and give rise to a current in the external circuit.
Therefore, the stack from 2 V, that in fig. 3-a is connected between the grid and cathode, It must not deliver any current and therefore does not provide energy to a circuit.
To be able to vary the anode current within fairly wide limits, the grid voltage is also brought to values lower than - 2 V and to know the behavior of the triode in such conditions are traced other characteristics, eg, for grid voltages - 4 V, di - 6V, etc.
To dispose of these different voltages may adopt the same system used for the anode voltage, connecting a potentiometer to the stack which provides the grid voltage, as seen in Fig. 4-a: the potentiometer cursor moves up to obtain the desired voltage, whose value is read on the voltmeter connected between the cathode and the grid.
The characteristics obtained for the various values of the grid voltage are shown in the diagram of fig. 4-b, from which it is seen that each characteristic is all the more shifted towards the right because the lower the voltage of the grid with which it was obtained.
The dot and dash line that crosses the characteristics delimits the area of the diagram, It situated below it, in which the voltage and the anode current have values with which is not exceeded the maximum anode dissipation.
The set of characteristics of Fig. 4-b constitutes a family of characteristic curves of the triode; these features are said anode because they indicate how varied the anode current as a result of variation of the anode voltage, when the grid voltage is assigned a value determined.
Since the grid serves to control the current flowing through the triode, interesting also be able to see directly as this current varies as a result of variation of the grid voltage, when the anode voltage is assigned a value determined.
For this purpose it determines the characteristics of the mutual family of triode, using the same circuit shown in Fig. 4-a, but proceeding in a different way.
In this case, indeed, it leaves unchanged the anode voltage to a value determined, e.g. 100 V, and it applies to a negative voltage grid by measuring it for different values of the corresponding anode current; reporting on a diagram of the grid voltage and the anode current can be drawn mutual characteristic relative to the anode voltage of 100 V.
It then increases the anode voltage, bringing, eg, a 170 V, and it is applied again to a negative voltage grid, by measuring it again for different values of the corresponding anode current; so you can draw a second mutual characteristic relative to the anode voltage of 170 V.
In the diagram of fig. 5 They show four mutual characteristics of the triode hitherto considered; these characteristics were obtained for the values of the most commonly used anode voltage, values that have been shown on each of them.
The anode current values are still shown on the vertical axis as in the case of the anodic characteristics, while the horizontal axis are now listed in the grid voltage values; since these values are negative, the axis has been plotted to the left of the vertical axis.
Now consider, eg, mutual characteristic relative to the anode voltage of 100 V and note that this characteristic meets the horizontal axis at point A, in correspondence to which it is marked the value of - 8-V of grid voltage.
When the triode is in the conditions indicated by the point A, its anode voltage is therefore of 100 V and its grid voltage - 8 V; since the point A lies on the horizontal axis, the anodic current is equal to zero.
Thus we see that, when the triode an anode voltage is applied to 100 V, the current can pass through only if the grid voltage has values higher than -8 V: in correspondence with this value the current can no longer cross the triode, evidently because the grid is sufficiently negative to repel all electrons emitted by the cathode, neutralizing the attraction exerted on them by the anode.
Under these conditions, it is said that the triode is interdicted and therefore the grid voltage which is reduced to zero, the anode current is said VOLTAGE OF INHIBITING GRID and is indicated by Vgi.
The cutoff voltage is different depending on the anode voltage applied to the triode: indeed, increasing the anode voltage also increases the attraction that the anode exerts on the electrons and thus the grid must become more negative to be able to neutralize this attraction, and to prevent the electrons to reach the anode.
In fig. 5 It is precisely sees that, the greater the anode voltage marked on each characteristic, the more negative is the grid voltage relative to the point where the characteristic, meets the horizontal axis.
The anodic and mutual considered characteristics are also called STATIC, because each of them indicates how varied the anode when current is made to vary only one of the two voltages from which depends on this current, while the other, voltage is maintained at a constant value.
It is seen, indeed, that each anodic characteristic is obtained for a given grid voltage, which remains constant at the variation of the anode voltage and the anode current, while each mutual characteristic is obtained for a given anode voltage, which remains constant to vary the grid voltage and the anode current.
