Transistor–Transistor Logic (TTL) - Seminar Paper

Introduction

Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT) and resistors. It is called transistor–transistor logic because both the logic gating function (e.g., AND) and the amplifying function are performed by transistors (contrast this with RTL and DTL).

TTL is notable for being a widespread integrated circuit (IC) family used in many applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, synthesizers, etc. The designation TTL is sometimes used to mean TTL-compatible logic levels, even when not associated directly with TTL integrated circuits, for example as a label on the inputs and outputs of electronic instruments. 

TTL was invented in 1961 by James L. Buie of TRW, "particularly suited to the newly developing integrated circuit design technology", and it was originally named transistor-coupled transistor logic (TCTL). The first commercial integrated-circuit TTL devices were manufactured by Sylvania in 1963, called the Sylvania Universal High-Level Logic family (SUHL). The Sylvania parts were used in the controls of the Phoenix missile. TTL became popular with electronic systems designers after Texas Instruments introduced the 5400 series of ICs, with military temperature range, in 1964 and the later 7400 series, specified over a narrower range, and with inexpensive plastic packages in 1966. 

The Texas Instruments 7400 family became an industry standard. Compatible parts were made by Motorola, AMD, Fairchild, Intel, Intersil, Signetics, Mullard, Siemens, SGS-Thomson and National Semiconductor, and many other companies, even in the Eastern Bloc (Soviet Union, GDR, Poland, Bulgaria). Not only did others make compatible TTL parts, but compatible parts were made using many other circuit technologies as well. At least one manufacturer, IBM, produced non-compatible TTL circuits for its own use; IBM used the technology in the IBM System/38, IBM 4300, and IBM 3081. 

The term "TTL" is applied to many successive generations of bipolar logic, with gradual improvements in speed and power consumption over about two decades. The most recently introduced family, 74AS/ALS Advanced Schottky, was introduced in 1985. As of 2008, Texas Instruments continues to supply the more general-purpose chips in numerous obsolete technology families, albeit at increased prices. Typically, TTL chips integrate no more than a few hundred transistors each. Functions within a single package generally range from a few logic gates to a microprocessor bit-slice. TTL also became important because its low cost made digital techniques economically practical for tasks previously done by analog methods. 

The Kenbak-1, ancestor to the first personal computers, used TTL for its CPU instead of a microprocessor chip, which was not available in 1971. The 1973 Xerox Alto and 1981 Star workstations, which introduced the graphical user interface, used TTL circuits integrated at the level of ALUs and bitslices, respectively. Most computers used TTL-compatible logic between larger chips well into the 1990s. Until the advent of programmable logic, discrete bipolar logic was used to prototype and emulate microarchitectures under development.

Implementation

Fundamental TTL gate

Two-input TTL NAND gate with a simple output stage (simplified).

TTL is a natural successor of DTL since it is based on the same fundamental concept - implementing the logic gate function by using the base-emitter junctions of a multiple-emitter transistor as switching elements like DTL input diodes. This IC structure is functionally equivalent to multiple transistors where the bases and collectors are tied together. The output of the simple TTL gate is buffered, like DTL, by a common emitter amplifier.

Input logical ones. When all the inputs are held at high voltage the base-emitter junctions of the multiple-emitter transistor are backward-biased. In contrast with DTL, small (about 10 μA) "collector" currents are drawn by the inputs since the transistor is in a reverse-active mode (with swapped collector and emitter). The base resistor in combination with the supply voltage acts as a substantially constant current source. It passes current through the base–collector junction of the multiple-emitter transistor and the base-emitter junction of the output transistor thus turning it on; the output voltage becomes low (logical zero).

Input logical zero. If one input voltage becomes zero, the corresponding base-emitter junction of the multiple-emitter transistor connects in parallel to the two connected in series junctions (the base-collector junction of the multiple-emitter transistor and the base-emitter junction of the second transistor). The input base-emitter junction steers all the base current of the output transistor to the input source (the ground). The base of the output transistor is deprived of current causing it to go into cut-off  and the output voltage becomes high (logical one). During the transition the input transistor is briefly in its active region; so it draws a large current away from the base of the output transistor and thus quickly discharges its base. This is a critical advantage of TTL over DTL that speeds up the transition over a diode input structure.

The main disadvantage of TTL with a simple output stage is the relatively high output resistance at output logical "1" that is completely determined by the output collector resistor. It limits the number of inputs that can be connected (the fanout). Some advantage of the simple output stage is the high voltage level (up to VCC) of the output logical "1" when the output is not loaded.

