The article will consider the TTL logic, which is still used in some branches of technology. In total there are several types of logic: transistor-transistor (TTL), diode-transistor (DTL), based on MOS transistors (CMOS), as well as based on bipolar transistors and CMOS. The very first microcircuits that were widely used were those that were built using TTL technologies. But other types of logic that are still used in technology cannot be ignored.
Diode-transistor logic
Using ordinary semiconductor diodes, you can get the simplest logic element (the diagram is shown below). This element in logic is called "2I". When zero potential is applied to any input (or both at once), then an electric current will begin to flow through the resistor. In this case, a significant voltage drop occurs. It can be concluded that at the output of the element the potential will be equal tounit, if this is exactly applied to both inputs at the same time. In other words, with the help of such a scheme, the logical operation "2AND" is implemented.
The number of semiconductor diodes determines how many inputs the element will have. When using two semiconductors, the “2I” circuit is implemented, three - “3I”, etc. In modern microcircuits, an element with eight diodes (“8I”) is produced. a huge disadvantage of DTL logic is a very small level of load capacity. For this reason, a bipolar transistor amplifier must be connected to the logic element.
But it is much more convenient to implement logic on transistors with several additional emitters. In such TTL logic circuits, a multi-emitter transistor is used, rather than semiconductor diodes connected in parallel. This element is similar in principle to "2I". but at the output a high level of potential can be obtained only if the same value is at the same time at the two inputs. In this case, there is no emitter current, and the transitions are blocked. The figure shows a typical logic circuit using transistors.
Inverter circuits on logic elements
With the help of an amplifier, it turns out to invert the signal at the output of the component. Elements of the "AND-NOT" type are indicated in the serial microcircuits of the aircraft. For example, a microcircuit of the K155LA3 series has in its design elements of the "2I-NOT" type in the amount of four pieces. Based on this element, an inverter device is made. This uses one semiconductor diode.
If you need to mergeseveral logic elements of the "AND" type according to the "OR" circuits (or if it is necessary to implement the logic elements "OR"), then the transistors must be connected in parallel at the points indicated on the diagram. In this case, only one cascade is obtained at the output. A logical element of the "2OR-NOT" type is shown in this photo:
These elements are available in microcircuits, which are denoted by the letters LR. But the TTL logic of the "OR-NOT" type is denoted by the abbreviation LE, for example, K153LE5. It has four logical elements “2OR-NOT” built in at once.
IC logic levels
In modern technology, microcircuits with TTL logic are used, which are powered by 3 and 5 V. But only the logical level of one and zero does not depend on voltage. It is for this reason that there is no need for additional matching of microcircuits. The graph below shows the allowable voltage level at the output of the element.
Voltage in an uncertain state at the input of the microcircuit, in comparison with the output, is permissible within smaller limits. And this graph shows the boundaries of the levels of a logical unit and zero for TTL-type microcircuits.
Turning on the Schottky diode
But simple transistor switches have one big drawback - they have a saturation mode when operating in the open state. In order for excess carriers to dissolve and the semiconductor not to be saturated, a semiconductor diode is switched on between the base and the collector. The figure showsway to connect Schottky diode and transistor.
A Schottky diode has a voltage threshold of about 0.2-0.4 V, while a silicon p-n junction has a voltage threshold of at least 0.7 V. And this is much less than the lifetime of a minority type of carriers in a semiconductor crystal. The Schottky diode allows you to keep the transistor due to the low threshold for opening the junction. It is for this reason that the triode is prevented from going into mode.
What are the families of TTL microcircuits
Usually, microcircuits of this type are powered by 5 V sources. There are foreign analogues of domestic elements - the SN74 series. But after the series comes a digital number, which indicates the number and type of logical components. The SN74S00 microcircuit contains 2I-NOT logic elements. There are microcircuits whose temperature range is more extended - domestic K133 and foreign SN54.
Russian microcircuits, similar in composition to SN74, were produced under the designation K134. Foreign microcircuits, whose power consumption and speed are low, have the letter L at the end. Foreign microcircuits with the letter S at the end have domestic counterparts in which the number 1 has been replaced by 5. For example, the well-known K555 or K531. Today, several types of K1533 series microcircuits are produced, in which the speed and power consumption are very low.
CMOS logic gates
Microcircuits that have complementary transistors are based on MOS elements with p- and n-channels. With the help of onepotential, a p-channel transistor opens. When a logical "1" is formed, the upper transistor opens and the lower one closes. In this case, no current flows through the microcircuit. When a "0" is formed, the lower transistor opens and the upper one closes. In this case, current flows through the microcircuit. An example of the simplest logic element is an inverter.
Please note that CMOS ICs do not draw current in static mode. Current consumption begins only when switching from one state to another logic element. TTL logic on such elements is characterized by low power consumption. The figure shows a diagram of an element of the "NAND" type, compiled on CMOS transistors.
An active load circuit is built on two transistors. If it is necessary to form a high potential, these semiconductors open, and a low one closes. Please note that transistor-transistor logic (TTL) is based on the operation of the keys. Semiconductors in the upper arm open, and in the lower arm they close. In this case, in static mode, the microcircuit will not consume current from the power source.
