What is TTL? – Transistor-Transistor Logic

In our some previous articles, in context of Arduino for beginners, we discussed about serial connection, UART, SPI, I2C protocol. Another commonly heard phrase is TTL. What is TTL? Transistor-Transistor Logic or TTL is a family of logic circuits used in electronics invented in the 1960s. We will try to keep the article as brief and easy as possible.

What is TTL?

TTL is a digital logic design where transistors act on DC pulses. Some TTL logic gates are fabricated onto an integrated circuit (IC). TTL ICs commonly bear 4 digit numbers beginning with 74 or 54. Like 7490 counter.

TTL family is made with bipolar transistor technology and tending to disappear because of their high energy consumption (compared to CMOS circuits). TTL technology is standardized to a supply voltage of 5 V. A TTL signal is defined as a low logic level between 0 and 1.4 V, and as a high logic level between 2.4 V and 5 V.

TTL is characterized by high switching speed, and some immunity to noise. Main drawback is the fact that circuits using TTL tend to draw more current than equivalent circuits using metal oxide semiconductor (MOS) logic.

Advantages

• This family of components combines good switching speed with low transfer time.
• The immunity is good provided that the power supply is decoupled from each circuit by a filtering capacitor.
• The air intakes, with no fixed state, are in logic state “1” by default (they only discharge a significant current if they are set to zero and they are almost equivalent to resistances towards Vcc = 5 V ) .

Disadvantages

• The TTL power supply must be accurate: +5 V ± 5%, compared to CMOS circuits which have a much larger supply voltage range (+3 to +18 V ). In case of non-compliance with this requirement, there is a risk, at best, erratic operation of the circuit, and at worst, a partial or complete destruction of the circuit.
• The bipolar technique is great consumer of electric current; TTL memories are certainly fast, but can hardly be rescued for a long time in case of power failure.
• The signals transmitted by the TTL circuits can not be transmitted without additional transmission circuits over long distances without loss: maximum length approximately 15 m.

Example

Two-input TTL NAND gate with a simple output stage : TTL inputs are the emitters of bipolar transistors. In the case of NAND inputs, the inputs are the emitters of multiple-emitter transistors, functionally equivalent to multiple transistors where the bases and collectors are tied together. The output is buffered by a common emitter amplifier.