Before starting the Astables, Monostable, Bistable transistor-based multivibrators to better understand. We will treat for the first time in this lesson that will be dedicated to a type of electronic tube, the TRIODE that can be compared to a tap because it allows to vary the current that passes through it like a tap to adjust the arrival of water.
1. - CONSTITUTION OF THE TRIODE
The triode is thus called because it comprises three electrodes : the anode, the cathode ; the tube is provided with a third electrode disposed between the first two. This one is called GRILLE because, in the first triode realized in 1907 by the American LEE DE FOREST (1873 - 1961), it was precisely constituted by a metal grid.
In the currently used triodes (Figure 1-a), the grid consists of a thin wire wound spirally around the cathode, without touching it ; around the grid, we find the anode that was drawn sectioned so that the grid is visible in the figure.
The gate, like the cathod and the anode, is connected to a pin for its connection to the circuit outside the tube.
The triode shown in Figure 1-a is very simplified ; the filament and the small pins are not represented, or the supports that hold the different electrodes in their positions : it has been shown that the essential elements of the tube which also appear on the graphic symbol of Figure 1-b represent the triode in the electrical diagrams.
As in the diode (without the Grid), the anode collects the electrons emitted by the cathode, electrons that are not impeded by the gate, because they can pass easily through its turns.
The grid can however influence the movement of the electrons going towards the anode if it is at a potential different from that of the cathode : in this way, the grid can be used to vary the current which passes through the triode, as will be seen in the next paragraph.
The effect of the gate on the current passing through a triode can be highlighted by plotting the characteristic curves of the tube.
For this, one proceeds by applying a voltage between the anode and the cathode (as for a tube diode) and by measuring for different values of this voltage the corresponding current which passes through the tube: for the triode also, the voltage and the current respectively constitute the anode voltage and the anode current.
To carry out the measurements, the same circuit is used as that already used for the diode and whose diagram is shown in Figure 2-a. In this diagram, we see that the gate is connected to the cathode so that it is at the same electrical potential as this electrode, this in order to study first how the triode behaves when the voltage between these two electrodes (called grid voltage) and that is indicated by Vg is equal to zero.
Under these conditions, the triode is equivalent to a diode, because the gate does not influence the anodic current : a characteristic curve is obtained (Figure 2-b) whose appearance is identical to that of the characteristic curve of the diode.
In this case too, the curve has been plotted with a solid line first line to indicate the values of the voltage and current for which the maximum anodic dissipation is not exceeded.
To see the effect of the gate on the anode current, it is necessary to bring this electrode to a potential different from that of the cathode : this can be achieved by connecting, for example, a battery between the two electrodes, as in Figure 3-a.
A 2V battery was used, connecting its positive pole to the cathode and its negative pole to the gate : thus the gate is at a potential 2V lower than that of the cathode and the gate voltage is now equal to - 2 V.
The behavior of the tube with a negative gate voltage is studied because, in most applications and especially in radio receivers, triodes are used under these conditions.
By applying anodic voltage again to the triode and again measuring the corresponding anode current for different values of this voltage, the characteristic (Figure 3-b), which is different from that obtained, can be plotted with zero gate voltage. As can be seen in Figure 3-b, we distinguish the characteristics by marking on each of them, the gate voltage with which it was obtained.
The characteristic obtained with a negative gate voltage is to the right of that obtained with a zero gate voltage : this implies that, for a given anode voltage applied to the triode, the anode current is even lower than the voltage of grid is more negative.
Figure 3-b shows, for example, that when the triode operates under the conditions indicated by the point A, that is to say with an anode voltage of 100 V and a gate voltage of 0 V, the current anodic is 12 mA ; on the contrary, the triode operates under the conditions indicated by point B, that is to say with an anode voltage always equal to 100 V, but a gate voltage of -2 V, the anode current is barely 6 mA.
This example clearly demonstrates that, in a triode, the anode current depends not only on the anode voltage, as in the diode, but also on the gate voltage : in fact, by not modifying the anode voltage, it was possible to vary the anode current by varying the gate voltage.
