Difference between TTL CMOS ECL | TTL vs CMOS vs ECL

These are logic familities used in IC (Integrated Circuit) designs. TTL (Transistor Transistor Logic) IC technology uses bipolar transistor as principal circuit element. CMOS (Complementary Metal Oxide Semiconductor) IC technology uses FET as principal circuit element. ECL (Emitter Coupled Logic) IC technology uses bipolar transistors configured as differential amplifier. Let us compare TTL vs CMOS vs ECL logic families in order to derive difference between TTL and CMOS and ECL based on various parameters such as fan-out, power consumption, noise immunity etc.

TTL | Transistor Transistor Logic

This logic family basic TTL gate has improvement over standard DTL gate. TTL gate has three different types of output configurations:
• Open collector output
• Totem-pole output
• Three state (or tristate) output

Totem pole provides less power dissipation, higher speed of operation and high fanout.

Standard TTL series of logic family starts with suffix 74.
For example: 7404, 74S86 and 74ALS161

The other two families are for military and industrial applications.
54 - military
84 - Industrial (Discontinued)

ECL | Emitter Coupled Logic

ECL is based on use of current steering switch realised using differential transistor pair. ECL eliminates saturation and improves speed but uses more power than other logic families such as TTL and CMOS.

ECL is fastest logic family among all and this is due to avoidance of saturation region for application.

Following are characteristics of ECL logic family.
• propagation rate is about 1 to 2ns
• Noise immunity and power dissipation is worst of all the logic families.
• High Level is -0.8V and Low Level is -1.8V
• It has differential input amplifier, Internal temperature and voltage compensated bias network, emitter follower outputs

ECL gates provide both true as well as complemented outputs.

CMOS | Complementary Metal Oxide Semiconductor

MOS logic families has more delay as 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 and CMOS and ECL

Following table mentions difference between TTL and CMOS and ECL logic families.

Specification TTL ECL CMOS
Fan in Typically, the fan-in for TTL gates is relatively low, often around 2 to 4 inputs. ECL gates generally have a higher fan-in compared to TTL CMOS gates typically have a high fan-in, allowing them to accommodate a relatively large number of input signals.
Fan out 10 25 >50
Power dissipation or Power per gate (mWatt) 1-22 4-55 1 @ 1MHz
Noise Immunity Very Good Good Excellent
Propagation delay, (tPD, ns) 1.5-33 1-4 1-200
Technology Bipolar transistors and resistors Bipolar transistors Complementary MOSFETs
Voltage levels Typically operates with a 5V power supply. Operates with a lower voltage level (around -5.2V) for high-speed applications. Supports a wide range of power supply voltages, commonly 3.3V or 5V.
Temperature sensitivity Sensitive Less sensitive than TTL Generally less sensitive than TTL
Advantages and disadvantages of TTL>> ECL>> CMOS>>

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.

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