Created it, 06/03/17
Update it, 06/03/21
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OPERATION OF THE BIPOLAR TRANSISTOR WITH SATURATED OR BLOCKED RATE
In the case which worries us, the transistor is used either in saturated mode, or in blocked mode, i.e. in commutation, with the manner of a switch, copying the two states of this one :
Closed
switch = saturated transistor
This state corresponds in the passing of a current in the circuit.
Open
switch = blocked transistor
This second state corresponds to the absence of current in the circuit.
The switch is with power-control whereas the transistor is with electric drive.
Figure 16 represents this analogy between switch and transistor used in commutation.
To continue in the way of the analogy between switch and transistor, this last functioning in commutation, two physical sizes are to be taken into account. You defer on figure 17, this one represents quadrant 1, i.e. the characteristic of exit IC function of VCE for IB given.
These two sizes are : VCE sat. (VCE saturation) and ICB0.
VCE sat. corresponds to the knee voltage of the characteristic, one still calls it tension of waste. It is about the minimal value which takes tension VCE for one IC imposed.
According to the current and type of transistor, this value evolves/moves between 0,15 V and 3 V approximately.
ICBO corresponds to the residual current of collector for an emitter current no one. It represents the value minimum of the collector current with beyond which one will not be able to go down when the transistor is in a blocked state. It is the current opposite one of the junction collector-bases (since it is about a reverse current of junction, he is due to the minority carriers, consequently, he will increase appreciably with an increase in temperature).
The order of magnitude of this current is a few microamperes.
These two limiting values correspond to defects. Indeed, with saturation, equivalent of the closed switch, a light tension persists, as if one were in the presence of a resistance of contact.
In addition, with blocking, a light residual current remains and fear being compared to a badly open switch which would have a resistance of parasitic escape in parallel on its contacts.
Another important value is the maximum power which can be dissipated on the collector. It is given by the manufacturer and can be related to quadrant 1, in the form of a curve known as of isopuissance.
Previously, during the description of quadrant 1, we indicated that this characteristic made it possible to determine the line of load.
To highlight the presence or the absence of the collector current, or generally, the presence of an electrical current, it is enough to place in the circuit a resistance. The tension appearing at the boundaries of this one informs us of this presence and the amplitude of this tension informs us about the importance of this current.
This corresponds to the law of Ohm, which we point out:
U = R x I
U : tension in volt
R : resistance in ohm
I = intensity of the current in amp.
It is a question of determining resistance R, to place in the collector, in order to collect in the output circuit of the transistor a useful signal, function of an input signal.
The determination of this resistance R is influenced by the electric need for the following current.
Figure 18 illustrates two possibilities of connection.
In the figure 18-a, the lamp (or element of load of the switch) is connected in series with the switch.
When this last is closed, the current circulates and the lamp is lit.
When it is open, the lamp dies out.
In the figure 18-b, the lamp and the switch are connected in parallel.
When the switch is closed, the lamp dies out.
When it is opened, the lamp is lit.
The result of operation is the reverse of that of the preceding assembly.
In addition, when the switch is closed, if one does not place in series a resistance R, the supply voltage VCC is shorted-circuit.
In electricity, as in electronics, the short-circuit is to be avoided!
It is necessary also to envisage a tension VCC higher than the tension of the lamp, bus so that the latter is lit, tension VCC must be equal to the tension of operation of the lamp, plus the voltage drop in R.
On the other hand, this assembly shows an interesting characteristic: the common trunk of potential.
In the case of association of switching functions fulfilled with transistors, this allows the design of modules, representing each one an elementary function and to lay out them end to end, the output signal of the one and the input signal of the other having the same potential of electric reference.
The circuit of the figure 18-a will be used preferably at the end of the assembly, to feed the body of exit, which will concretize the logical operations carried out upstream.
On the figure 18-c, this circuit is the transposition, with transistor, of that of the figure 18-a.
The figure 18-d represents the transistor transposition of the assembly of the figure 18-b.
Figure 19 represents the more general cases. It does not act any more a lamp, but of a resistance of load RL (L = load = charge).
The resistance of collector is located by RC.
Let us examine what occurs in the circuit from the figure 19-a, where the resistance of load is confused with that of collector RC.
In general, it is necessary well to start from known data and in this example, one supposes that we know the tension necessary to the operation of receiver RL and, either its current, or its resistance.
