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SEMICONDUCTORS 5 “9th PART”
This lesson follows upon the examination of the transistor started in the preceding lessons.
The first chapter will be devoted to the examination of the curves characteristic of the transistor.
The second chapter will indicate the use which one makes of these characteristic curves. Lastly, the final chapter will be devoted to the effect of the temperature on the operation of the transistor.
1. - CURVES CHARACTERISTIC OF THE TRANSISTOR
1. 1. - CURVES AND NETWORKS OF CHARACTERISTICS OF THE TRANSISTOR
You saw that the curve characteristic of the diode makes it possible to know its operation. Indeed, for each tension applied at the boundaries of the diode, the characteristic makes it possible to know the current which crosses it.
It is the same for the transistor ; i.e. its properties can be given thanks to characteristic curves.
Nevertheless, the case of the transistor is different from that of the diode. First of all, it can be used in the three fundamental assemblies (common transmitter, common collector, bases common) and consequently, the characteristic curves will be a function of the examined particular assembly.
Then, the transistor has four electric quantities whereas the diode has only two of them. That appears clearly on figure 1.
These four electric quantities are the two tensions Ve and Vs, and the two currents Ie and Is.
The Ve tension, applied between the common electrode and control electrode and current Ie traversing the control electrode define the input circuit of the transistor.
In the same way, the Vs tension, applied between the common electrode and output electrode, and the Is current circulating in the output electrode define the output circuit of the transistor.
A characteristic curve represents the relation between two electric quantities. In this case, there are four electric quantities. It is thus possible to plot six characteristic curves (Ve - Ie, Vs - Is, Ve - Is, Vs - Ie, Ve - Vs, Ie - Is).
In addition, the four electric quantities all are dependant one on the other. Consequently, for a couple of sizes given, there are several characteristics which one calls a network of characteristics. Let us take the example of the couple Vs - Is. You know that the Is current is a function of current Ie (Is is also a function of the Ve tension since this one determines Ie).
Consequently, it is possible to plot a curve characteristic for each value of current Ie.
Ie is the parameter relating to the network of characteristics Vs - Is. Figure 2 represents the network of characteristics relating to the couple Vs - Is (network of characteristics of exit).
For each of the six couples enumerated previously, there is a network of characteristics which one traces according to a parameter. Thus, one can trace the network of characteristics relating to the Ve couple - Ie having for parameter Vs (characteristics of entry). We will reconsider this example later on.
Until now, we made correspond only one parameter for a couple of electric quantities given (for example parameter Ie for the couple Vs - Is).
However, it is possible to make correspond a second parameter for each of the six above mentioned couples. That is explained by the fact why there are four electric quantities. For example for the characteristics of exit (couple Vs - Is), the parameter can be Ve. In this case, for each particular value of Ve, there will be a characteristic curve. Thus, it is possible to trace 12 (6 x 2) different networks of characteristics.
However, to know the properties of a transistor, it is enough to have two networks of characteristics. The ten others result some graphically.
We will speak in this chapter about the networks about characteristics usually provided by the manufacturers in technical documentations.
In addition, we will take into account the transmitter assembly common bus we saw that it had many advantages compared to that at common base.
The electric quantities used appear on figure 3.
1. 2. - CURVES CHARACTERISTIC RELATING TO THE TRANSMITTING ASSEMBLY COMMON OF A TRANSISTOR NPN TO SILICON
1. 2. 1. - EXAMINATION OF THE OUTPUT CIRCUIT
In order to plot these curves, it is necessary to carry out the assembly of figure 4.
This last comprises four measuring apparatus and two food of D.C. current B1 and B2.
The B1 pile makes it possible to polarize the junction base-transmitter on line. The variable resistor R1 makes it possible to vary basic current IB.
is a millivoltmeter and allows to measure the tension of entry VBE, while basic current IB is measured thanks to the microammeter .
The B2 pile polarizes in reverse the junction collector-bases. R2 resistance makes it possible to vary tension VCE. This tension VCE is measured with the voltmeter , while the output current IC is measured with the milliammeter .
