Digital Logic Families PDF

**UNIT -- 4 Digital Logic Families** 1. **Classification of logic families:** C:\\Users\\Dell\\Desktop\\logic-families-classification.jpg [ A **logic family** is a collection of different ][integrated circuit chips](https://en.wikipedia.org/wiki/Integrated_circuit)[ that have similar input,...

**UNIT -- 4 Digital Logic Families** 1. **Classification of logic families:** C:\\Users\\Dell\\Desktop\\logic-families-classification.jpg [ A **logic family** is a collection of different ][integrated circuit chips](https://en.wikipedia.org/wiki/Integrated_circuit)[ that have similar input, output, and internal circuit characteristics, but they perform different **logic gate** functions such as **AND**, **OR**, **NOT**, etc.] The idea is that different logic gate functions, when fabricated in the form of an integrated circuit with the same approach, or which belongs to the same logic family, will have identical electrical characteristics (electrically compatible with each other). These families may vary by speed, power consumption, cost, voltage and current levels. In case of integrated circuits belonging to different logic families, [digital system](https://en.wikipedia.org/wiki/Digital_Systems) should ensure compatibility interfacing techniques. And that is the reason why we must be clear about different logic families and use the best combination of integrated circuits during the design of a digital system. **CLASSIFICATION OF LOGIC FAMILY** Logic families are mainly classified as Bipolar Logic Families and Unipolar Logic Families. 1. **Bipolar Logic Families:** It mainly uses bipolar devices like diodes, transistors in addition to passive elements like resistors and capacitors. These are sub classified as saturated bipolar logic family and unsaturated bipolar logic family. 1. Saturated Bipolar Logic Family: In this family the transistors used in ICs are driven into saturation. For example: 1. Transistor-Transistor Logic (TTL) 2. Resistor-Transistor Logic (RTL) 3. Direct Coupled Transistor Logic (DCTL) 4. Diode Transistor Logic (DTL) 5. High Threshold Logic(HTL) 6. Integrated Injection Logic (IIL or I 2 L) 2. **Unsaturated bipolar logic family:** In this family the transistors used in IC is not driven into saturation. For example: 7. Schottky TTL 8. Emitter Coupled Logic(ECL) 2. **Unipolar Logic Families:** It mainly uses Unipolar devices like MOSFETs in addition to  passive elements like resistors and capacitors. These logic families have the advantages of high speed and lower power consumption than Bipolar families. These are classified as: 3. PMOS or P-Channel MOS Logic Family 4. NMOS or N-Channel MOS Logic Family 5. CMOS Logic Family 3. **Characteristics of digital ICs-** **1) Speed of operation:**\ The speed of a digital circuit is specified in terms of the propagation delay time. If the propagation delay time is lower, the speed of the IC is higher.\ \ **2) Power of dissipation:**\ This is the amount of power dissipated in an IC. It is determined by the current, I~CC~, that it draws from the V~CC~ supply, and is given by V~CC~ \* I~CC~.\ I~CC~ is the average value of I~CC~(0) and I~CC~(1). This power is specified in milliwatts.\ \ **3) Figure of merit:**\ The figure of merit of a digital IC is defined as the product of speed and power. The speed is specified in terms of propagation delay time expressed in nanoseconds. *Figure of merit = propagation delay time (ns) \* power (mW)* It is specified in pico joules (ns \* mW = pJ).\ A low value of speed-power product is desirable. In a digital circuit, if it is desired to have a high speed, low propagation delay time, then there is a corresponding increase in the power dissipation and vice versa.\ \ **4) Fan Out:**\ Fan out is the number if similar gates which can be driven by a gate. High fan-out is advantages because it reduces the need for additional drivers to drive more gates.\ \ **5) Current and voltage parameters:** - - **6) Noise immunity:**\ The input and output voltage levels are defined above are shown in figure. Stray electric and magnetic fields may induce unwanted voltages, known as noise, on the connecting wires between logic circuits. This may cause the voltage at the input to a logic circuit to drop below V~lH~ or rise above V~lL~ and may produce undesired operation.  -------------------------------------- **Noise immunity** So, the circuit\'s ability to tolerate noise signals is referred to as the noise immunity, a quantitative measure of which is called Noise Margin.