RFTTL vs CMOS vs ECL: Logic Family Comparison
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These are logic families used in IC (Integrated Circuit) designs.
- TTL (Transistor Transistor Logic): IC technology that uses bipolar transistors as the principal circuit element.
- CMOS (Complementary Metal Oxide Semiconductor): IC technology that uses FETs as the principal circuit element.
- ECL (Emitter Coupled Logic): IC technology that uses bipolar transistors configured as a differential amplifier.
Let’s compare TTL, CMOS, and ECL logic families to highlight the differences between them based on various parameters such as fan-out, power consumption, and noise immunity.
TTL (Transistor Transistor Logic)
This logic family’s basic TTL gate is an improvement over the standard DTL gate.
TTL gates have three different types of output configurations:
- Open collector output
- Totem-pole output
- Three-state (or tristate) output
Totem-pole outputs provide less power dissipation, higher speed of operation, and high fanout.
Standard TTL series of logic families start with the suffix 74. For example: 7404, 74S86, and 74ALS161. The other two families, 54 (military) and 84 (Industrial - Discontinued), were used for military and industrial applications, respectively.
ECL (Emitter Coupled Logic)
ECL is based on the use of a current steering switch realized using a differential transistor pair.
ECL eliminates saturation and improves speed but uses more power than other logic families such as TTL and CMOS. ECL is the fastest logic family among all, and this is due to the avoidance of the saturation region for application.
Following are characteristics of ECL logic family:
- Propagation rate is about 1 to 2 ns.
- Noise immunity and power dissipation are the worst of all the logic families.
- High Level is -0.8V and Low Level is -1.8V.
- It has a differential input amplifier, internal temperature and voltage compensated bias network, and emitter follower outputs.
ECL gates provide both true as well as complemented outputs.
CMOS (Complementary Metal Oxide Semiconductor)
MOS logic families have more delay compared to transistor logic families.
Following are the typical features of CMOS logic family:
- Lowest power dissipation
- Excellent noise immunity
- High packing density
- Wide range of supply voltage
- High speed
- Highest fan out among all logic families
Difference between TTL, CMOS, and ECL
The following table outlines the differences between TTL, CMOS, and ECL logic families.
| Specification | TTL | ECL | CMOS |
|---|---|---|---|
| Basic gate | NAND | OR/NOR | NAND/NOR |
| Fan in | Typically low, around 2-4 | Generally higher than TTL | Typically high |
| Fan out | 10 | 25 | >50 |
| Power dissipation (mW) | 1-22 | 4-55 | 1 @ 1MHz |
| Noise Immunity | Very Good | Good | Excellent |
| Propagation delay (ns) | 1.5-33 | 1-4 | 1-200 |
| Technology | Bipolar transistors & resistors | Bipolar transistors | Complementary MOSFETs |
| Voltage levels | Typically 5V | Around -5.2V | Commonly 3.3V or 5V |
| Temperature sensitivity | Sensitive | Less sensitive than TTL | Generally less sensitive than TTL |
Conclusion
It’s important to note that technology has evolved, and newer variations of these logic families, such as Advanced TTL, Low-power TTL, Positive ECL (PECL), Low-voltage positive ECL (LVPECL), and Advanced CMOS, have been developed to address specific requirements in different applications.
The choice of logic family depends on factors such as power consumption, speed, noise immunity, and the specific requirements of the application.