Created it, 05/10/15
Update it, 06/01/16
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SEMICONDUCTORS 5 “11th PART”
3. - EFFECTS OF THE TEMPERATURE ON THE OPERATION OF THE TRANSISTOR
3. 1. - EFFECTS OF THE TEMPERATURE ON THE CHARACTERISTICS OF THE TRANSISTOR
As saw it to you, if one polarizes only the junction collector-bases, a residual current ICBO circulates in the circuit of the collector.
This junction being polarized in opposite direction, current ICBO is due to the minority carriers consisted the couples electron-positron pair. The number of these couples increase when the temperature grows. Consequently, current ICBO increases too.
This increase in current ICBO can influence the normal operation of the transistor.
To determine this influence, it is first of all necessary to know the relation between ICBO and the ambient temperature. For that, it is necessary to carry out the assembly of figure 19.
Current ICBO is measured with a microammeter. One heats the transistor to follow the evolution of ICBO.
One realizes whereas ICBO doubles when the temperature increases 10° C (transistor with germanium). For a transistor with silicon, ICBO doubles for an increase in temperature of 6° C.
Let us give an example for the transistor to germanium.
For a temperature of 25° C, ICBO = 5 µA ; with 35° C, ICBO = 5 x 2 = 10 µA with 45° C, ICBO = 10 x 2 = 20 µA.
One can thus plot a curve representing the variation of ICBO according to the temperature for a given transistor. Indeed, for a given temperature, ICBO can be different according to the examined transistor.
One can then plot the graphics of figure 20 which represents the relative increase in current ICBO according to the temperature.
It is seen that ICBO is multiplied by 30 when the temperature passes from 25° C to approximately 75° C.
Now let us see the influence of the temperature on the collector current during the normal operation of the transistor.
Let us consider the transistor assembled in base common, illustrated to the figure 21-a.
The current of transmitter IE is worth 2 mA, ICBO are worth 5 µA with 25° C and IC are worth 1,965 mA.
The following relation binds these various values.
|
IC = ( |
with |
Therefore, it is worth 5 µA x 6 = 30 µA = 0,030 mA with 50° C.
From where IC = (0,98 x 2) + 0,030 = 1,990 mA. (Figure 21-b)
Current IC increased by 25 µA, which corresponds to the increase in residual current ICBO.
As the residual current is always weak compared to the collector current in the assembly bases common, one can say that the temperature has little influence on the collector current.
Now let us consider the transistor assembled out of common transmitter (figure 21-c).
IB = 35 µA, ICEO = 250 µA with 25° C and IC = 1,965 mA.
Recall :
These values are bound by the relation :
x IB + ICEOIt is known that :
= 
/ (1 - )
|
However
|
from where |
|
Current ICEO can be calculated using the following relation already seen :
|
ICEO = ( |
|
|
ICEO = (49 + 1) x 5 µA = 250 µA |
|
|
From where |
IC = (49 x 0,035) + 0,250 |
|
IC = 1,965 mA |
This value relates identical to that to the assembly bases common (figure 21-a).
Let us calculate the value of IC for a temperature of 50° C.
For that, one must calculate ICEO
However, ICEO = (
+ 1) x ICBO
Thus ICEO = (49 + 1) x 30 µA = 1500 µA = 1, 5 mA
And IC = 49 x 0,035 + 1,5 mA = 3,215 mA (Figure 21-d)
The absolute increase in current IC is worth :
Under the same conditions of temperature, it was not that of 25 µA for the assembly bases common.
In conclusion, the collector current of a transistor assembled out of common transmitter is notably influenced by the temperature.
It is in addition the greatest disadvantage of the common transmitting assembly.
Figure 22 makes it possible to see the effect of the temperature on the network of characteristics of exit.
You notice that when the temperature passes from 25° C to 55° C, the whole of the characteristics is shifted upwards. The temperature also has an effect on the position of the point of operation.
It is what we will see with the assembly of figure 23.
In figure 22, one plotted the two straight lines of load relating to this assembly.
Current IB is worth 20 µA.
Consequently, to 25° C, the point of operation A corresponds to :
VCE = 5,4 volts and with IC = 2,4 mA
With 50° C, the point of operation of the assembly moved (point A'). VCE decreased (4 volts) and IC increased (3,3 mA). If one wanted to preserve the same values for VCE and IC, it would be necessary that current IB is 10 µA (point A").
In certain cases, the increase in current IC involves an increase in the power dissipated by the transistor. That causes to increase the temperature of the transistor, from where increase in current IC and the dissipated power and so on. This phenomenon is the thermal runaway and can lead to the destruction of the transistor.
In order to avoid this phenomenon, it is necessary to resort to suitable assemblies.
Note that these problems involved in the temperature are especially sensitive with the transistors to germanium. In the case of the transistors with silicon, the residual currents are definitely lower and consequently, the effect of the temperature is more reduced.
3. 2. - THERMAL STABILIZATION AND FACTOR OF STABILITY
It is necessary to limit the effects of the temperature. For that, there are two solutions: either to prevent the increase in the temperature, or to use an assembly which neutralizes the effects of the temperature.
In general, one seeks to reduce the basic current (common transmitting assembly) when the temperature increases.
In the case of figure 22 above, for example, one will seek to fix IB = 10 µA for T = 50° C. Thus, the point of operation will not change.
If it is wanted that the point of operation does not vary, it is necessary that current IB is directly related to the temperature. If this one increases, IB decreases and vice versa.