\ \ **7) Operating temperature range:**\ The temperature range by which a IC functions properly must be known. The accepted temperature ranges are: 0 to 70 degree Celsius for consumer and industrial applications and -55 degree Celsius to 125 degree Celsius.\ \ **8) Power supply requirements:**\ The supply voltage and the amount of power required by an IC are important characteristics required to choose the proper power supply. **3.TTL logic** The Transistor-Transistor Logic (TTL) is a logic family made up of BJTs (bipolar junction transistors). As the name suggests, the transistor performs two functions like logic as well as amplifying. The best examples of TTL are logic gates namely the 7402 NOR Gate & the 7400 NAND gate. TTL logic includes several transistors that have several emitters as well as several inputs. The types of TTL or transistor-transistor logic mainly include Standard TTL, Fast TTL, Schottky TTL, High power TTL, Low power TTL & Advanced Schottky TTL. #### Characteristics of TTL The characteristics of TTL include the following. 1. 2. 3. 4. **4. Operation of TTL NAND gate** C:\\Users\\Dell\\Desktop\\ma8TguN.png - A two input TTL NAND is shown above. A and B are two inputs while Y is the output. - Operation of the gate:  These two diagrams are only for your understanding, need not draw in exam. #### #### 4.Open Collector Output The main feature is that its output is 0 when low and floating when high. Usually, an external Vcc may be applied.  Open Collector Output of Transistor-Transistor Logic Transistor Q1 behaves as a cluster of diodes placed back to back. With any of the input at logic low, the corresponding emitter-base junction is forward biased and the voltage drop across the base of Q1 is around 0.9V, not enough for the transistors Q2 and Q3 to conduct. Thus the output is either floating or Vcc, i.e. High level. Similarly, when all inputs are high, all base-emitter junctions of Q1 are reverse biased and transistor Q2 and Q3 get enough base current and are in saturation mode. The output is at logic low. (For a transistor to go to saturation, collector current should be greater than β times the base current). #### Applications The applications of open collector output include the following. - - - **5. The wired AND** - A **wired logic connection** is a [logic gate](https://en.wikipedia.org/wiki/Logic_gate) that implements [boolean algebra (logic)](https://en.wikipedia.org/wiki/Boolean_algebra_(logic)) using only passive components such as [diodes](https://en.wikipedia.org/wiki/Diode) and [resistors](https://en.wikipedia.org/wiki/Resistors). - A wired logic connection can create an [AND](https://en.wikipedia.org/wiki/AND_gate) or an [OR gate](https://en.wikipedia.org/wiki/OR_gate). - The limitations are the inability to create a [NOT](https://en.wikipedia.org/wiki/NOT_gate) gate and the lack of level restoration. **6.Tri-State logic**.  It provides 3 state output like the following - - - - **7. CMOS logic** The logic gates are the basic building blocks of all digital circuits and computers. These logic gates are implemented using transistors called MOSFETs. A MOSFET transistor is a voltage-controlled switch. The MOSFET acts as a switch and turns on or off depending on whether the voltage on it is either high or low. There are two types of MOSFETs: NMOS and PMOS. The NMOS turns on when the voltage is high and off when the voltage is low. The PMOS, on the other hand, turns on whenever the voltage is low and goes off as the voltage goes high. When the two are used together to realize the logic gates, they are called CMOS (Complementary MOS). The reason they are called complementary is that NMOS and PMOS work in a complementary fashion. When the NMOS switch turns on, the PMOS gets off, and vice-versa.  ### 8. CMOS Inverter: The CMOS inverter is shown below. It consists of a series connection of a PMOS and an NMOS. VDD represents the voltage of logic 1, while the ground represents logic 0. Whenever the input is high or 1, the NMOS is switched on while the PMOS is turned off. Thus output Y is directly connected to the ground and thus comes to be logic 0. When the input is logic 0, the reverse happens -- NMOS goes off and PMOS goes on. This provides a direct path between VDD and output Y. Hence Y becomes high. This is the basic principle of operation of a CMOS inverter.  ### 9.NAND Gate and NOR Gate: NAND and NOR gates can be easily realized using CMOS logic as shown below. C:\\Users\\Dell\\Desktop\\nandnor.gif **10. Interfacing CMOS and TTL:**  **11. Comparison of Logic Families** ------------------------------------