Advantages and Disadvantages of TTL over CMOS and ECL

TTL stands for Transistor Transistor Logic. It is a type of digital logic family that uses bipolar junction transistors and resistors to implement logic functions. Developed in the 1960s, TTL has been widely used in various electronic applications and is known for its simplicity and versatility. Let us explore advantages and disadvantages of TTL over ECL and CMOS.

• The basic TTL logic circuit is NAND gate. It uses BJTs internally.
• Binary value of logic 1 is represented by 2.4 V (between 2.4V and Vcc ( usually 5V), must be more than 2V ) and logic 0 is represented by 0.4 V (between 0V and 0.8V, must be less than 0.8V) .

Advantages of TTL

Following points summarize TTL advantages over ECL and CMOS:
1. It is most rugged meaning least susceptible to electrical damage.
2. It has strong drive capability.
3. It is faster in some versions.
4. TTL requires only one supply voltage unlike CMOS which needs two (i.e. Vdd and Vss).
Note : Vdd is between 3V and 16V and Vss is ground.
5. It has noise immunity better than ECL but lesser than CMOS.
6. It has power per gate of about 1-22 mW, which is in between CMOS and ECL.
7. The average propagation delay time is nominal and between 1.5 to 33 ns. This is better compare to CMOS. It refers to the time taken for a change in the input of a logic gate to result in a corresponding change in the output. It should be as short as possible.

Disadvantages of TTL

Following points summarize TTL disadvantages over ECL and CMOS:
1. Cosumes more power compares to CMOS family and hence it is not suitable for battery powered devices.
2. It has limited fan out (~ 10) , which is less compare to CMOS. It has to be noted that any logic circuit having 'N' fan out can drive 'N' number of logic inputs.
3. TTL logic has limited speed compared to ECL, hence TTL is not preferable for applications requiring very high speed operation.
4. TTL circuits are sensitive to temperature variations, which may affect their performance in environments with varying temperature conditions.

Conclusion : TTL (Transistor-Transistor Logic) offers advantages such as simplicity, ease of use, and versatility in various applications. However, TTL has some notable disadvantages, including moderate power consumption, limited speed compared to other logic families like ECL, and sensitivity to temperature variations.

Also refer difference between TTL ECL and CMOS>>.

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