This is because the electrons emitted by the cathode now exert not only the attractive force of the anode, but also a repulsive force on the part of the negative grid : therefore, only the most Fast emitted electrons can pass through the gate and reach the anode, thus constituting the anode current.
In fact, in the triode as in the diode, a cloud of electrons forms around the cathode which, with its negative spatial charge, concurs with the grid to hinder the passage of electrons towards the anode.
It should be noted, however, that while it is not possible to control the repulsive action that the electron cloud exerts on the electrons, this is possible for the gate, because it suffices to vary its voltage.
The two characteristics of Figure 3-b, highlight a much more important fact.
As we have seen, the anode current of 12 mA at 6 mA can be reduced by increasing the gate voltage from 0 V to -2 V and leaving unchanged the 100 V value of the anode voltage.
In the same way, the anode current can be reduced by varying the anode voltage and leaving the value of 0 V of the gate voltage unchanged.
In this case, the triode operates under the conditions indicated by the point C, which corresponds precisely to an anode current of 6 mA and a gate voltage of 0 V : in Figure 3-b, it can be seen that this can be achieved by reducing the anodic voltage from 100 V to 60 V.
These remarks allow us to deduce that, in order to reduce the anode current from 12 mA to 6 mA, a variation of 2 V (from 0 V to -2 V) is sufficient if the gate voltage is affected, whereas if it acts on the anode voltage, it requires a variation of at least 40 V (from 100 V to 60 V), twenty times higher than the previous one.
This is due to the small distance between the gate and the cathode and allows the gate to act on the anode current more efficiently than the anode further away from the cathode.
It can therefore be concluded that the gate of a triode is able to control the anode current by means of variations in its voltage.
It should also be noted that the control of the anode current by the gate occurs without energy expenditure : indeed, since the gate is generally negative with respect to the cathode, no electron can be carried on it and give rise to a current in the outdoor circuit.
The 2 V battery, which in Figure 3-a is connected between the gate and the cathode, thus does not deliver any current and does not supply energy to the circuit.
In order to be able to vary the anode current between fairly wide limits, the gate voltage is brought to values below -2 V and, to know the behavior of the triode under these conditions, other characteristics are recorded (for example, for grid voltages of -4 V, -6 V, etc ...).
To have these different voltages, we can adopt the system used for the anode voltage, by connecting a potentiometer to the battery that provides the gate voltage, as seen in Figure 4-a : the fader of the potentiometer moves up to to obtain the desired voltage, whose value is read on the voltmeter connected between the cathode and the gate.
The characteristics obtained for the different values of the gate voltage are shown in the diagram of Figure 4-b, which makes it possible to see that each characteristic is all the more displaced to the right as the grid voltage with which it has been obtained is more negative.
The line in dots and points which crosses the characteristics delimits the zone of the diagram, located below her, where the voltage and the anodic current have values which do not make it possible to exceed the maximum anodic dissipation.
The set of characteristics of Figure 4-b constitutes a family of characteristic curves of the triode ; these characteristics are called anodic because they indicate how the anode current varies when the anode voltage varies, and that the gate voltage has a determined value.
Since the gate serves to control the current flowing through the triode, it is interesting to see directly how this current varies as a result of the variation of the gate voltage, when the anode voltage has a certain determined value.
For this, we determine the family of mutual characteristics of the triode, using the same circuit as that of Figure 4-a, but proceeding in a different way.
In this case, in fact, the anode voltage is left unchanged at a determined value, for example 100 V, and a negative voltage is applied to the gate by measuring the corresponding anode current for different values of this voltage ; by plotting the values of the gate voltage and the anode current on the diagram, the mutual characteristic relative to the anodic voltage of 100 V can be plotted.
The anode voltage is then increased, for example to 170 V, and a negative voltage is again applied to the gate, again measuring the corresponding anode current for different values thereof ; it is thus possible to draw a second mutual characteristic relating to the anode voltage of 170 V.
In the diagram of Figure 5, four characteristics of the triode studied so far are reported ; these characteristics were obtained for the values of the anode voltage most often used, values which were indicated on each of them.
The values of the anodic current are still reported on the vertical axis as in the case of the anode characteristics, while on the horizontal axis are now reported the values of the gate voltage; since these values are negative, the axis has been drawn to the left of the vertical axis.