When the triode is operated in its normal working circuit, instead simultaneously vary both the grid voltage is the anode voltage, over, naturally, the anodic current; to know the behavior of the triode in these operating conditions using another feature, which we will be discussed considering the use circuit of the triode.
Circuit of use of the triode
In fig. 6-It is shown in the circuit diagram in which is used the triode, using its property of allowing the anode current control by means of the grid voltage.
In order to understand the operation of the triode it is necessary to distinguish in its use circuit and the GRID CIRCUIT CIRCUIT ANODIC.
The grid circuit is drawn with solid line in Fig. 6-b, from which one can see that includes two generators connected in series between the grid and the cathode.
The AC voltage generator provides the voltage which is made to vary the anode current; in order to avoid that during the positive half-waves of this voltage the grid becomes positive, in series with the generator arises the battery GRID Bg, which it is connected so as to make the negative grid relative to the cathode.
The DC voltage supplied by the battery grid is said bias voltage and has a value such that the grid can not become positive even when the AC voltage reaches its maximum positive value, as we shall see later: thereby no current circulates in the grid circuit.
The plate circuit, drawn with a continuous line in fig. 6-c, It includes Ba anodic battery having in series the anode resistor Ra, which is also called load resistor; these two elements are connected in series between the anode and the cathode of the triode.
The anode battery provides the anode current, whose intensity varies when it is made to vary the grid voltage; the load resistor serves to vary the anode voltage as a result of variations of the anode current.
Indeed, the anodic current flowing through the resistor Ra gives rise to a voltage drop at its terminals: accordingly the anode voltage that occurs between the anode and the cathode of the triode is much less than that provided by the higher anodic battery is the voltage drop that occurs at the ends of the load resistor.
To vary the intensity of the anodic current, obviously it varies the voltage across the load resistor fall and therefore also varies the anode voltage.
By suitably selecting the values of the voltages provided by Bg batteries and Ba, and the value of the resistor Ra, can be done in such a way that the various anode voltage with the same trend of the grid voltage but with greater amplitude.
Basically, between the anode and cathode of the triode you can be obtained a voltage which is similar to that applied to the grid but that has a greater value; therefore it says that the triode gives rise to a VOLTAGE AMPLIFICATION.
Given the great importance of voltage amplification, should see in more detail how the triode in this its typical application.
For this purpose it is appropriate to first consider the circuit of the triode when no signal is applied to the grid, and then you have only the direct current voltage supplied by the batteries and Bg Ba: in this case it says that the triode is in rest conditions and the scheme of its circuit is represented without the AC voltage generator, as seen in Fig. 7.
In this figure it is assumed that the bias voltage, also called VOLTAGE TO REST GRID (which is denoted by V g0), has the value of - 4V and that the voltage Vb supplied by the battery anode has the value of 250 V.
In the anode circuit it circulates the anode current of rest (which indicates IA0): according to the conventional sense, this current starts from the positive pole of the battery Ba, through the load resistor 25 kΩ, then the triode in series to it and returns to the negative pole of the battery Ba.
Because of the voltage drop which occurs at the ends of the load resistor, the anode voltage of rest (which indicates VA0) It is less than the voltage Vb supplied by the battery anodic.
To determine the current value of IA0 and VA0 voltage can not use Ohm's law since the triode, as the diode, He does not obey this law: It is therefore adopts a graphical method, resorting to the anodic characteristics of the triode and proceeding in the following way.
First, they consider two extreme cases, ie the case in which the anode voltage Va would have the same value of 250 V of voltage Vb and the case in which the same voltage Va would have a value of zero.
The voltage Va would have the value of 250 V if the triode he was interdiction, as it is supposed, in Figure. 8-a, in which it is shown the anode current equal to zero.
In this case, indeed, not circulating current, there would not be any voltage drop at the terminals of the load resistor and the entire voltage Vb would be applied between the anode and cathode of the triode.
The triode would thus be in the conditions indicated by point A in Figure. 8-c, which correspond precisely an anode current Ia = 0 and an anode voltage Va = 250 V.
The anode voltage Va would, instead, the value zero in the case in which the triode place was short-circuited by a conductor externally connected between its anode and its cathode, as seen in Fig. 8-b.