Logic of this type is most frequently encountered with the collector resistor of the output transistor omitted, making an open collector output. This allows the designer to fabricate logic by connecting the open collector outputs of several logic gates together and providing a single external pull-up resistor. If any of the logic gates becomes logic low (transistor conducting), the combined output will be low. Examples of this type of gate are the 7401 and 7403 series.

TTL with a "totem-pole" output stage

Standard TTL NAND with a "totem-pole" output stage, one of four in 7400

To solve the problem with the high output resistance of the simple output stage the second schematic adds to this a "totem-pole" ("push-pull") output. It consists of the two n-p-n transistors V3 and V4, the "lifting" diode V5 and the current-limiting resistor R3 (see the figure on the right). It is driven by applying the same current steering idea. 

When V2 is "off", V4 is "off" as well and V3 operates in active region as a voltage follower producing high output voltage (logical "1"). When V2 is "on", it activates V4, driving low voltage (logical "0") to the output. V2 and V4 collector–emitter junctions connect V4 base-emitter junction in parallel to the series-connected V3 base-emitter and V5 anode-cathode junctions. V3 base current is deprived; the transistor turns "off" and it does not impact on the output. In the middle of the transition, the resistor R3 limits the current flowing directly through the series connected transistor V3, diode V5 and transistor V4 that all are conducting. It also limits the output current in the case of output logical "1" and short connection to the ground. The strength of the gate may be increased without proportionally affecting the power consumption by removing the pull-up and pull-down resistors from the output stage. 

The main advantage of TTL with a "totem-pole" output stage is the low output resistance at output logical "1". It is determined by the upper output transistor V3 operating in active region as a voltage follower. The resistor R3 does not increase the output resistance since it is connected in the V3 collector and its influence is compensated by the negative feedback. A disadvantage of the "totem-pole" output stage is the decreased voltage level (no more than 3.5 V) of the output logical "1" (even, if the output is unloaded). The reason of this reduction are the voltage drops across the V3 base-emitter and V5 anode-cathode junctions.

Interfacing considerations

Like DTL, TTL is a current-sinking logic since a current must be drawn from inputs to bring them to a logic 0 level. At low input voltage, the TTL input sources current which must be absorbed by the previous stage. The maximum value of this current is about 1.6 mA for a standard TTL gate. The input source has to be low-resistive enough (< 800 Ω) so that the flowing current creates only a negligible voltage drop (< 0.8 V) across it, for the input to be considered as a logical "0". TTL inputs are sometimes simply left floating to provide a logical "1", though this usage is not recommended.

Standard TTL circuits operate with a 5-volt power supply. A TTL input signal is defined as "low" when between 0 V and 0.8 V with respect to the ground terminal, and "high" when between 2.2 V and 5 V (precise logic levels vary slightly between sub-types and by temperature). TTL outputs are typically restricted to narrower limits of between 0 V and 0.4 V for a "low" and between 2.6 V and 5 V for a "high", providing 0.4V of noise immunity. Standardization of the TTL levels was so ubiquitous that complex circuit boards often contained TTL chips made by many different manufacturers selected for availability and cost, compatibility being assured; two circuit board units off the same assembly line on different successive days or weeks might have a different mix of brands of chips in the same positions on the board; repair was possible with chips manufactured years (sometimes over a decade) later than original components. Within usefully broad limits, logic gates could be treated as ideal Boolean devices without concern for electrical limitations.

In some cases (e.g., when the output of a TTL logic gate needs to be used for driving the input of a CMOS gate), the voltage level of the "totem-pole" output stage at output logical "1" can be increased up to VCC by connecting an external resistor between the V3 collector and the positive rail. It pulls up the V5 cathode and cuts-off the diode. However, this technique actually converts the sophisticated "totem-pole" output into the simple one having significant output resistance (determined by the external resistor).

Packaging

Like most integrated circuits of the period 1965–1990, TTL devices were usually packaged in through-hole, dual in-line packages with between 14 and 24 lead wires, usually made of epoxy plastic (PDIP) or sometimes of ceramic (CDIP). Beam-lead chip dice without packages were made for assembly into larger arrays as hybrid integrated circuits. Parts for military and aerospace applications were packaged in flat packs, a form of surface-mount package, with leads suitable for welding or soldering to printed circuit boards. Today, many TTL-compatible devices are available in surface-mount packages, which are available in a wider array of types than through-hole packages.

