Created it, 06/03/17
Update it, 06/03/22
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4. - SWITCHING FUNCTIONS A BIPOLAR TRANSISTORS “TTL”
As an introduction to the TTL (Transistor - Transistor - Logic), we will return with the DTL with figure 20.
We see there a great similarity between the input circuit of an operator in technology DTL and the symbol of the transistor (symbol very little used, but which image well two junctions it is made up).
To obtain several entries, it is enough to associate several diodes on the transmitting side. We come to the transistor multi-transmitters, that one meets in almost all operators TTL.
Figure 21 represents, in this technology, operators NAND and NOR as well as the truth tables.
Figure 22 shows the preceding circuits, NAND and NOR to which are associated circuits NOT in order to obtain it AND and it OR with the truth tables which are referred to it.
Some modifications were made to these operators in a preoccupation with an improvement.
We will examine them hereafter.
Improvements compared to the basic assembly.
One of the characteristics of this technology is to commutate quickly, i.e. to pass very quickly from a state to another. It is an advantage, but with these very short times, wiring and connections represent coils and capacities which enter in oscillations in the presence of these signals.
These damped oscillations are recorded by the following operator like a succession of states 0 and states 1.
To obviate this disadvantage, a diode is placed on each entry as it is indicated on the figure 23.
In addition, these operators using a transistor assembled out of common transmitter, have a “resistance” of too high exit compared to the “resistance” of entry of the operators who they are likely to order.
Figure 24 represents the stage of exit of an operator on whom one connected a load consisted the entries of other operators.
This load is different depending on the state from exit of T (state 1 or high and state 0 or low).
On the bottom grade, the transistors of entry of the following stages are conductive and the current IL is important.
At the high level, these same transistors are blocked, there is however a current IL' (due amongst other things to the reverse currents of the junctions of the diodes about which one spoke previously).
On the bottom grade, the transistor T will have to support its clean running collecting amplified current transmitting following operators.
One is tempted to say that it would be enough to decrease to the clean maximum its running collecting to attenuate this phenomenon by increasing RC.
However, if RC is increased, at the high level, the reverse currents of the junctions walk on as it is indicated on the figure 24-b and one realizes that if RC increases and RL' decreases, level high VS becomes so low that there will be little difference between high level and bottom grade.
You can note that the currents IL and IL' are opposite directions. We will speak again about it further.
It is necessary, consequently, to give up this assembly with the profit of an other whose resistance of exit will be lower. This new assembly is called: “Totem pole”.
The figure 25-a represents in a simplified way this assembly which can be compared to two switches, one being opened, the other closed and vice versa, both not being able to be opened or closed at the same time.
One can already include/understand the interest of this assembly.
The two switches are in fact of the transistors, as indicated on the figure 25-b, gone up in series and ordered in opposition, i.e. one imposes the saturation of the one and the blocking of the other.
In the high state, T1 is saturated and T2 is blocked.
Exit VS is with the potential + VCC.
In the low state, T1 is blocked and saturated T2.
Exit VS is with potential 0.
Actually, the things are not so simple. Let us take again the figure 25-b and suppose that T1 is saturated.
The potential of point A goes down, that of the point B will go up.
Tension VBE of T3 is reached and this one led.
Tension VS is equal to VCE SAT of T3.
The tension out of B is equal to VBE of T3.
The tension in A is equal to the tension out of B plus tension VCE SAT of T1.
VS = VCE SAT (T3) --------------> tension on the transmitter of T2
VA = VBE (T3) + VCE SAT (T1)
from where tension VBE of T2 :
VA - VS = VBE (T3) + VCE SAT (T1) - VCE SAT (T3)
Two VCE SAT are cancelled and tension VBE of T2 is equal to tension VBE of T3.
T2 also leads, whereas it should be blocked.
It is thus necessary to have recourse to an artifice (which one used previously), by adding a tension equal to this VBE but of contrary sign. For that, we will use the tension of a junction (diode), polarized in the busy direction. The diagram becomes that of the figure 26-a.
The tensions on this assembly are as follows :
VA = VBE (T3) + VCE SAT (T1)
VB = VBE (T3)
VC = VCE SAT (T3) + VD
VBE (T2) = VA - VC = [VBE (T3) + VCE SAT (T1)] - [VCE SAT (T3) + VD]
since : VBE (T3) @ VD
VCE SAT (T1) @ VCE SAT (T3)
from where : VBE (T2) = 0 V.
The T2 transistor is well blocked, whereas T3 is conductive, VS is with potential 0 (low state).
If T1 is blocked, the potential of the point B tends towards 0 and T3 is blocked.
The potential of A goes up towards the + VCC is tension VBE of T2 is reached, a current bases is established and T2 is saturated.
The potential of VS goes up towards the + VCC (high state).
There is however still a small defect at the time of the transition, i.e. before T3 is completely blocked, T2 starts to lead, which results in a call of current that VCC must provide. This call of current will be slowed down by the addition of a resistance of low value as indicated in the figure 26-b.
Figure 27 represents an operator NAND
such as one can meet it in practice.
On this assembly, R2 resistance disappears, the current bases of T2 is provided by the reverse current of the junction collector-bases T1.
As an indication, the following table provides the values of resistances of this circuit :
R1 = 4 kW
R3 = 1,6 kW
R4 = 1 kW
R5 = 130 W
the supply voltage is of : VCC = + 5 V
the current of entry in a high state : 40 µA
the current of entry in a low state : 1,6 mA
the output current in a high state : 400 µA
the output current in a low state : 16 mA
the output voltage in a high state : 2,4 to 3,4 V
the output voltage in a low state : 0,2 to 0,4 V
the tension of entry considered as high state : 2 V
the tension of entry considered as low state : 0,8 V
These parameters are, in the catalogs manufacturers, represented by abbreviations. In next TECHNOLOGY 4, we will indicate a list of these abbreviations.
Right now, we can speak and give some explanations concerning the TTL about them.
the supply voltage : VCC
the current of entry in a high state :
I IH
I symbol of the current, index IH corresponds to :
I : input “entered”
H : hight-level “high level”
The current of entry in a low state:
I IL
The index L corresponds to: low-level “bottom grade”
the output current in a high state:
I OH
the index O corresponds to: output “left”
the output current in a low state:
I OL
the output voltage in a high state: V
OH
V is the symbol of a tension
the output voltage in a low state:
V OL
the tension of entry taken into account like a high
level: V IH
the tension of entry taken into account like a
bottom grade: V IL
In logic, it is rare that only one operator is necessary to fulfill a function. It is thus necessary to associate several of these operators to lead to the result.
There is thus to standardize these entries and these exits whatever the operators in order to carry out these associations without shelf as for the levels of tension and current.
If one uses, in the same assembly, different technologies, for example operators with diodes with TTL, one is brought to borrow circuits known as of interface.
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