In this case the load resistor is connected directly across the battery Ba and therefore the current flowing through it can be calculated using Ohm's Law, dividing the voltage Vb of 250 V for the value of 25 kΩ load resistor and obtaining 250 : 25 = 10 mA.
The triode would now be in the conditions indicated by the point B of fig. 8-c, which correspond precisely an anode current Ia = 10 mA and an anode voltage Va = 0.
When the triode is neither interdiction nor short, but it is located in the conditions of fig. 7, its anode current must have a value between those extremes of 0 mA and 10 mA and its anode voltage must also have a value between those extremes of 0 You see 250 V.
To find these values unite the points A and B with a straight line, as seen in Fig. 8-c, and it considers the point, indicated with P0 in Figure, in which this straight line intersects the anodic characteristic relative to the grid voltage Vg = -4 V, and that in fact the rest of the grid voltage, as shown in Fig. 7.
The straight line and said RETTA LOAD, while the point Po and said OPERATING POINT of the triode, as it indicates the conditions under which pipe works.
Indeed, in correspondence to the point Po can be read on the vertical axis the value of the anodic current of rest, which is Iao = 5 mA, while the horizontal axis reads the value of the anode voltage of rest, which is Vao = 125 V.
To find a confirmation of the accuracy of these results is sufficient to observe that the voltage drop produced by the Iao current to the load resistor, added to the anode voltage Vao, It must be equal to the voltage Vb supplied by the battery anodic.
Since the voltage drop across the load resistor is 5 x 25 = 125 V and also because the anode voltage has the value of 125 V, the sum of these two voltages and precisely equal to the value of 250 V the voltage Vb.
Further confirmation of the accuracy of this way of proceeding will forward with other examples relating to the same load line.
Established so all the values of the quantities relating to the triode in rest conditions, you can move on to consider how to change these values when the grid voltage is made to vary by means of an alternating voltage generator in series with the battery grid, as seen in Fig. 9-a: the alternating voltage supplied by this generator represents the signal to be amplified.
The grid voltage Vg (uppercase) present between the grid and the cathode is now equal to the sum of the voltage continues vg0 polarization and the AC voltage which is denoted by vg (lowercase).
The voltage Vg is therefore called VOLTAGE TOTAL grid, vg while the voltage and the bias voltage are respectively called COMPONENT COMPONENT CONTINUE ALTERNATE and the grid voltage.
Assuming that the AC component has a sinusoidal shape and a maximum value of 2 V, It can represent it as is done in Fig. 9-b for two complete cycles.
The DC component can be graphically represented by a straight line parallel to the horizontal axis (fig. 9-c), as consistently maintained over time the value of -4 V; since this value is negative, and the straight line was plotted below the horizontal axis.
To know which value assumes the grid voltage Vg at a given moment, just add up the values assumed in this same instant by the alternating components and continues.
In particular, the instant in which the alternating component reaches the maximum positive value of +2 V, the total voltage of the grid is equal to -2 V.
In fact we can say, quite simply, that the two volts positive of the AC component neutralize two of the four volts negative of the DC component, for which the grid are applied only two volts negative.
instead, the instant in which the alternating component reaches the maximum negative value of -2 V, these two negative volts are added to the four volt negative of the DC component, for which the grid are applied to a total of six volts negative.
By performing the same amount in several other instants, one can determine the trend of the total of the grid voltage, which is as shown in fig. 9-d; since this tension has always negative values, the curve that represents and drawn below the horizontal axis.
It appears evident that the total voltage has the same trend of the AC component, But with the difference that, while the alternating component varies 2 V in more and less than the value zero, the total voltage varies 2 V in more and less than -4 V to the value of the DC component (dotted line of Fig. 9-d).
The total voltage of the grid can thus be considered as an alternating voltage superimposed on a DC voltage: because of that, the alternating component can never make positive the grid, and as you mentioned earlier.
In this regard and well to observe that the bias voltage does not simply aim to avoid that the positive signal render the grid, but essentially it serves to operate the triode in the desired conditions: later Vedrà, indeed, that in some cases the bias voltage may have a value such that the grid becomes positive during the operation of the triode.
After seeing how varies the grid voltage, It can consider what happens in the anode of the triode circuit, in correspondence to the two extreme values of - 2V and - 6V assumed by said voltage.
For this purpose, it still uses the anodic characteristics of the triode and the load line drawn on them.
