Created it, 05/10/15
Update it, 06/01/15
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SEMICONDUCTORS 5 “10th PART”
2. - USE OF THE CURVES CHARACTERISTIC OF A TRANSISTOR
2. 1. - DETERMINATION OF A POINT OF OPERATION
The characteristics of the transistors are used to determine the operating conditions of these transistors.
One determines thus the value of the electric quantities intervening in this operation.
The most used characteristic is that which relates to the output circuit.
Let us consider the network of characteristics of figure 11.
That is to say a tension VCE equalizes with 10 volts and a current IB equal to 70 µA.
To determine current IC correspondent with these two parameters given, it is enough to trace a vertical starting from the tension VCE = 10 V ; this vertical cuts the characteristic relative to IB = 70 µA to point A. From this point A, it remains to trace a horizontal line which cuts the vertical axis of the Cartesian reference mark. In this case, one finds IC = 20 mA.
Point A is called not operation. Indeed, this point A makes it possible to know the three parameters relating to the operation of the transistor.
You notice that it is enough to know two parameters among three to determine this point of operation.
Let us suppose for example that one knows IC = 14 mA and IB = 50 µA.
It is enough to trace a horizontal line corresponding to IC = 14 mA ; this one meets the characteristic IB = 50 µA at the point B. One then traces the vertical resulting from the point B, which makes it possible to determine a tension VCE equal to 9 volts.
We now will examine the operation of the assembly located on figure 12.
The common transmitting assembly is fed by a pile of 24 volts. The variable resistor RB makes it possible to vary basic current IB.
Resistance RC is the resistance of load located in the collector of the transistor.
The terminal voltage of the resistance of load is VR. The sum of tensions VR and VCE is equal to VCC is 24 volts.
We wish to know the report/ratio which exists between three sizes VCE, IC and IB. We have for example the network of characteristics of exit with parameter IB.
It is enough to choose a certain number of values for current IC, then to calculate tension VCE for each one of these values. That will enable us to as many place points on the network of characteristics.
That gives us the co-ordinates of a first point which we name A on figure 13.
In the same way, for IC = 10 mA, VR = 8 volts and VCE = 16 volts (not B).
For IC = 15 mA, VCE = 12 volts (point C)
For IC = 20 mA, VCE = 8 volts (point D)
For IC = 25 mA, VCE = 4 volts (point E)
You notice that these five points are aligned ; also, one can make pass a line by these points which one calls right-hand side of load.
This line of load represents the whole of the points of operation for the transistor. I.e. for each point of the right-hand side, one can determine three sizes VCE, IC and corresponding IB.
This line meets the two axes of the reference mark at the points P and Q. the point Q corresponds to a tension VCE of 24 volts and to a current IC no one. The point P corresponds to IC = 30 mA and VCE = 0 volt ; in this case, tension VR is equal to tension VCC.
As you know it, it is enough to know two points to determine a line. However, in this case, it is possible to know these two points to plot the straight line of load.
We will take an example. That is to say a resistance of load RC equalizes to 1,3 KW and a voltage supply VCC equal to 16 volts.
When IC are null, we know that VR = 0 volt and VCE = VCC = 16 volts. We have thus to determine the Q' point on the horizontal axis (figure 13). To determine the second point, we pose VCE = 0 volt. In this case VR = 16 volts and IC = VR / RC = 16 / 1,3 x 103 = 12,3 mA.
The second P' point thus corresponds to IC = 12,3 mA. (Application of the law of Ohm).
It remains to connect these two P' points and Q' and thus, one plotted the straight line of load relative to RC = 1,3 KW and VCC = 16 volts.
Its position is definitely different from that examined before.
These two examples thus show that the position of the right-hand side of load is related to VCC and of RC.
It is not always possible to determine the two points located out of the two axes of the reference mark.
Let us take the case of the network located on the figure 14.
That is to say VCC = 24 volts and RC = 0,3 kW.
When IC = 0 mA, VCE = VCC = 24 volts. We thus know the point Q located on the horizontal axis.
For VCE = 0 volt, we have IC = VR / RC = VCC / RC is IC = 24 / 300 = 80 mA.
However, the point corresponding to IC = 80 mA is located apart from the graph, therefore it is advisable to seek another point.
One thus takes IC = 50 mA.
One deduces VR = RC x IC = 300 x 50 x 10-3 = 15 volts
and VCE = VCC - VR = 24 - 15 = 9 volts.
Thus, the second point S corresponds to VCE = 9 volts and to IC = 50 mA.
