Transistor–transistor logic

The transistor-transistor logic (TTL) is a circuitry ( logic family) for logic circuits ( gates), in which are used as active component of the circuit planar NPN bipolar transistors. Usually a multi-emitter transistor used at the input so that only one transistor is required for a plurality of inputs.

• 3.1 Low-power Schottky TTL
• 3.2 Low Voltage TTL
• 3.3 Old types and their designations
• 3.4 Current Types

History

The TTL technology was invented in 1961 by James L. Buie at TRW. The first commercial circuits, the company produced Sylvania Electric Products. A further development is also based on bipolar transistors with lower power consumption, the integrated injection logic ( I 2 L ) dar.

Standard TTL

Standard TTL circuitry is designed to operate at a supply voltage of 5 V with an error of 5%. Against low supply voltage responsive to the TTL technology relatively tolerant so that in most cases the operation is possible, for example, at a 4.5 volt flat battery. Even lower values ​​affect, for example, the capacity of the outputs from ( fanout ), so that in general more stable operation is guaranteed.

A high voltage is defined as a high level ( positive logic, a logic 1 ), a low voltage is referred to as low level ( logical 0 in positive logic ). The circuits are so dimensioned that the input voltages UE <0.8 V as a low level, and the UE > 2.0 V to be recognized as a high level. The output voltage UA is typically <0.4 V for the low level and > 2.4 V for the high level when the permitted load. The static noise margin is therefore both for high- and for low level 0.4 V.

Logical blocks in TTL technology have the advantage that they are less sensitive to electrostatic discharge as CMOS devices. The disadvantage is due to the current-controlled transistors in a CMOS compared to significantly higher power consumption ( power consumption) in a static operation.

The picture shows the structure of a TTL NAND gate. V1 is the multi-emitter transistor, U1 and U2 are the input voltages. A special feature of the TTL circuit is that unconnected inputs act as if they were on a high level. Are practically unused inputs are set to a fixed potential, so as to ensure that the circuit is operating correctly. Unconnected inputs can worsen the massive passive noise immunity of a circuit.

Operation

TTL gates work like DTL gate. One difference is in the execution of the diode gate and the amplifier. The amplifier consists of the driving transistor V2 and a push-pull output stage ( totem-pole circuit ).

The unit is controlled by U1 or U2 to a low level ( or mass ) are placed. Thus V1 is conductive because now flows through R1, a base current. The base of V2 is thereby placed near ground ( UV1Sat ), which inhibits V2. This is the basis of V3 on high, the V4 to low. So V3 directs and sets the output high. Are connected with the inputs is high, V2 is supplied through the base- collector path of V1 and current conducting. V3 and V4 is disabled conductive. Only in this state, the output is set to low.

In the "open collector " type (open collector ) is missing V3, V4 of the collector is thus open out to the exit. In this case, an external "pull up" resistor must be connected in place of R3. This design allows multiple outputs to be connected in parallel to a " wired-AND " ( wired AND ). Each of the thus parallel gates adapted to receive the flow of a subsequent gate, without being influenced by the other, and so turn the following input to low.

Variants

Low -power Schottky TTL

To prevent saturation of the transistors can be connected in parallel in the base -collector path of the Schottky diode, so that the voltage of the base can never be more than 0.3 V above the potential of the collector. This results in a Schottky transistor. This parallel circuit prevents a decrease of the collector-emitter voltage drops below 0.3 V. The dimensioning can be made ​​much higher impedance for this type of circuit, resulting in a significantly lower static power consumption. At the gate delay has no influence. Is necessary for the level shift diode is replaced by a Darlington circuit in the example shown.

Low Voltage TTL

Low voltage TTL ( LVTTL ) is a special type of transistor-transistor logic ( logic family), wherein the supply voltage of 5 V is reduced to 3.3V.

Old types and their designations

Standard TTL ICs can be recognized by a designation of the form 74xx or 74xxx, which refers " 74 " on the logic family and xx / xxx on the gate type ( eg xx = "00" corresponds to NAND). Most modules are also available as 54xx for the military temperature range or as 84xx for the industrial temperature range. Built in TTL technology are also the less common 49xx series and the 75xx series, the interface comprising primarily level converter and other matching circuits.

The names of the variants are based generally on the standard type to which the block is pin-and function- compatible, which variant is characterized by inserted letters. The supply voltage range, and the signal levels are not necessary compatible. Besides the previously mentioned there are many more TTL variants. For example, for 7400 compatible:

• 74L00: Low-power TTL with lower power consumption at a lower switching speed
• 74H00: High-speed TTL with much higher switching speed at higher power consumption
• 74S00: Schottky TTL with higher switching speed in a higher power consumption
• 74F00: Fast- Schottky
• 74AS00: Advanced Schottky

The CMOS 40xx series from the 1980s are characterized by a wide supply voltage of 3 V to 15 V. The cutoff frequency is 1 MHz, which is why playing 40xx devices in new developments virtually no role.

Current types

Common TTL modules are labeled 74nnxx, with the number specified xx the block. Blocks of type 74xx are obsolete.

• 74LSxx: ( low power Schottky ): successor to the 74xx, precursor of the 74- CMOS devices
• 74HCxx: Standard Logic Devices (2012 ). U_V = 2 V - 6 V,
• All unused inputs must lie on defined potential ( or -).
• Max frequency: 25 MHz

• U_V = 4.5 V - 5.5 V

Related logic families

As a precursor of the TTL logic families, the family resistor -transistor logic, and the diode - transistor logic can be viewed. These two logic families are outdated and have now virtually irrelevant.

Close to the TTL family, the slow interference-free logic is used that has been used in the past for special applications. Today, this logic family also has virtually no appreciable significance.