When the grid voltage assumes the value of - 2V, the point representing the conditions of the triode operation must be located at the intersection of the load line with the characteristic relative to the voltage Vg = -2 V: in the diagram of fig. 10-at this point it has been indicated by P '.
In correspondence to this point, the anodic current is Ia = 6 mA and the anode voltage is Va = 100 V.
Because the grid has become less negative, -4 passing from V (fig. 8-c) a —2V (fig. 10-a), the anodic current is increased, passing the 5 mA and 6 mA.
It is increased the current, It is also increased to the voltage drop of the load resistor and consequently the anode voltage is decreased, who went from 125 V a 100 V.
Since the voltage Vb supplied by the battery is always anodic 250 V, having been reduced to 100 V, the anode voltage Va should be increased to 150 V the voltage across the load resistor fall: multiplying the value of 25 This KΩ resistor for the current 6 mA, You are precisely obtained 25 X 6 = 150 V.
In the diagram in fig. 10-They are shown in the values of all the variables that affect the operation of the triode in these conditions.
To see in what conditions does the triode, when the grid voltage assumes the value of - 6 V, You can refer to Figure. 10-b.
In the diagram of this figure it is seen that the operating point, indicated with P”, It is now located at the intersection of the load line with the characteristic relative to the voltage Vg = -6 V: at this point the anode current is Ia = 4 mA and the anode voltage is Va = 150 V.
still Comparing these values with those relating to the triode in rest conditions (fig. 8-c), it is seen that the anodic current is decreased, passing the 5 mA and 4 mA, because the grid has become more negative, going from - 4V to - 6 V.
With the diminishing current, is also decreased to the voltage drop of the load resistor and as a result has increased the anode voltage, who went from 125 V a 150 V.
Since the voltage Vb supplied by the battery is still anodic 250 V, It is increased to 150 V, the anode voltage Va should be decreased to 100 V the voltage across the load resistor fall: multiplying the value of 25 This kΩ resistor for the current 4 mA, You are precisely obtained 25 X 4 = 100 V.
The values of all the variables that affect the operation of the triode in these conditions were reported in the diagram in fig. 10-b.
In fig. 10 They are determined extreme values assumed by the anode current and the anode voltage in correspondence to the extreme values of - 2V and -6 V of the grid voltage, but with the same procedure could determine other values of these magnitudes between those extremes.
The load on a straight line connecting the anodic characteristics therefore allows to see how it behaves when the triode, during its operation, vary the grid voltage simultaneously, the anode current and the anode voltage: the load line is therefore also called operating characteristic.
By determining different values of anode current and the anode voltage and bringing them on a diagram you can be plotted the curves that show the evolution over time of these magnitudes.
As seen in Fig. 11, these curves have sinusoidal shape like the one that represents the grid voltage in fig. 9-d and that has again been shown in fig. 11-a, to have a complete view of all the variables that affect the operation of the triode.
You can see how, while the grid voltage varies between -2V values and - 6 V, the anodic current (fig. 11-b) varies between values 6 mA and 4 mA and the anode voltage (fig. 11-c) varies between values 100 You see 150 V.
E’ Importantly,, as the total voltage V grid varies 2 V in more and less than the value of - 4V of vg0 rest of the grid voltage, so also the anode current Ia varies 1 mA in more and less than the value of 5 mA of the anode current of rest IA0 and similarly the anode voltage Va varies 25 V in more and less than the value of 125 V of anodic voltage VA0 rest.
Consequently, Having considered the total voltage Vg of the grid formed by an alternating component superimposed on a DC component vg vg0, you can also consider the anode current Ia as a total anode current Ia formed by an alternating component superimposed on a DC component IA0; similarly, can be considered the anode voltage Va as a total anode voltage formed by an alternating component superimposed on a DC component goes VA0.
In the case of grid voltage it is evident that the two components actually exist, because each of them is supplied by a generator inserted in the grid circuit.
There can also convince that the two components of the anode current and the anode voltage actually exist, Whereas it is possible to obtain separate.
Eg, measuring the total anode current or the total anode voltage by means of a moving coil instrument used as an ammeter or voltmeter as, you get the value of only the DC component of these quantities, because the moving coil instruments do not indicate AC.
instead, if the total anode current is sent into the primary winding of a transformer, to the secondary winding is obtained a voltage which depends only on the alternating component of this current, because the processors do not work with direct current.