The line of load is thus determined by the points Q and S.
Consequently, it is easy to determine the values of sizes VCE and IC if current IB. is fixed
For example, the characteristic corresponding to IB = 110 µA meets the straight lines of load at the point T (figure 14).
This point makes it possible to know VCE = 14 volts and IC = 33,5 mA.
2. 2. - THE VALUES LIMIT FOR A TRANSISTOR
Like any component, the transistor can function only between certain limiting values of the electric quantities which are dependant for him.
These values limit are dependant on the type of the transistor, its dimensions, its constitution…
For example, a low-size transistor will dissipate less power than another of more important size.
For each type of transistor, the manufacturer provides limiting values which should not be exceeded under the normal conditions of use. In the contrary case, the transistor can be irremediably damaged, or all at least to cause an abnormal operation.
The values limit generally provided by the manufacturers are as follows :
Other limiting values relating to the base and the transmitter can be provided.
a) The maximum value of the collector current
This value is given so that the transistor functions correctly and so that the junction cannot be damaged.
In figure 15, this value limits IC max is represented by a horizontal line in dotted line. In this case, IC max = 10 mA.
The point of operation of the transistor must be located in lower part of this dotted line.
According to the value of resistance RC, two situations are to be considered.
All the points of operation are thus located in lower part of this dotted line.
b) The maximum value of the tension of collector
During the examination of the diode with junction, you saw that there was opposite a tension known as “tension of breakdown” beyond which the diode was damaged. However, in the transistor, the junction collector-bases is equivalent to a polarized diode in reverse.
There is thus a maximum tension between the collector and the base of the transistor not to be exceeded. In other words, there is a max. tension VCE.
On figure 16, tension VCE max is indicated by the tear line.
The point of operation of the transistor must be located on the left of this dotted line.
If tension VCE exceeds notably the value of VCE max, IC become very important and the transistor is likely to be destroyed.
c) the maximum value of the power of dissipation of the collector
The transistor absorbs a power equal to the product of tension VCE by current IC.
We have the relation :
PC will be expressed in mW, if VCE is in volts and IC in mA.
This power absorptive by the transistor corresponds to a consumption of electric power.
This electric power is converted in the transistor into calorific energy by Joule effect. Consequently, there is heating of the transistor; in particular the temperature of the junction increases.
However, as the temperature of junction cannot exceed a certain value, it is necessary to limit the power dissipated by the transistor.
The increase in the temperature of the junction can result from the following formula :
The Rth coefficient is expressed in °C / W and indicates the increase in temperature of the junction in °C for a dissipation of power of one Watt.
Let us take an example :
|
From where |
T'j = 400 x 0,015 = 6° C |
The temperature of the junction increases 6° C.
To calculate the temperature of the junction, the relation should be applied :
|
Tj = Ta + T'j |
(2) |
By taking again the example above with Ta = 25° C, we have :
The acceptable maximum temperature on the level of the junction is between 150° C and 200° C for a transistor at silicon.
The temperature of the Tj junction depends on the ambient temperature Ta and the increase in temperature T'j which is related to PC. Consequently, maximum power acceptable PC max depends not only on the type of transistor but also on Ta.
A manufacturer thus indicates a value of PC max for a fixed temperature Ta.
Generally, the values of Rth and Tj max are provided by the manufacturer.
Under these conditions, one can calculate PC max starting from the relations (1) and (2) :
If one knows the values of Tj max, Ta and Rth, it is easy to calculate max. PC.
It is interesting to see how the knowledge of PC max limits the zone of use of the network of characteristics.
Since the value of PC max is known, it is easy to plot a curve representing the relation :
Figure 17 represents an example with three curves relating to three values of PC max (400 mW, 200 mW and 100 mW).
These curves are called curved of isopuissance, because for all the points of operation located on a curve, the power dissipated by the transistor is identical.
Generally, one defers on the network of characteristics the curve of isopuissance, IC max and VCE max as that appears on figure 18.
Any point of operation will have to be located in the zone ranging between the two axes of the reference mark, the two segments of right-hand side representing IC max and VCE max and the curve of isopuissance corresponding to the transistor used.
For an ambient temperature of 25° C, PC max is equal to 125 mW and consequently, the useful zone is delimited by items 0, A, B, C and D.
This useful zone decrease quickly if the ambient temperature increases : If Ta = 55° C, PC max = 50 mW ; in this case, the useful surface area is delimited by items 0, A, B', C' and D.
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