3. - THE EFFECTS OF TEMPERATURE ON THE OPERATION OF THE TRANSISTOR
3. 1.- EFFECTS OF TEMPERATURE ON THE CHARACTERISTICS OF THE TRANSISTOR
As you have seen, if we only polarize the collector-base junction, a residual current ICBO flows in the collector circuit.
This junction being polarized in opposite direction, the current ICBO is due to the minority carriers constituted by the electron-hole pairs. The number of these couples increases when the temperature increases. As a result, the ICBO current also increases.
This increase in ICBO current can influence the normal operation of the transistor.
To determine this influence, one must first know the relation between ICBO and the ambient temperature. For this, it is necessary to carry out the assembly of Figure 19.
The ICBO current is measured with a microamperometer. The transistor is heated to follow the evolution of ICBO.
We then see that ICBO doubles when the temperature increases by 10° C (germanium transistor). For a silicon transistor, ICBO doubles for a temperature rise of 6° C.
Let's give an example for the germanium transistor.
For a temperature of 25° C., ICBO = 5 µA ; at 35° C, ICBO = 5 x 2 = 10 µA at 45° C, ICBO = 10 x 2 = 20 °A.
It is thus possible to draw a curve representing the variation of ICBO as a function of the temperature for a given transistor. Indeed, for a given temperature, ICBO may be different depending on the transistor under consideration.
It is then possible to draw the graph of Figure 20 which represents the relative increase of ICBO current as a function of temperature.
It can be seen that ICBO is multiplied by 30 when the temperature goes from 25° C to about 75° C.
Let us now see the influence of the temperature on the collector current during normal operation of the transistor.
Consider the transistor mounted in common base, shown in Figure 21-a.
The emitter current IE is 2 mA, ICBO is 5 µA at 25° C and IC is 1, 965 mA.
The following relationship links these different values.
IC = (
x IE) + ICBO
with
= 0,98
Let's calculate the IC current at 50° C. The ICBO current is multiplied by about 6 (see Figure 20).
Therefore, it is worth 5 µA x 6 = 30 µA = 0.030 mA at 50° C.
Hence IC = (0.98 x 2) + 0.030 = 1.990 mA. (Figure 21-b)
The IC current has increased by 25 µA, which corresponds to the ICBO residual current increase.
Since the residual current is always small compared to the collector current in the common base circuit, it can be said that the temperature has little influence on the collector current.
Now consider the transistor mounted as a common emitter (Figure 21-c).
IB = 35 µA, ICEO = 250 µA à
25° C et
IC = 1,965 mA.
Recall :
ICEO
is the residual collector-emitter current when the base is in the air.
These values are linked by the relation :
IC =
x IB + ICEO
We know that :
=
/ (1 -)
Or
= 0,98
from where
= 49.
The ICEO current can be calculated using the following relation already seen :
ICEO = (
+ 1) x ICBO
ICEO = (49 + 1) x 5 µA = 250 µA
From where
IC = (49 x 0,035) + 0,250
IC = 1,965 mA
This value is identical to that relating to the common base assembly (Figure 21-a).
Let's calculate the value of IC for a temperature of 50° C.
For this, we must calculate ICEO
Or, ICEO =
(+1) x ICBO
Therefore 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 of the IC current is worth :
3,215 - 1,965 = 1,250 mA whether 1250 µA
Under the same temperature conditions, it was only 25 µA for the common base assembly.
In conclusion, the collector current of a common emitter transistor is significantly influenced by temperature.
This is also the biggest drawback of the common transmitter assembly.
Figure 22 shows the effect of temperature on the output characteristics network.
You notice that when the temperature goes from 25° C to 55° C, all the features are shifted upwards. The temperature also has an effect on the position of the operating point.
This is what we will see with the assembly of Figure 23.
In Figure 22, the two load lines relating to this arrangement have been drawn.
Current IB is worth 20 µA.
Therefore, at 25° C, the operating point A corresponds to :
VCE = 5,4 volts and at IC = 2,4 mA
At 50° C, the operating point of the assembly has moved (point A'). VCE decreased (4 volts) and IC increased (3.3 mA). If we wanted to keep the same values for VCE and IC, the current IB should be 10 µA (point A").
In some cases, the increase of the current IC causes an increase in the power dissipated by the transistor. This has the effect of increasing the temperature of the transistor, hence the increase in current IC and the power dissipated and so on. This phenomenon is thermal runaway and can lead to the destruction of the transistor.
In order to avoid this phenomenon, appropriate arrangements must be made.
Note that these temperature-related problems are especially sensitive with germanium transistors. In the case of silicon transistors, the residual currents are much lower and therefore the effect of the temperature is smaller.
3. 2. - THERMAL STABILIZATION AND COEFFICIENT OF STABILITY
It is necessary to limit the effects of temperature. For that, there are two solutions : either to prevent the increase of the temperature, or to use an assembly which neutralizes the effects of the temperature.
In general, it is sought to reduce the base current (common emitter assembly) when the temperature increases.
In the case of Figure 22 above, for example, we will try to fix IB = 10 µA for T = 50° C. Thus, the operating point will not change.
If it is desired that the operating point does not vary, it is necessary that the current IB is directly related to the temperature. If this increases, IB decreases and vice versa.
To obtain this automatic correction of the basic current, it is necessary to use a particular polarization circuit.