Let us now study, for example, the mutual characteristic relative to the anode voltage of 100 V and note that this characteristic meets the horizontal axis at point A, opposite which is indicated the value of -8 V of the gate voltage.
When the triode is in the conditions indicated by point A, its anode voltage is therefore 100 V and its gate voltage of -8 V ; since point A is on the horizontal axis, the anode current is zero.
It can thus be seen that when an anode voltage of 100 V is applied to the triode, the current can only flow through it if the gate voltage has values greater than -8 V : for this value, the current can obviously not cross the triode, because the grid is sufficiently negative to repel all the electrons emitted by the cathode, neutralizing the attraction exerted on them by the anode.
Under these conditions, it is said that the triode is blocked ; the gate voltage which reduces the anode current to zero is therefore referred to as the Blocking Grid Voltage and is indicated by Vgc.
The so-called blocking voltage (cut-off voltage) is different according to the anode voltage applied to the triode : in fact, by increasing the anode voltage, the attraction that the anode exerts on the electrons is also increased. therefore, the grid must become more negative in order to neutralize this attraction and prevent the electrons from reaching the anode.
In Figure 5, it is precisely seen that the higher the anode voltage marked on each characteristic, the greater the relative gate voltage at the point where the characteristic meets the horizontal axis is negative.
The anodic and mutual characteristics studied are also called STATIC, because each of them indicates how the anode current varies when one of the two voltages on which this current depends, while the other voltage is maintained at a constant value.
It has been seen that each anode characteristic is obtained for a determined gate voltage, which remains constant when the anode voltage and the anode current vary while each mutual characteristic is obtained for a given anode voltage, which remains constant when the gate voltage and anode current vary.
When the triode operates in its normal use circuit, the gate voltage as the anode voltage vary on the contrary simultaneously in addition to naturally, the anode current ; in order to know the behavior of the triode in these operating conditions, another characteristic is used, which we will treat when we study the circuit of use of the triode.
In Figure 6-a, we see the circuit diagram in which the triode is used, using its property to allow the control of the anode current by means of the gate voltage.
To understand the operation of the triode, it is necessary to distinguish in this circuit the gate circuit and the anode circuit.
The gate circuit is drawn in solid lines in Figure 6-b, and it can be seen that it comprises two generators in series connected between the gate and the cathode.
The Alternate Voltage generator provides the voltage that varies the anode current ; to avoid that during the positive half of this voltage grid becomes positive, it is in series with this generator grid battery Bg, connected so as to make the negative grid relative to the cathode.
The DC voltage supplied by the gate battery is called the bias voltage, and it has a value such that the gate can not become positive even when the AC voltage reaches its positive maximum value, as we shall see later : no current flows in the gate circuit.
The anodic circuit, drawn in line continues in Figure 6-c, comprises the anode battery Ba with series anode resistance Ra, also called load resistor ; these two elements connected in series are placed between the anode and the cathode of the triode.
The anode battery provides the anode current, the intensity of which varies when the voltage of the gate is varied ; the load resistor serves to vary the anode voltage in accordance with the variations of the anode current.
Indeed, the anode current flowing through the resistor Ra gives rise to a voltage drop at its ends : consequently, the anode voltage obtained between the anode and the cathode of the triode is all the lower than that provided by the battery anodic, that the voltage drop that occurs at the ends of the load resistance is greater.
When the intensity of the anode current varies, the voltage drop at the ends of the charge resistance varies, so obviously as the anode voltage.
By opportunely choosing the values of the voltages provided by the batteries Bg and Ba and the value of the resistor Ra, it is possible to make the anode voltage vary in the same way as the gate voltage but with a greater amplitude.
In short, between the anode and the cathode of the triode, a voltage similar to that applied to the gate but of a higher value can be obtained ; it is said that the triode gives rise to a voltage amplification.
Voltage amplification is of fundamental importance in radio technology, because in the circuits of radio equipment, one often has very small amplitude voltages which must be amplified to be able to use them properly.