This tension roughly determines the value of the supply voltage VCC :
VCC = VRL + VCE sat.
Tension VCC and the current IRL which becomes current collector, allow the choice of the transistor.
When the transistor is blocked (interruptory open) the tension on its terminals VCE is as follows :
VCE = VCC - (RL x ICBO)
It should be remembered that the transistor is not a perfect switch and that in the absence of polarization of the junction transmitter-bases, circulates in the collector, reverse current ICBO of the junction collector-bases.
For the layout of the right-hand side of load, one supposes the transistor like a perfect switch and the tension becomes: VCE = VCC
When the transistor is saturated (interruptory closed), the current which crosses the circuit is given by the relation :
IC = (VCC - VCE SAT) / RL
In the case of the right-hand side of load, one neglects tension VCE of saturation (certain transistors of commutation have a tension of waste lower than the 1 / 10th of volt).
Consequently :
IC @ VCC / RL
Let us carry these two values on the network of quadrant 1, representing the characteristic of exit of a transistor of which we know from now on that it will have to support a tension equal to VCC and one current IRL = IC.
On figure 20, the line D which connects these two values, is called the line of load.
The current IC which circulates in RL when the transistor is saturated corresponds, according to the network, with a current bases IB. We know the relation :
IC = bIB (b = h21E = profit while running)
In the case of figure 20, this relation is not checked any more bus :
IC < bIB or IB > IC / b
This relation checks well that we are in mode of saturation. Any variation of IB does not involve any more one variation of IC in the report/ratio b.
In commutation, without signal at the entry, the point of operation is in A. With a signal at the entry, the point of operation passes out of B.
The transitory passage of A with B must be carried out as soon as possible. For the moment, we will consider that it is carried out instantaneously.
The reverse, i.e. the passage of B towards A, must be done under the same conditions.
Current IB, given on the network of exit, brings a major element to us referring to the input signal, i.e. with the output signal of the preceding circuit.
On the basis of the quadrant 4 which represents the opposite characteristic of transfer, one will draw, for VCE SAT in question, the value of VBE according to current IB previously found.
We are in possession of the characteristics of the V1 signal, which it is necessary to apply to the entry to obtain the point of operation B (i.e. current IB under tension VBE)
Point A is obtained for a tension of null entry VBE, therefore a null current IB.
Let us come in on the figure 21 in which resistances R1 and R2 appear.
They are assembled in tension divider bridge, because one uses tension VCC to obtain VBE.
In R1, current IP circulates, of the dividing bridge, plus the current bases IB :
VR1 = R1 x (IP + IB)
In R2, circulates the current bridge IP, tension VR2 is known since it is about VBE :
VR2 = VBE = R2 x IP
For reasons of stability of the point of operation, one takes a current bridge IP equal to 10 times the current bases IB (variation of IB due to the dispersion of the profit while running or a variation in temperature).
IP = 10 IB
Consequently, in R1 eleven times circulate the current bases and the ohmic value of each one of these resistances is as follows :
R1 = (VCC - VBE) / (11 x IB)
R2 = VBE / (10 x IB)
When switch I is opened, tension VBE appears at the boundaries of R2, current IB is injected into the base of the transistor. Current IC circulates in RL.
We are on the line of load at the point B. the transistor is saturated. Practically, all tension VCC is at the boundaries of RL, whereas tension VCE is almost null.
When switch i is closed, tension VBE disappears, current IB is null involving the cancellation of IC (except for ICBO).
We are on the line of load, at point A. the transistor is blocked. All tension VCC is transferred between collector and transmitter of the transistor, the terminal voltage of RL is practically null.
Let us return to the assembly of the figure 19-b and examine its operation. For that, it is necessary to know :
line of load of the assembly which is built by
regarding the transistor as a perfect switch.
limits, on this line of load, of the point of
operation in commutation (blocked or saturated), the transistor being regarded
as a nonperfect switch.
In this type of assembly, we know that tension VCC must be higher than the tension necessary to the receiver consisted RL.
This minimal value of VCC is determined by :
VCC = VL + (RC x IL)
It is quite obvious that we know VL and IL thus RL.
The choice of RC allows us with IL defining VCC and if required taking another value for RC in order not to lead to a too high tension VCC.
The choice of RC conditions the value of IC by regarding the transistor as a perfect switch.
