2.6 Logic Families and ICs

2.6 Logic Families and ICs

Introduction to Logic Families and ICs

Digital circuits form the core of modern computing, communication, and control systems. These circuits are built from basic logic gates (AND, OR, NOT, etc.), and their physical implementation technology defines a logic family. Each family has distinct characteristics in speed, power consumption, noise immunity, and integration density. The evolution from simple Resistor-Transistor Logic (RTL) to sophisticated CMOS-based families represents a relentless pursuit of higher performance and lower power. This unit traces this technological journey, examines the key parameters that define a logic family's suitability for an application, and delves into the circuit-level operation of foundational families like DTL and TTL, providing insight into the hardware basis of digital logic.

---

1. Logic Family Evolution and Classification

1. Definition: A logic family is a collection of different integrated circuit (IC) chips that share a common fabrication technology and have compatible input/output logic voltage levels, enabling them to be interconnected to build complex digital systems.

2. Evolution Timeline and Classification:

   • Discrete Component Era:

     • Diode Logic (DL): Used diodes to perform AND/OR functions. Lacked signal restoration and isolation.

   • Early Bipolar Families:

     • Resistor-Transistor Logic (RTL): First family to use transistors for inversion. Simple but slow.

     • Diode-Transistor Logic (DTL): Improved noise margin over RTL by adding diodes for level shifting.

     • Transistor-Transistor Logic (TTL): Became the industry standard for decades. Used multi-emitter transistors for higher speed and better fan-out.

   • MOS-Based Families:

     • PMOS/NMOS: Used only P-type or N-type MOSFETs. Simpler fabrication than CMOS but had static power dissipation.

     • Complementary MOS (CMOS): Used both P-type and N-type MOSFETs. Revolutionized digital design with its extremely low static power consumption, high noise margin, and scalability. Became the dominant technology.

   • Advanced/BiCMOS Families:

     • BiCMOS: Combines bipolar transistors (for high-speed output drive) and CMOS (for low-power logic). Used in high-performance applications.

     • Advanced TTL (Schottky TTL - 74S, Low-Power Schottky - 74LS): TTL variants using Schottky diodes to prevent transistor saturation, improving speed.

     • Advanced CMOS (HC, HCT, AC, ACT): Successors to the original 4000-series CMOS, offering speeds comparable to TTL while maintaining low power.

3. Broad Classification:

   • Bipolar Families: RTL, DTL, TTL, ECL. Use bipolar junction transistors (BJTs). Generally faster but consume more power.

   • MOS Families: PMOS, NMOS, CMOS. Use Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). Generally offer higher packing density and lower power.

   • BiCMOS Families: Combine bipolar and MOS on the same chip for optimal performance.

---

2. Logic Family Characteristics

The choice of a logic family for a specific application depends on evaluating the following key parameters:

1. Speed (Propagation Delay, $t_{pd}$):

   • Definition: The average time delay for a signal to propagate from the input to the output of a logic gate. $t_{pd} = \frac{t_{PHL} + t_{PLH}}{2}$.

   • Significance: Determines the maximum operating frequency of the system. Lower $t_{pd}$ means higher speed.

   • Typical Ranges: TTL: ~10 ns, CMOS (HC): ~10 ns, Advanced CMOS (AC): <5 ns.

2. Power Dissipation:

   • Static Power (Quiescent Power): Power consumed when the gate is in a stable logic state (no switching). CMOS has negligible static power; TTL has significant static power.

   • Dynamic Power: Power consumed during the switching transition, due to charging/discharging of load capacitances and short-circuit current. Proportional to the switching frequency ($f$) and load capacitance ($C_L$). $P_{dynamic} = C_L V_{DD}^2 f$

   • Power-Delay Product (PDP): A figure of merit combining speed and power. $PDP = P_{avg} \times t_{pd}$ (measured in picojoules, pJ). Lower PDP is better.

3. Noise Margin:

   • Definition: The maximum amount of spurious voltage (noise) that can be superimposed on a logic input signal without causing an undesirable change in the output state. It measures the circuit's immunity to noise.

   • Key Levels:

     • $V_{OH(min)}$: Minimum guaranteed output voltage in HIGH state.

     • $V_{OL(max)}$: Maximum guaranteed output voltage in LOW state.

     • $V_{IH(min)}$: Minimum input voltage guaranteed to be recognized as a HIGH.

     • $V_{IL(max)}$: Maximum input voltage guaranteed to be recognized as a LOW.

   • Calculations:

     • High-Level Noise Margin ($NM_H$): $NM_H = V_{OH(min)} - V_{IH(min)}$

     • Low-Level Noise Margin ($NM_L$): $NM_L = V_{IL(max)} - V_{OL(max)}$

   • Significance: CMOS typically has larger noise margins (~30% of $V_{DD}$) than TTL (~0.4V), making it more robust.

4. Fan-out:

   • Definition: The maximum number of standard logic inputs of the same family that a single gate output can drive reliably while maintaining correct logic levels.

   • Determined by the ratio of the output current capability (source/sink) to the input current requirement of a driven gate.

   • TTL has a typical fan-out of 10. CMOS has a very high fan-out for DC operation (due to high input impedance), but is limited by capacitive loading at high frequencies.

5. Power Supply Voltage and Compatibility:

   • TTL: Standardized at 5V ±10%. Inputs are not tolerant of voltages above $V_{CC}$.

   • CMOS: Originally 3-15V (4000 series). Modern families (74HC) operate at 2-6V. Many are designed to be TTL-compatible (e.g., 74HCT accepts TTL input levels with a 5V supply).

