Understanding IC Logic Family Performance

Comparing different Integrated Circuit (IC) logic families is crucial for selecting the right components for a digital circuit design. One of the key performance parameters used for comparison is the rise time. Rise time is defined as the time it takes for the output voltage of a logic gate to transition from 10% to 90% of its final high level when the input changes state.

A shorter rise time indicates faster switching speed for the logic gate. The question asks us to compare the given logic families based on their rise time in ascending order, which means listing them from the fastest (lowest rise time) to the slowest (highest rise time).

Comparing Logic Families by Rise Time

Let's look at the characteristics of each logic family mentioned:

• (A) MECL (Motorola Emitter Coupled Logic): MECL is a bipolar logic family known for its extremely high speed. It achieves fast switching by avoiding saturation of transistors. MECL families typically have very low rise times, often in the sub-nanosecond range.
• (B) CMOS (Complementary Metal-Oxide Semiconductor): CMOS logic uses both NMOS and PMOS transistors. While known for very low power consumption and high noise immunity, traditional or older CMOS families are generally slower than bipolar logic like TTL and ECL. Their rise times can be significantly higher, often in the range of several nanoseconds to tens of nanoseconds, although modern high-speed CMOS variants are much faster. For a general comparison like this, standard CMOS speed is usually considered.
• (C) Low power Schottky TTL (LS-TTL): This is a variant of the standard Transistor-Transistor Logic (TTL) family. LS-TTL uses Schottky diodes to prevent transistor saturation, improving speed compared to standard TTL, while also reducing power consumption compared to Schottky TTL (S-TTL). LS-TTL offers a good balance of speed and power, with rise times typically in the range of a few nanoseconds.
• (D) PECL (Positive Emitter Coupled Logic): PECL is a modern form of ECL logic that uses a positive power supply voltage, making it more compatible with standard positive power supply systems compared to the negative voltage required by older MECL. PECL maintains the high-speed characteristics of ECL and is often considered one of the fastest logic families available today, with rise times typically comparable to or slightly lower than MECL, in the sub-nanosecond range.

Ordering by Ascending Rise Time

Based on the typical speeds and rise times:

• ECL families (PECL, MECL) are the fastest, having the lowest rise times (sub-nanosecond). PECL is often considered slightly faster than MECL.
• TTL families like LS-TTL are slower than ECL but faster than general-purpose CMOS (few nanoseconds).
• Standard CMOS is generally the slowest among these for switching speed (tens of nanoseconds).

So, ordering from lowest rise time (fastest) to highest rise time ( slowest) we get:

1. PECL (D) - Fastest, lowest rise time
2. MECL (A) - Very fast, slightly higher rise time than PECL
3. Low power Schottky TTL (C) - Faster than CMOS, slower than ECL
4. CMOS (B) - Slowest, highest rise time

The ascending order of rise time is therefore (D), (A), (C), (B).

Comparing with Options

Let's compare our derived order (D), (A), (C), (B) with the given options:

• Option 1: (D), (A), (C), (B)
• Option 2: (B), (C), (A), (D)
• Option 3: (C), (B), (D), (A)
• Option 4: (B), (D), (A), (C)

Our derived order matches Option 1.

Based on the comparison and typical characteristics, the correct order of logic families based on ascending rise time (fastest to slowest) is PECL, MECL, Low power Schottky TTL, and CMOS. This corresponds to the option sequence (D), (A), (C), (B).

Logic Family Rise Time Comparison

The final answer is $\boxed{(D), (A), (C), (B)}$.

Revision Table: Logic Family Comparison

Additional Information on Digital Logic Families

Digital logic families are groups of logic gates fabricated using the same basic circuit technology. They define electrical characteristics such as voltage levels for HIGH and LOW states, power consumption, speed (propagation delay, rise time, fall time), noise immunity, and fan-out. Understanding these characteristics is essential for designing reliable digital systems.

• Propagation Delay: The time it takes for the output of a logic gate to change state after the input has changed. This is often specified as average propagation delay ($\tau_{pd}$). Shorter propagation delay means higher speed.
• Rise Time ($\tau_r$): Time for the output to go from 10% to 90% of its final high value.
• Fall Time ($\tau_f$): Time for the output to go from 90% to 10% of its final low value.
• Noise Immunity: The ability of a gate to withstand unwanted voltage fluctuations (noise) on its inputs without changing its output state incorrectly.
• Fan-out: The maximum number of standard logic inputs that the output of a gate can drive reliably.

ECL and PECL are known for their high speed due to their non-saturating nature, but they consume more power and have lower noise margins compared to CMOS. CMOS offers excellent power efficiency and noise immunity, making it suitable for battery-powered devices, but historically has been slower than bipolar logic. TTL, like LS-TTL, represents a middle ground in terms of speed, power, and noise immunity, and was widely used for many years.