In Figure 6, an alternating voltage generator has been indicated for the sake of simplicity, but in practice the voltage applied to the gate will be due to an RF signal obtained, for example, with an antenna, or to a signal BF obtained, with a microphone : between the anode and the cathode of the triode, one will thus obtain a signal HF or BF with an amplitude higher than that which was applied to the grid.
Given the great importance of voltage amplification, it is necessary to see in more detail how the triode works in this typical application.
To do this, let us first study the triode circuit when no signal is applied to the gate and that the DC voltages are supplied by the batteries Bg and Ba : in this case, we say that the triode is in rest condition, and the circuit diagram is shown without the AC voltage generator, as seen in Figure 7.
In this figure, it has been assumed that the bias voltage, also referred to as the gate quiescent voltage (which is indicated by Vgo), has the value of -4 V and that the voltage Vb supplied by the anode battery has the value of 250 V.
In the anode circuit, circulates the anode current of rest (which is indicated by Iao) : according to the conventional sense, this current starts from the positive pole of the battery Ba, crosses the load resistance of 25 kΩ, and therefore also the triode connected to it in series, and returns to the negative pole of the battery Ba.
As a result of the voltage drop at the ends of the load resistor, the anode voltage of rest (which is indicated by Vao) is lower than the voltage Vb supplied by the anode battery.
To determine the value of current Iao, Ohm's law can no longer be used because the triode, like the tube diode, does not obey this law : one thus adopts a graphical method, by resorting to the anodic characteristics of the triode and proceeding as follows.
We first study two extreme cases, that is to say the case where the anode voltage Va would have the same value of 250 V as the voltage Vb, and the case where the voltage Va would be zero.
Voltage Va would be 250 V if the triode was blocked as assumed in Figure 8-a, where the anode current of zero was indicated.
In this case, because the current is not flowing, there would be no voltage drop at the ends of the load resistor, and the entire voltage Vb would be applied between the anode and the cathode of the triode.
The triode would therefore be in the conditions indicated by point A of Figure 8-c, to which correspond an anode current Ia = 0 mA and anode voltage Va = 250 V.
The anode voltage Va would, on the contrary, be zero if the triode was short-circuited by means of a conductor connected externally between its anode and its cathode, as in Figure 8-b.
In this case, the load resistor is connected directly to the ends of the battery Ba and the current that passes through it with Ohm's law can be calculated by dividing the voltage Vb of 250 V by the value of 25 kΩ of the load resistance and 250 / 25 = 10 mA is obtained.
The triode would now be in the conditions indicated by point B of Figure 8-c, to which correspond precisely an anode current Ia = 10 mA and anode voltage Va = 0 V.
When the triode is neither blocked nor short-circuited, but is in the conditions of Figure 7, its anode current must have a value between the extreme values of 0 mA and 10 mA and its Anode voltage must also have a value between the extreme values of 0 V and 250 V.
To find these values, we join the points A and B by a line, as in Figure 8-c and we study the point, indicated by Po in the figure, where this line meets the anode characteristic relative to the voltage of gate Vg = -4 V, which is precisely the gate voltage indicated in Figure 7.
The line is called the charge line, while the point Po is called the operating point of the triode because it indicates the conditions that allow the operation of the tube.
Indeed, opposite the point Po, one can read on the vertical axis the value of the anodic current of rest, which is Iao = 5 mA, while on the horizontal axis, one reads the value of the anodic tension of rest which is Vao = 125 V.
To find a confirmation of the accuracy of these results, it is sufficient to observe that the voltage drop produced by the current Iao at the ends of the load resistor, added to the anode voltage Vao, must be equal to the voltage Vb supplied. by the anode battery.
Since the voltage drop at the ends of the load resistor is 5 x 25 = 125 V and since the anode voltage is also 125 V, the sum of these two voltages is equal to the value of 250 V of the voltage Vb.
Later, with other examples relating to the same load line, we will find confirmations of the correctness of this way of proceeding.
Now that all the values of the quantities relative to the triode have been established in the quiescent conditions, it is possible to examine how these values change when the gate voltage is varied by means of a series connected alternating voltage generator. with the gate battery, as seen in Figure 9-a : the AC voltage supplied by this generator represents the signal to be amplified.