The two components can also be separated by means of a capacitor, that passes only the alternating current, stopping the continuous.
It will be seen that very often used precisely the use of a capacitor or a transformer to obtain the alternating component of the total anode voltage, which constitutes the amplified signal.
This signal has the same trend as that applied to the triode grid, but it has greater amplitude: in effect, these two signals differ not only in the amplitude, but also for another reason.
This is evident by examining the fig. 12. in which shows alternating components of the grid voltage and the anode voltage derived from the diagrams in Figs. 11.
In fig. 12 you immediately notice that the positive half-waves of the signal applied to the grid, semiwaves drawn with more marked line, correspond to the negative half-waves of the amplified signal, semiwaves also designed with more marked line; the same thing also happens to the half-waves drawn with thinner line.
In this case it says that the two voltages are in opposition, because when one reaches the maximum positive value, the other reaches the maximum negative value and vice versa, as is seen in Fig precisely. 12.
It 'good to remember that the amplified signal obtained from a triode is in opposition with the signal applied to the grid.
To know how much is amplified the signal applied to the grid of the triode, just divide the maximum value of the alternating component of the anode voltage for the maximum value of the alternating component of the grid voltage: , resulting in the VOLTAGE GAIN, which is denoted by G.
In the case of fig. 12, since the maximum values of the alternating components of the anode voltage and the grid voltage are respectively 25 V e 2 V, the voltage gain is G = 25 : 2 = 12,5; this means that the signal applied to the grid of the triode is amplified 12,5 times.
differentials of the triode Parameters
In all the considerations set out so far it has always made reference to the triode ECC82 built by Philips; considering other types of triode you would get different results, while running these tubes under the same conditions of the triode ECC82.
Eg, with the same variation of the grid voltage, you might get a variation of the anode current greater or lesser, or, with the same variation of the anode current, you might get a variation of greater or lesser anode voltage; Finally, it could also be different gain.
Each type of triode is therefore characterized by means of suitable sizes adapted to indicate its performance when operating under certain conditions, ie with a given grid bias voltage, with an anodic current date of rest and with a given anodic resting tension.
For each type of triode will indicate three sizes, He said PARMETRI DIFFERENTIAL the triode, each of which is defined according to the variation of two of the three electrical quantities relating to the same triode (grid voltage, anode current and anode voltage), while the third is considered constant.
A first parameter is said SLOPE of the triode (symbol S), or TRANSCONDUTTAZA, or even MUTUAL CONDUTTAZA; this parameter indicates how much varies the anode current when the grid voltage is made to vary from 1 V, while the anode voltage remains constant.
Expressing in milliamperes the variation of the anode current corresponding to the variation of 1 V of the grid voltage, the slope is expressed in milliampere per volt (symbol mA / V).
The slope allows to know the attitude of a triode to control the anode current by means of the grid voltage: between two types of triodes having different gradients, It will be better than the one with higher slope.
The slope of a triode can be determined experimentally with the circuit shown in Fig. 13-a.
For this purpose, regulates the grid voltage so that the triode functions in the desired conditions, and then reads the milliammeter the value of the anodic current flowing through the tube.
It then acts on the potentiometer slider doing vary from 1 V the grid voltage and then again on milliammeter reads the value of the anodic current: the difference between this value and the previously read clearly indicates how much the anode current is varied by varying the 1 V the grid voltage and thus gives directly the slope of the tube.
During this test the anode voltage does not vary, because there is the anode resistance that produces voltage variations; the small internal resistance of the milliammeter inserted in the anode circuit may be considered negligible.
As it is seen previously, the anode current of a triode can also be controlled by means of the anode voltage, leaving the grid voltage unchanged.
The second differential parameter that indicates for a triode precisely allows to know the attitude of the tube to control the anode current by means of the anode voltage.
This parameter is said internal differential resistance of the triode (symbol ra): it indicates than the anode voltage must vary in order to obtain the variation of 1 mA of the anode current, while the grid voltage remains constant.
Expressing in volts the variation of the anode voltage required to vary the 1 the anode current mA, the internal resistance is expressed in chiloohm.