We now will pass to tracing network of characteristics relating to the couple of electric quantities VCE / IC by using for parameter basic current IB
This network of characteristics, which one of is used, is represented on figure 5.
Tension VCE is related to the horizontal axis of the Cartesian reference mark, while current IC is related to the vertical axis.
To trace the characteristic relative to IB = 0, it is enough to open the input circuit (the base is in the air). Then, one varies VCE using R2 resistance ; For example of volt in volt (0, 1, 2, 3… volts). For each value of VCE, one measures the value of current IC correspondent. You notice that the characteristic for IB = 0 is practically confused with the horizontal axis. There is a current very weak ICEO which is current of escape.
Then, to trace the second characteristic (IB = 2 µA), one closes again the circuit input and one acts on R1 so that current IB is equal to 2 µA. It is then enough to make a series of measurements like previously while acting on R2, which makes it possible to trace the second characteristic.
Then, one passes to the third characteristic for IB = 4 µA and so on…
In this case, the network includes/understands 13 characteristics and IB varies from 0 to 24 µA.
We will detail the examination of this network.
1. 2. 2. - EXAMINATION OF THE INPUT CIRCUIT
The network of characteristics relating to the input circuit (sizes VBE and IB) is given thanks to the assembly located on figure 4, (diagram deferred below) with a procedure different from the preceding one. The microammeter is replaced by a milliammeter.
Initially, one fixes tension VCE using R2 resistance.
In the second time, one raises the characteristic for various values of IB. For that, one varies R1 and for each value of IB, one raises tension VBE. That determines a characteristic. For one second characteristic, one changes the value of VCE and one starts again the same series of measurements. Several characteristics thus are traced as that appears on figure 6.
You notice immediately that these characteristics are similar to that of a diode. Indeed, the junction base-transmitter is on line polarized.
However, when IB is null (bases in the air), tension VBE is not null, but is worth approximately 0,6 volt.
You also note that tension VCE has very little influence on tension VBE. If for the same basic current VCE from 1 volt passes to 10 volts, tension VBE varies only few tens of mV.
Consequently, the manufacturers are satisfied to provide only one characteristic of entry, corresponding to an average value of VCE.
1. 2. 3. - NETWORK OF CHARACTERISTICS RELATING TO SIZES IC AND IB (PARAMETER VCE)
The assembly used is always that represented on figure 4 above.
Initially, one fixes tension VCE using R2. Then in the second time, one regulates R1 in order to vary current IB. It is enough to record the value of current IC for each particular value to current IB.
One obtains thus the network of figure 7.
These characteristics are called characteristic of transfer because they connect the output current IC with the current of entry IB.
You notice that IC are proportional to IB (with constant VCE).
The higher VCE is, the more the proportionality factor between IC and IB is high.
Actually, the characteristics of transfer are not exactly lines, but practically one can compare them to lines.
1. 2. 4. - GENERAL DIAGRAM OF the CHARACTERISTICS Of a TRANSISTOR
It is possible to join together the three networks of characteristics seen previously.
This regrouping is represented on figure 8.
There are four quadrants.
The characteristics of exit are laid out in the quadrant (I) located in top and on the right.
Quadrant II laid out in top and on the left represents a characteristic of transfer. Compared to figure 7, increasing currents IB are counted towards the left of origin 0.
Lastly, quadrant III represents a characteristic of entry.
In quadrant IV, one represents the network of characteristics relating to the reaction of the output circuit on the input circuit.
In the case of the assembly out of common transmitter, these networks of characteristics give the following values :
A manufacturer can provide other characteristics that those already seen up to now. In particular, the characteristic relating to current IC and tension VBE can be given (figure 9). Tension VCE can be constant.
As there is a dispersion related to the manufacture of the transistors, the manufacturer indicates a typical characteristic as well as the minimal characteristic and the maximum characteristic.
The majority of the transistors however have characteristics which coincide appreciably with the typical characteristic.
This characteristic does not belong to the group of the 12 networks of characteristics mentioned above.
It is given according to parameter VCE.
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