The gate voltage Vg obtained between the gate and the cathode is now equal to the sum of the DC bias voltage Vgo and the alternating voltage indicated by Vg.
The voltage Vg is therefore called the total gate voltage, while the voltage Vg and the bias voltage Vgo are called the AC component and the DC component of the gate voltage.
Assuming that the AC component has a sinusoidal pitch and a maximum value of 2 V, it can be represented as in Figure 9-b for two complete cycles.
The DC component can be represented graphically by means of a line parallel to the horizontal axis (Figure 9-c), because it constantly keeps the value of -4 V ; since this value is negative, the line has been drawn below the horizontal axis.
To know the value taken by the gate voltage Vg at a given instant, it is sufficient to sum the values taken at each instant by the components AC and DC.
In particular, at the moment when the alternating component reaches the maximum positive value of + 2 V, the total gate voltage is equal to - 2 V.
Indeed, it can be said very simply that the two positive volts of the AC component neutralize two of the four negative volts of the DC component ; that is why at the grid are only applied two negative volts.
On the contrary, at the moment when the AC component reaches the maximum negative value of -2 V, these two negative volts are added to the four negative volts of the DC component : a total of six negative volts are therefore applied to the grid.
By making the same sum in various other instants, one can determine the pace of the total voltage of the grid, which is seen in Figure 9-d ; since this voltage always has negative values, the curve which represents it is under the horizontal axis.
It is obvious that the total voltage has the same pace as the AC component but with the difference that, while the AC component varies by 2 V more and less than the zero value, the total voltage varies from 2 V in plus and minus the value of - 4 V of the DC component (dashed line in Figure 9-d).
The total gate voltage can therefore be considered as an alternating voltage superimposed on a DC voltage : consequently, the AC component can never make the grid positive, as has already been said previously.
In this connection, it should be observed that the polarization voltage is not simply intended to prevent the gate from being made positive by the signal, but essentially serves to make the triode operate under the desired conditions : the next lessons, we will see that in some cases, the bias voltage may have a value such that the grid becomes positive during the operation of the triode.
After having seen how the gate voltage varies, we can study what happens in the anode circuit of the triode, opposite the two extreme values of - 2 V and - 6 V taken by this voltage.
For this purpose, the anode characteristics of the triode and the charge line drawn on them are still used.
When the grid voltage is set to - 2 V, the point representing the operating conditions of the triode shall be at the intersection of the load line with the characteristic relating to the voltage Vg = - 2 V : on the diagram of Figure 10-a, this point was indicated by P'.
The anode current corresponding to this point is Ia = 6 mA and the anode voltage is Va = 100 V.
Since the gate became less negative, from -4 V (Figure 8-c) to -2 V (Figure 10-a), the anode current is increased from 5 mA to 6 mA.
As the current increases, the voltage drop at the ends of the load resistance also increases, and therefore the anode voltage decreases from 125 V to 100 V.
Since the voltage Vb supplied by the anode battery is still 250 V, the anode voltage Va having been reduced to 100 V, the voltage drop at the ends of the load resistance must have increased to 150 V : by multiplying the value of 25 kΩ of this resistance by the current of 6 mA, we obtain just 25 x 6 = 150 V.
In the diagram of Figure 10-a, the values of all the magnitudes which participate under these conditions in the operation of the triode have been reported.
To see under which conditions the triode operates when the grid voltage is set to - 6V, one can refer to Figure 10-b
In the diagram of this figure, it can be seen that the operating point, indicated by P', is now at the intersection of the load line of the characteristic relating to the voltage Vg = -6 V : the anode current corresponding to this point is Ia = 4 mA and the anode voltage is Va = 150 V.
Comparing these values with the values relative to the triode in the rest conditions (Figure 8-c), we see that the anode current decreased from 5 mA to 4 mA because the grid became more negative, passing from - 4 V to - 6 V.
As the current decreased, the voltage drop at the ends of the load resistor also decreased, and as a result the anode voltage increased from 125 V to 150 V.
Since the voltage Vb supplied by the anode battery is still 250 V, and the anode voltage has increased to 150 V, the voltage drop at the ends of the load resistor must have decreased ; by multiplying the value of 25 kΩ of this resistance by the current of 4 mA, we obtain just 25 x 4 = 100 V.