To obtain this autocorrection of the basic current, it is necessary to employ a particular circuit of polarization.
One defines a factor of stability (S) for a in the following way determined circuit :
This coefficient measures the relative increase in the collector current IC compared to the increase in residual current ICBO.
The value of S is inversely proportional to thermal stability.
In the example of the assembly bases common, where the increase of IC is equal to that of ICEO, one has S = 1.
For the common transmitting assembly, current ICEO
undergoes an increase (
+ 1) time more important than that of ICBO.
+ 1) x ICBOConsequently, current IC increases (
+ 1) time more than current ICBO.
|
IC = |
|
|
= |
|
|
Thus |
S = |
In the example chosen, S = 49 + 1 = 50.
Thermal stabilization is based on the phenomenon of negative feedback
This stabilization having for goal to maintain current IC constant, when the transistor is replaced, current IC remains identical so that it was front.
The advantage of a circuit of stabilization is thus double : it allows a stability in temperature and the replacement of a transistor in spite of dispersion of the characteristics of these components.
3. 2. 1. - STABILIZATION BY NEGATIVE FEEDBACK OF THE COLLECTOR
This simple assembly is represented on the figure 24.
Resistance RB is not connected any more to the tension + VCC, but to the collector of the transistor.
If the transistor warms up, current
IC tends to increase, the terminal voltage of RC
tends to increase and VCE tends to decrease.
However, IB 
VCE / RB thus IB tends to also
decrease. It results from it that IC tend to decrease.
This assembly is thus opposed to a variation of current IB.
There is reaction of the output voltage VCE on the current of entry IB.
One can make the opposite reasoning if IC tend to decrease. One realizes in this case that IB tends to increase, therefore that current IC tends to be maintained constant.
This assembly is interesting if RC is rather high (or VCE lower than VCC / 2). Indeed, a small variation of IC must involve a sufficient variation of VCE.
This assembly will thus not be suitable when a transformer (primary winding) is assembled in series with the collector. The resistance of the primary winding is too low.
3. 2. 2. - STABILIZATION BY NEGATIVE FEEDBACK OF THE TRANSMITTER.
The circuit of the figure 25-a also makes it possible to have a constant current IC.
The principle is as follows. When IC tend to increase, IE also tends to increase and consequently, VE and VB too. Therefore, the terminal voltage RB tends to decrease like IB.
Consequently, IC tend to decrease. There is thus a reaction of the tension of transmitter VE on the current of entry IB. resistance RE must be rather high so that the variations of IC induce sufficient variations of VE.
This assembly presents nevertheless several disadvantages. First of all, VE has a value close to VCC / 2 bus RE has a raised value, consequently, tension VCC will be much higher than in the case of a common transmitting assembly. Then, resistance RE dissipates a great part of the consumption by the assembly, therefore the output of the circuit is rather weak.
This assembly could be appropriate if the consumption is not too high and if the factor of stability (S) is not too low.
If not, it is preferable to use the assembly of the figure 25-b.
The base is polarized by a tension divider bridge consisted R2 and R3. Current IB will be much more sensitive to the variations of VE (or of current IC).
This assembly makes it possible to limit VE from 10 to 20 % of tension VCC. The power dissipated by RE will be thus definitely lower than that of the preceding assembly (figure 25-a).
Current IP will be 5 to 10 times superior with current IB, because tension VB must be practically constant.
3. 2. 3. - STABILIZATION BY THERMISTERS
The assembly is represented on figure 26 below.
Thermister RT is a resistance whose value is a function of the temperature. It is consisted semiconductor elements.
These thermisters are of two types. In a first case, the value of the thermister increases with the temperature; it is called a thermister CTP or thermister with Positive Temperature coefficient.
Conversely, the value of the thermister can decrease when the temperature increases; it is about a thermister CTN to Negative Temperature coefficient. This second type is used more.
It is that used in the assembly proposed. The operation of this assembly is as follows.
The terminal voltage RB is practically constant bus it is equal to VCC - VBE. Consequently, the current of basic bridge IP is constant.
However IP = IB + IT, therefore when the temperature increases, RT decreases, IT increases and consequently IB decreases. This causes to decrease current IC, therefore to be opposed to the rise in this current under the effect of the temperature.
This circuit is particularly indicated when one cannot insert a resistance of sufficient value in the transmitter.
It is thus generally used for the final stage of power of an amplifier.
The thermister must be located near the transistor in order to collect the variations in temperature.
3. 2. 4. - STABILIZATION BY DIODE
It is enough to replace thermister CTN of the preceding assembly by a diode (figure 27).
The principle of operation is identical to that of the assembly with thermister.
In this case, IP = IDi + IB.
When the temperature rises, the current reverses IDi increases.
The diode D must be located near the transistor.
We finished the examination of the various circuits of thermal stabilization.
To limit the effects of the temperature, it is necessary to evacuate the heat produced by a transistor.
That is all the more necessary as the dissipated power is high (case of the transistors of power). These transistors are thus fixed on radiators.
The radiators are metal parts in which the heat produced by the transistors is transmitted thanks to the phenomenon conduction. Thus, the rise in temperature of the junction is limited.
Under normal operation, the temperature of the junction rises up to a certain value of balance. When the transistor reaches this balance, the quantity of heat produced by the junction is equal to the heat loss in the environment (case of the transistor, radiator and ambient air).
In the next lesson of the semiconductors n° 6, we will approach the resistance of entry and exit of the transistors in D.C. current and alternative as well as the hybrid parameters, and well of others still …
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