Even the internal differential resistance can be determined experimentally, by means of the circuit shown in fig. 13-b.
For this purpose, it carries the triode to function in the desired conditions, by suitably adjusting its anode voltage, whose value is read on the voltmeter.
It then varies the anode voltage until the anode current has changed by 1 mA and at this point again on the voltmeter reads the value of the anode voltage: the difference between this value and the previously read clearly indicates by how much is due to vary the anode voltage to vary from 1 the anode current mA and then directly from the differential internal resistance of the triode.
Previously it is also seen that the grid controls more effective anode the anode current, being closer to the cathode: the third differential parameter that indicates for a triode allows to know precisely what the grid is more effective in controlling the anode current of the anode.
This parameter is said amplification coefficient of the triode (simbolo m): it indicates how much you must vary the anode voltage to maintain the anode current constant despite the variation of 1 V of the grid voltage.
To get a better idea of this parameter, should consider as you proceed to determine experimentally.
For this purpose we use the circuit shown in Fig. 13-c: after adjusting the grid voltage and the anode voltage in such a way as to bring the triode to function in the desired conditions, They read the values of anode current and the anode voltage.
At this point varies 1 V the grid voltage and, in this way, It will also vary a certain amount the anode current; It varies then the anode voltage up to bring the anodic current to the initial value: the variation of the anode voltage necessary to obtain this directly indicates the amplification coefficient of the triode.
Unlike the other two parameters, the amplification coefficient is simply expressed with a number.
Since the three parameters of the triode differential cover all of the anode current control by the grid voltage or the anode voltage, You can assume that there is a relation that ties together the three parameters themselves: indeed, it was found that the amplification coefficient is equal to the product of the slope for the differential internal resistance.
The differential parameters of a triode can also be determined graphically by means of the mutual characteristics and anode of the triode same.
In fig. 14 display, eg, as you can be seen the slope of a triode type ECC83 from its mutual characteristics.
It was assumed that the triode it has in the rest state indicated by point P0, ie, with the bias voltage vg0 = - 2 V, with an anode voltage of VA0 rest = 250 V and anode current of rest IA0 = 1,2 mA.
Since the slope is given by the variation of the anode current consequent to the variation of 1 V of the grid voltage, while the anode voltage remains constant, considering the values of the anode current corresponding to the values of -1,5 You see -2,5 V of the grid voltage.
The change in 1 V of the grid voltage is thus obtained by increasing and decreasing of 0,5 V the value of - 2V of the bias voltage.
From the points of the horizontal axis on which the values of -1.5 V and -2.5 V are plotted two vertical lines are marked up to meet at the points P’ and P” mutual characteristic relative to the anode voltage of 250 V, on which there is also the point P0.
In this way the anode voltage is not varied because the three points corresponds to the same value of this voltage, while it varies the anode current: can be seen in fact that, at the points P’ and P”, the current assumes the values respectively 2,2 mA and 0,5 mA.
The difference between these two values (2,2 — 0,5 = 1,7), indicating to what the anode current is varied to vary from 1 V of the grid voltage, directly from the slope of the triode: it can be concluded that the triode considered, in the rest state indicated by the point Po, It has a slope of 1,7 mA / W.
The slope of the triode can also be derived from the anodic characteristics of the hose, proceeding as seen in Fig. 15.
In this case, the point P0, which indicates the conditions of the triode rest, is located on the anodic characteristic relative to the grid voltage Vg = - 2 V, in correspondence to the anode voltage of VA0 rest = 250 V and the anode current of rest IA0 = 1,2 mA.
Since Po point draw a vertical line until it meets at the points P’ and P” the anodic characteristics relating respectively to -1.5 V grid voltages and -2,5 V, so that also in this case the variation of 1 V of the grid voltage is obtained by increasing and decreasing of 0,5 V the value of - 2 Of the bias voltage V.
So doing not the anode voltage is varied because the three points corresponds still the same value of 250 V of this voltage, while it varies the anode current.
Also in this case, indeed, which is located in correspondence to the points P’ e .P” the current assumes the values respectively 2,2 mA and 0,5 mA, from whose difference is still gets the same value of 1.7mA / V slope.