A stability coefficient (S) is defined for a circuit determined as follows :
This coefficient measures the relative increase of the collector current IC with respect to the increase of the residual current ICBO.
The value of S is inversely proportional to the thermal stability.
In the example of the common base assembly, where the increase of IC is equal to that of ICEO, we have S = 1.
For the common transmitter assembly, the ICEO current is increased + 1) more important than that of ICBO.
ICEO= (+ 1) x ICBO
As a result, the current IC increases (+1) times more than the current
ICBO.
IC=
x IB + ICEO
=x IB + (+ 1) x ICBO
therefore
S=
+ 1
In the example chosen, S = 49 + 1 = 50.
Thermal stabilization is based on the phenomenon of feedback
This stabilization is intended to maintain a constant current IC, when replacing the transistor, the current IC remains identical to what it was before.
The advantage of a stabilization circuit is therefore twofold : it permits a temperature stability and the replacement of a transistor despite the dispersion of the characteristics of these components.
3. 2. 1. -STABILIZATION BY COUNTER-REACTION OF THE COLLECTOR
This simple assembly is shown in Figure 24.
The resistor RB is no longer connected to the voltage + VCC, but to the collector of the transistor.
If the transistor heats up, the current IC tends to increase, the voltage across RC tends to increase and VCE tends to decrease. Now, IB has slightly equal VCE / RB so IB tends to decrease as well. As a result, IC tends to decrease.
This assembly therefore opposes a variation of the current IB.
There is a reaction of the output voltage VCE on the input current IB.
We can do the opposite reasoning if IC tends to decrease. It can be seen in this case that IB tends to increase, so that the current IC tends to remain constant.
This arrangement is interesting if RC is high enough (or VCE less than VCC / 2). Indeed, a small variation of IC must result in a sufficient variation of VCE.
This assembly will not be appropriate when a transformer (primary winding) will be connected in series with the collector. The resistance of the primary winding is too low.
3. 2. 2. - STABILIZATION BY COUNTER-REACTION OF THE TRANSMITTER..
The circuit of Figure 25-a also allows to have a constant current IC.
The principle is the following. When IC tends to increase, IE also tends to increase and therefore VE and VB also. So, the voltage across RB tends to decrease as well as IB.
From then on, IC tends to decrease. There is therefore a reaction of the emitter voltage VE on the input current IB. The RE resistance must be high enough that variations in IC induce sufficient variations in EV.
This arrangement nevertheless has several disadvantages. First, VE has a value close to VCC / 2 because RE has a high value, therefore the VCC voltage will be much higher than in the case of a common transmitter assembly. Then, the RE resistor dissipates a significant part of the power consumed by the assembly, so the efficiency of the circuit is quite low.
This assembly may be suitable if the power consumed is not too high and if the stability coefficient (S) is not too low.
Otherwise, it is best to use the assembly of Figure 25-b.
The base is biased by a voltage divider bridge consisting of R2 and R3. Current IB will be much more sensitive to changes in EV (or IC current).
This assembly makes it possible to limit VE from 10 to 20% of the voltage VCC. The power dissipated by RE will be significantly lower than that of the previous assembly (Figure 25-a).
The IP current will be 5 to 10 times higher than the current IB, because the voltage VB must be practically constant.
3. 2. 3. -THERMISTOR STABILIZATION
The assembly is shown in Figure 26 below.
The thermistor RT is a resistance whose value is a function of the temperature. It consists of semiconductor elements.
These thermistors are of two types. In a first case, the value of the thermistor increases with temperature ; it is called a PTC thermistor or Positive Temperature Coefficient Thermistor.
Conversely, the value of the thermistor may decrease as the temperature increases ; it is a NTC thermistor with Negative Temperature Coefficient. This second type is no longer used.
This is the one used in the proposed editing. The operation of this assembly is as follows.
The voltage across RB is nearly constant because it is equal to VCC - VBE. As a result, the current of the IP base bridge is constant.
Now IP = IB + IT, so when the temperature increases, RT decreases, IT increases and consequently IB decreases. This has the effect of reducing the current IC, so to oppose the rise of this current under the effect of temperature.
This circuit is particularly indicated when one can not insert a resistance of sufficient value in the transmitter.
It is therefore generally used for the final power stage of an amplifier.
The thermistor should be located near the transistor to capture temperature changes.
3. 2. 4. - DIODE STABILIZATION
Simply replace the NTC thermistor from the previous assembly with a diode (Figure 27).
The operating principle is identical to that of thermistor mounting.
In this case, IP = IDi + IB.
When the temperature rises, the reverse current IDi increases.
Diode D must be located near the transistor.
We have completed the examination of the various thermal stabilization circuits.
To limit the effects of temperature, it is necessary to evacuate the heat produced by a transistor.
This is all the more necessary as the power dissipated is high (case of power transistors). These transistors are therefore fixed on radiators.
Radiators are metal parts in which the heat produced by the transistors is transmitted through the conduction phenomenon. Thus, the temperature rise of the junction is limited.
In normal operation, the temperature of the junction rises to a certain equilibrium value. When the transistor reaches this equilibrium, the amount of heat produced by the junction is equal to the heat dissipated in the environment (transistor housing, radiator and ambient air).
In the next lesson of Semiconductors N° 6, we will discuss the input and output resistance of DC and AC transistors as well as hybrid parameters, and many more ...
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x IE) + ICBO
x IB + ICEO
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