---

3. Simple Gate Circuits

3.1 Diode-Transistor Logic (DTL)

1. Basic NAND Gate Circuit:

   • Input Stage: Multiple input diodes ($D_A, D_B$) connected to a positive supply via a resistor ($R_1$). This performs an AND function on the inputs (anode common).

   • Level-Shifting Stage: One or two diodes ($D_1, D_2$) and a resistor ($R_2$) connected between the input stage and the base of a transistor. This shifts the voltage level to ensure proper transistor biasing and improves noise margin.

   • Output/Inverting Stage: A single npn bipolar transistor ($Q_1$) with a collector resistor ($R_C$). Acts as a NOT gate/inverter.

2. Operation:

   • Output LOW (0): If all inputs are HIGH ($V_{CC}$), all input diodes are reverse-biased. Current flows through $R_1$, the level-shifting diodes, and into the base of $Q_1$, saturating it. Output $V_{CE(sat)} \approx 0.2V$.

   • Output HIGH (1): If any input is LOW (ground), the corresponding input diode conducts. The voltage at point P drops to ~0.7V, which is insufficient to forward bias the level-shifting diodes and the transistor's base-emitter junction. $Q_1$ is cut off, and output pulls HIGH through $R_C$.

3. Characteristics:

   • Advantage: Better noise margin than RTL due to the level-shifting diodes.

   • Disadvantage: Slower than TTL due to charge storage in the saturated transistor and the use of resistors.

3.2 Resistor-Transistor Logic (RTL)

1. Basic NOR Gate Circuit:

   • Input Stage: Multiple input resistors ($R_{in}$) connecting the inputs to the base of a single npn transistor.

   • Output Stage: A collector resistor ($R_C$) connected to $V_{CC}$.

2. Operation:

   • Output LOW (0): If all inputs are LOW (0V), all transistor base-emitter junctions are reverse-biased or at zero bias. The transistor is in cut-off. Output pulls HIGH ($V_{CC}$) through $R_C$.

   • Output HIGH (1): If any input is HIGH ($V_{CC}$), current flows through the corresponding input resistor into the base, driving the transistor into saturation. Output drops to $V_{CE(sat)} \approx 0.2V$.

   • Note: This is a positive-logic NOR gate. (If any input is HIGH, output is LOW).

3. Characteristics:

   • Advantage: Simple structure, low component count.

   • Disadvantages:

     • Poor Noise Margin: Input LOW level is too close to the transistor's turn-on threshold (~0.6V).

     • Low Fan-out: Input loads the driving source.

     • High Power Consumption: Due to resistor current paths when the transistor is on.

     • Slow Speed: Due to saturated transistor operation and RC time constants.

3.3 Transistor-Transistor Logic (TTL)

1. Fundamental Innovation: Replacement of the input diodes and level-shifting network of DTL with a multi-emitter bipolar transistor (for NAND) acting as the input stage. This is the defining feature of TTL.

2. Basic TTL NAND Gate (Standard 7400) - Three Stages:

   • Input Stage ($Q_1$): A multi-emitter npn transistor.

     • Each emitter acts as an input. The base-collector junction of $Q_1$ acts like a forward-biased diode.

   • Phase-Splitter Stage ($Q_2$): Provides complementary signals to drive the output stage.

   • Totem-Pole Output Stage ($Q_3, Q_4, D_1$):

     • Active Pull-Up: $Q_3$ and $D_1$ pull the output HIGH quickly when $Q_4$ is OFF.

     • Active Pull-Down: $Q_4$ (saturating) pulls the output LOW strongly.

     • Prevents Current Shoot-Through: $Q_3$ and $Q_4$ are never fully ON simultaneously, minimizing short-circuit current.

3. Operation (NAND):

   • Output LOW (0): If all inputs are HIGH (>2V), the base-emitter junctions of $Q_1$ are reverse-biased. Current flows from $V_{CC}$ through $R_1$ and the base-collector junction of $Q_1$ (forward-biased), turning ON $Q_2$ and $Q_4$. $Q_4$ saturates, pulling output LOW. $Q_3$ is OFF.

   • Output HIGH (1): If any input is LOW (~0.2V), the corresponding base-emitter junction of $Q_1$ is forward-biased, clamping its base voltage to ~0.9V. This is insufficient to turn on the series combination of $Q_1$'s B-C junction, $Q_2$'s B-E, and $Q_4$'s B-E. $Q_2$ and $Q_4$ turn OFF. $Q_3$ turns ON via $R_2$, pulling the output HIGH through $D_1$.

4. Characteristics:

   • High Speed: Active pull-up/pull-down provides fast switching and good drive capability. Use of non-saturating variants (Schottky TTL) further increases speed.

   • Good Fan-out: Typically 10.

   • Moderate Power Dissipation: Higher than CMOS but improved over RTL/DTL.

   • Standardized Logic Levels: $V_{OH(min)} = 2.4V$, $V_{OL(max)} = 0.4V$, $V_{IH(min)} = 2.0V$, $V_{IL(max)} = 0.8V$.

5. Sub-families:

   • 74LS (Low-Power Schottky): Used Schottky-clamped transistors and higher resistor values. Popular compromise between speed and power.

   • 74S (Schottky): Faster but higher power.

   • 74ALS (Advanced Low-Power Schottky): Improved version of LS.

Conclusion: The study of logic families reveals the critical engineering trade-offs inherent in digital hardware design. From the simplicity of RTL to the optimized performance of TTL and the power efficiency of CMOS, each family represents a solution tailored to the technological and application constraints of its era. Understanding their internal circuitry—like the multi-emitter transistor of TTL—provides a deep appreciation for how abstract Boolean logic is physically realized, forming the essential foundation for designing and interfacing digital systems.