This second method to derive the slope of a triode can be taken if you do not have mutual tracked feature for the desired value of the anode voltage: indeed, on the anode characteristics, you can run the graphical determination of the slope for any value of the anode voltage, which is read directly on the horizontal axis.
From the same anodic characteristics, it can also derive the differential internal resistance of the same triode, proceeding as seen in Fig. 16.
Even in this case the point P0 indicates the conditions of the triode rest, which are the same as those already considered above.
Since the differential internal resistance is given by the variation of the anode voltage necessary to obtain the variation of 1 mA of the anode current while the grid voltage remains constant we consider the values that must assume the anode voltage to vary the anode current between the values of 1,7 mA and 0,7 mA, ie to make it increase and decrease of 0,5 mA with respect to the value of 1,2 mA of the anode current of rest.
From the vertical axis points on which the values are marked by 1,7 mA and 0,7 mA two horizontal lines are drawn up to meet at the points P’ and P” the anodic characteristic relative to the grid voltage of - 2 V, on which there is also the point P0.
In this way it has not changed because the grid voltage to the three points corresponds to the same value of this voltage, while it varies the anode voltage, which passes from the value of 277 V, in correspondence to the point P ', the value of 218 V, in correspondence to the point P”
The difference between these two values (211 — 218 = 59), indicating to what is the anode voltage must vary in order to vary the 1 the anode current mA, directly from the internal differential resistance de! triode: it can be concluded that the triode consider it, in the rest state indicated by point P0, It has an internal differential resistance 59 KΩ.
By means of the anodic characteristics of the triode it can also derive its amplification coefficient, proceeding as seen in Fig. 17, in which the point P0 still indicates the same resting conditions considered in the previous cases.
Since the amplification coefficient is given by the variation of the anode voltage necessary to maintain the anode current constant to vary from 1 V of the grid voltage, They consider the values that must assume the anode voltage so that the anodic current unaltered retains its value 1,2 mA when the grid voltage varies 0,5 V -2 V with respect to the value of the bias voltage, passing from -1.5 V to - 2,5 V.
From the point P0 will trace a horizontal line until meeting at the points P’ and P” the anodic characteristics relating to the grid voltage and -1,5V -2,5 V.
In this way the anode current is not varied because the three points corresponds to the same value of 1,2 mA of this current, while the anode voltage must vary to compensate for the variation of the grid voltage: It can be seen in fact that the anode voltage assumes the values of 200 You see 300 V indicated in correspondence respectively to the points P’ and P”.
The difference between these two values (300 – 200 = 100), indicating to what you have to vary the anode voltage to maintain constant the current to vary from 1 V of the grid voltage, directly from the amplification coefficient of the triode: it can be concluded that the triode considered, in the rest state indicated by point P0, It has an amplification coefficient equal to 100.
At this point can occur if the amplification coefficient thus obtained is equal to the product of the other two differential parameters of certain triode previously.
Since the slope has the 1.7mA / V value and the internal differential resistance has the value of 59 kΩ, you get: 1,7 X 59 = 100,3.
The result is certainly acceptable, Whereas the graphic methods will always commit minor inaccuracies, deriving from the difficulty to read with accuracy the values plotted on the axes of the diagrams; for this same reason, in some cases you may even encounter some; small difference between the determined parameters graphically and those, obtained experimentally, which they are indicated by the manufacturers of the tubes.
The data relating to the triode considered can be indicated in the following way:
anodic resting tension: Va0 = 250 V
anodic current of rest: Ia0 = 1,2 mA
bias voltage: Vg0 = -2 V
slope: S = 1,7mA / V
internal differential resistance: out = 59 kΩ
coefficient of amplification: m = 100.
Note that along with the three differential parameters the data relating to the operating point must always indicate also (anode voltage, anodic current and bias voltage) because the differential parameters of a triode are different depending on the operating point of the tube, due to the fact that the characteristics are not straight but curved.
To confirm this, It would be enough to choose on the anode characteristics and mutual another point P0 for it and repeat the same operations as shown in Fig. 14, in Figure. 15, in Figure. 16 and in Figure. 17: It would be seen that so that the values of the differential parameters are actually different.
Now the graphs accompanying the characteristics of the valve are not so incomprehensible. For now I'll stop here, the article came long enough.
I start to prepare for the next article on more than three elements valves.