TTL is particularly well suited to integrated circuits because the inputs of a gate may all be integrated into a single base region to form a multiple-emitter transistor. Such a highly customized part might increase the cost of a circuit where each transistor is in a separate package, but, by combining several small on-chip components into one larger device, it conversely reduces the cost of implementation on an IC.

Comparison with other logic families

TTL devices consume substantially more power than equivalent CMOS devices at rest, but power consumption does not increase with clock speed as rapidly as for CMOS devices.[21] Compared to contemporary ECL circuits, TTL uses less power and has easier design rules but is substantially slower. Designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy, but level-shifting devices are required between the two logic families. TTL is less sensitive to damage from electrostatic discharge than early CMOS devices.

Due to the output structure of TTL devices, the output impedance is asymmetrical between the high and low state, making them unsuitable for driving transmission lines. This drawback is usually overcome by buffering the outputs with special line-driver devices where signals need to be sent through cables. ECL, by virtue of its symmetric low-impedance output structure, does not have this drawback.

The TTL "totem-pole" output structure often has a momentary overlap when both the upper and lower transistors are conducting, resulting in a substantial pulse of current drawn from the supply. These pulses can couple in unexpected ways between multiple integrated circuit packages, resulting in reduced noise margin and lower performance. TTL systems usually have a decoupling capacitor for every one or two IC packages, so that a current pulse from one chip does not momentarily reduce the supply voltage to the others.

Several manufacturers now supply CMOS logic equivalents with TTL-compatible input and output levels, usually bearing part numbers similar to the equivalent TTL component and with the same pinouts. For example, the 74HCT00 series provides many drop-in replacements for bipolar 7400 series parts, but uses CMOS technology.

Sub-types

Successive generations of technology produced compatible parts with improved power consumption or switching speed, or both. Although vendors uniformly marketed these various product lines as TTL with Schottky diodes, some of the underlying circuits, such as used in the LS family, could rather be considered DTL.

Variations of and successors to the basic TTL family, which has a typical gate propagation delay of 10ns and a power dissipation of 10 mW per gate, for a power-delay product (PDP) or switching energy of about 100 pJ, include:

· Low-power TTL (L), which traded switching speed (33ns) for a reduction in power consumption (1 mW) (now essentially replaced by CMOS logic)

· High-speed TTL (H), with faster switching than standard TTL (6ns) but significantly higher power dissipation (22 mW)

· Schottky TTL (S), introduced in 1969, which used Schottky diode clamps at gate inputs to prevent charge storage and improve switching time. These gates operated more quickly (3ns) but had higher power dissipation (19 mW)

· Low-power Schottky TTL (LS) — used the higher resistance values of low-power TTL and the Schottky diodes to provide a good combination of speed (9.5ns) and reduced power consumption (2 mW), and PDP of about 20 pJ. Probably the most common type of TTL, these were used as glue logic in microcomputers, essentially replacing the former H, L, and S sub-families.

· Fast (F) and Advanced-Schottky (AS) variants of LS from Fairchild and TI, respectively, circa 1985, with "Miller-killer" circuits to speed up the low-to-high transition. These families achieved PDPs of 10 pJ and 4 pJ, respectively, the lowest of all the TTL families.

· Most manufacturers offer commercial and extended temperature ranges: for example Texas Instruments 7400 series parts are rated from 0 to 70°C, and 5400 series devices over the military-specification temperature range of −55 to +125°C.

· Radiation-hardened devices are offered for space applications

· Special quality levels and high-reliability parts are available for military and aerospace applications.

· Low-voltage TTL (LVTTL) for 3.3-volt power supplies and memory interfacing.

Applications

Before the advent of VLSI devices, TTL integrated circuits were a standard method of construction for the processors of mini-computer and mainframe processors; such as the DEC VAX and Data General Eclipse, and for equipment such as machine tool numerical controls, printers and video display terminals. As microprocessors became more functional, TTL devices became important for "glue logic" applications, such as fast bus drivers on a motherboard, which tie together the function blocks realized in VLSI elements.

Analog applications

While originally designed to handle logic-level digital signals, a TTL inverter can be biased as an analog amplifier. Connecting a resistor between the output and the input biases the TTL element as a negative feedback amplifier. Such amplifiers may be useful to convert analog signals to the digital domain but would not ordinarily be used where analog amplification is the primary purpose. TTL inverters can also be used in crystal oscillators where their analog amplification ability is significant.