IC = VCC / RC 1
Always on the basis of the same considerations, we can find VCE :
VCE = VCC - (RC x IL) 2
It should be noted that :
VCC - (RC x IL) = VL
Current IC and tension VCE allow the choice of the transistor.
This choice being made, one has the characteristic of exit on which one defers values 1 and 2 that it is enough to connect to obtain the line of load.
Now, we will seek the limits of operation by not reconsidering more the transistor like a perfect switch.
In the saturated state, current IC becomes :
IC = (VCC - VCE SAT) / RC
While considering, under tension VCE SAT, the current in RL like negligible.
In the blocked state, tension VCE becomes :
VCE = VCC - [ (RC x IL) + (RC x ICBO) ] = VCC - RC (IL + ICBO)
The figure 22-a, illustrates the characteristic of exit of a transistor on which we carried these values.
As previously, we deduce, starting from this network, current IB, then quadrant 4 tension VBE corresponding according to VCE SAT.
The figure 22-b represents the assembly with its control circuit R1, R2 and i (identical to that of figure 21).
We know to calculate R1 and R2 because it is about the same reasoning as for the preceding assembly.
The presence of a base current generates a collector current.
The base current is the consequence of a tension VBE which polarizes the junction base-transmitter in the busy direction.
In this type of assembly, common transmitter (figure 22-b), one recovers the output signal between collector and transmitter (tension VCE).
Between the entry and the exit, there is inversion of the signal. It is said that assembly E-C (transmitting commun run) is phase-shifting.
On the figure 22-a, the line in dotted line represents the line of load which one would obtain if RL were removed.
Figure 23 illustrates the limits of operation of the transistor.
Line 1 determines, in the zone of saturation, the tension of waste below which, for one IC given, one will not be able to go down.
Line 2 determines, in the zone of blocking, the residual current ICBO below which, for a given VCE, one will not be able to go down.
Formless curve 3 on the maximum capacity that the transistor can dissipate.
The lines a and b correspond to the limiting values which the line of load can take.
a corresponds to
an infinite RC (infinite ohmic value:
cut resistance).
b
corresponds to
a null RC (null ohmic value:
resistance in short-circuit). This line cannot be practically reached without
destruction of the transistor or food VCC.
We have just flown over the problem concerning the transistor in commutation (operation out of switch). The goal of this lesson is not the study of the transistor assemblies, but to provide a sufficient knowledge to approach the realization of logical processes on the basis of a material which has its physical limits and, consequently, to divide the shelves which had with a bad logical interpretation and those which had with risks inherent in technology.
The practical realization of an assembly must always comprise at least two phases :
a pure and theoretical logical phase
a practical phase implying the knowledge of technology employed.
These two stages are as important one as the other.
The second is too often retracted and a great number of vexations are ascribable for him.
The manufacturers place at the disposal of the technicians notes relating to the parameters of the circuits which they manufacture, it is necessary to take note of it.
The transistor is not only used in commutation but also for the analogical assemblies, in the linear zone (amplification), i.e. apart from the zones of saturation and blocking.
On this subject, in our case, the passage from one zone to another (of point A at the point B on the line of load) is not carried out instantaneously, but with a finished, consecutive speed with certain stray capacities.
This way borrows the linear zone of amplification which is a disadvantage in commutation, and more time of transit is short, plus we minimize this disadvantage (this one results in a degradation of the rising and downward faces of the signal, as well as an increased sensitivity, at this time there, with the external parasites).
This speed limit is bound, amongst other things, with two stray capacities: Cbe and Ccb.
These capacities are those of the junctions of the transistor. Cbe is allotted to the junction base-transmitter, Ccb with the junction collector-bases.
Resistance interns rbb base (made up by the zone of semiconductor of the base and the ohmic contact of this one) and the above mentioned capacities constitute time-constants which delay the departure of a point towards the other and slow down the rate of travel on the line of load.
The bipolar transistor is used for the logical operators in technologies known as:
RTL
: (logic
with resistance - transistor)
DTL : (logic
with diode - transistor)
TTL : (logic with transistor - transistor).
The latter is used for its speed but its consumption and its dissipation do not allow a very thorough integration. Also one has resort to another type of transistor, from which operation is a little different and which one names : field-effect transistor.
It is the latter which we will examine in the next lesson technology 2.
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