The values of all the quantities that participate in the operation of the triode under these conditions have been reported in the diagram of Figure 10-b.
In Figure 10, the extreme values taken by the anode current and the anode voltage for the extreme values of -2 V and -6 V of the gate voltage have been determined ; with the same method, it would be possible to determine other values of the quantities between these extremes.
The load line drawn on the anode characteristics makes it possible to see how the triode behaves when, during its operation, the gate voltage, the anode current and the anode voltage simultaneously vary ; the load line is therefore also called the operating characteristic.
By determining different values of the anode current and the anode voltage and plotting them on a diagram, the curves which show the shape of these quantities can be drawn.
As can be seen in Figure 11, these curves have a sinusoidal shape like that which represents the gate voltage in Figure 9-d and that is shown in Figure 11-a, to have a complete view of all the magnitudes involved in the operation of the triode.
It can thus be seen that while the gate voltage varies between the values of -2 V and -6 V, the anode current (Figure 11-b) varies between the values of 6 mA and 4 mA and the anode voltage (Figure 11-c) varies between values of 100 V and 150 V.
It is important to note that, just as the total gate voltage Vg varies by 2 V more and less than the value of - 4 V of the gate voltage Vgo, so the anode current Ia varies also 1 mA more and less than the value of 5 mA of the anode quiescent current Iao, and likewise the anode voltage Va varies by 25 V more and less with respect to the value of 125 V of the anode voltage Vao rest.
Therefore, now that the total gate voltage Vg has been considered to be formed of an alternating component Vg superimposed on a DC component Vgo, the anode current Ia can be considered as being a total anode current formed of an alternative component Ia superimposed on a DC component Iao ; in the same way, the anode voltage can be considered as being a total anode voltage Va formed of an alternating component Va superimposed on a DC component Vao.
In the case of the gate voltage, it is obvious that the two components actually exist, since each of them is provided by a generator inserted into the gate circuit.
One can be convinced of the existence of the two components of the anode current and the anode voltage, seeing that it is possible to obtain them separately.
For example, by measuring the total anode current or the total anode voltage by means of a voice coil apparatus used as a milliammeter or as a voltmeter, the value of the DC component is obtained only because the voice coil instruments give no indication in alternating current.
On the contrary, if the total anode current is sent into the primary of a transformer, there is obtained at the ends of the secondary a voltage that depends only on the AC component of this current, because the transformers do not operate in direct current.
We can separate the two components by means of a capacitor, which allows only the alternating current, and which stops the direct current.
In previous lessons, we have seen that it is very often used the use of a capacitor or a transformer to obtain the AC component of the total anode voltage for example, which is the amplified signal.
This signal has the same pace as that applied to the gate of the triode, but its amplitude is greater. These two signals differ elsewhere on another point.
This can be seen very clearly by examining Figure 12, which shows the alternative components of the gate voltage and the anode voltage obtained from the diagrams of Figure 11.
In Figure 12, it can be seen immediately that at the positive half-waves of the signal applied to the grid, alternations designated by a more marked line, correspond to the negative half-waves of the amplified signal, alternations also designated by a more marked line; the same thing also happens for alternations designated by a finer line.
In this case, it is said that the two voltages are in opposition, because when one reaches the positive maximum value, the other reaches the negative maximum value and vice versa, as seen in Figure 12.
It should be remembered that the amplified signal obtained from a triode is in phase opposition with the signal applied to the gate, because this fact is very important to understand the operation of electronic oscillators and feedback circuits, as we have seen in previous lessons based on transistors or based on integrated circuits.
To know how much is amplified the signal applied to the gate of the triode, it is sufficient to divide the maximum value of the AC component of the anode voltage by the maximum value of the AC component of the gate voltage ; this gives the voltage gain, which is indicated by G.
In the case of Figure 12, since the maximum values of the alternating components of the anode voltage and the gate voltage are 25 V and 2 V, the voltage gain is G = 25 / 2 = 12.5 ; which means that the signal applied to the gate of the triode is amplified 12.5 times.
Characteristics of the Triode
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