TTL technology

TTL stands for transistor-transistor logic, meaning transistor-to-transistor logic. It is a digital electronic circuit construction technology. In components manufactured with TTLRS technology, the input and output elements of the device are bipolar transistors.

Features

• Its characteristic feeding voltage is between 4.75V and 5.25V (as seen, a very narrow range). Normally TTL works with 5V.
• The logical levels are defined by the voltage range between 0.0V and 0.8V for the L (low) state and 2,2V and Vcc for the H (high) state.
• The transmission rate between the logical states is its best base, although this feature makes you increase your consumption as your biggest enemy. Reason for which different versions of TTL have appeared such as FAST, LS, S, among others and lately the CMOS: HC, HCT and HCTLS. In some cases you can reach just over 400 MHz.
• TTL output signals are degraded quickly if not transmitted through additional transmission circuits (they cannot travel more than 2 m by cable without serious losses).

History

Although TTL technology originates from the Sylvania studios, it was popularized by Signetics due to its higher speed and noise immunity than its DTL predecessor, mainly offered by Fairchild Semiconductor and Texas Instruments. Texas Instruments immediately went on to manufacture TTL, with its 74xx family that would become an industry standard.

Families

TTL technology circuits are usually prefixed with the number 74 (54 in military and industrial series). Next, a code of one or several figures that represents the family and later one of 2 to 4 with the model of the circuit.

With respect to families, it is worth distinguishing:

• TTL: standard series.
• TTL-L (low power): low consumption series.
• TTL-S (schottky): Quick series (schotky diodes).
• TTL-AS (advanced schottky): enhanced version of the previous series.
• TTL-LS (low power schottky): combination of L and S technologies (is the most extended family).
• TTL-ALS (advanced low power schottky): enhanced version of the LS series.
• TTL-F (FAST: fairchild advanced schotky).
• TTL-AF (advanced FAST): enhanced version of the F series.
• TTL-HCT (high speed C-MOS): HC series equipped with logical levels compatible with TTL.

Versions

To the initial 7400 family, or 74N, was soon added a slower but low-power version, the 74L and its fast counterpart, the 74H, which had the base of the transistors doped with gold to produce recombination centers and decrease the half-life of minority carriers in the base. But the speed problem comes from the fact that it is a saturated family, that is, the transistors go from cutting to saturation. But a saturated transistor contains an excess charge at its base that must be removed before it starts to cut off, prolonging its response time. The saturation state is characterized by having the collector at less voltage than the base. Then a diode between the base and the collector diverts the excess current, preventing the introduction of excess charges in the base. Due to their low forward voltage, Schottky barrier diodes are used. This is how you have the 74S and 74LS, Schottky and low power Schottky families. The 74S and 74LS completely displaced the 74L and 74H, due to their better product delay·consumption. Improvements in the manufacturing process led to the reduction in the size of the transistors that allowed the development of three new families: 74F (FAST: Fairchild Advanced Schottky Technology) from Fairchild and 74AS (Advanced Schottky) and 74ALS (Advanced Low Power Schottky) from Texas Instruments. Later, National Semiconductor redefined the 74F for the case of buffers and interfaces, becoming 74F(r).

NAND door in standard TTL technology (N).

Technology

TTL technology is characterized by having three stages, the first being the one that names it:

• Input Stage per issuer: A multi-layer transistor is used instead of the DTL diode matrix.
• Phase Separator: it is a transistor connected in common emitter that produces in its collector and emitter signals in contraphase.
• Driver: consists of several transistors, separated into two groups. The first is connected to the phase separator emitter and drain the current to produce the low level to the output. The second group is connected to the phase divider collector and produces the high level.

This general configuration varies slightly between devices in each family, mainly the output stage, which depends on whether they are buffers or not and whether they are open collector, ThreeState, etc.

There are major variations between the different families: 74N, 74L and 74H that differ mainly in the value of the bias resistors, but the 74LS (and not 74S) lack the characteristic multi-emitter transistor of TTL. Instead they carry a Schottky diode array (like DTL). This allows them to accept a wider range of input voltages, up to 15V on some devices, to facilitate their interface with CMOS.

It is also quite common, in circuits connected to buses, to place a PNP transistor at the input of each line to decrease the input current and thus load the bus less. There are interface devices that integrate matching impedances to the bus to reduce reflections or increase speed.

Applications

In addition to the LSI and MSI circuitry described here, LS and S technologies have also been used in:

• Microprocessors, such as 8X300, Signetics, AMD 2900 family and others.
• RAM memories.
• PROM memories.
• Programmable array logicor PAL, consisting of a PROM that interconnects inputs and a certain number of logical doors.