--- title: "CMOS characteristics; TTL 2-input AND" subject: "Digital Logic" chapter: 10 key: "semester_3/digital_logic:cmos-characteristics-ttl-and" source: https://www.bctengineer.com/q/semester_3/digital_logic/cmos-characteristics-ttl-and site: BCT Engineering --- # CMOS characteristics; TTL 2-input AND Digital Logic · Chapter 10: Digital Integrated Circuits - Asked in: 2078 Bhadra, 3+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/cmos-characteristics-ttl-and - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-10 ## Question Explain the characteristics of CMOS logic families. Draw the schematic diagram of TTL 2-input AND gate and explain with necessary diagrams. ## Short answer CMOS features very low static power (nW), dynamic power CL·VDD²·f, a wide supply range (3–15 V for 4000B), high noise margin (~30% VDD), fan-out ~50, and is sensitive to static. A TTL 2-input AND gate has a NAND front-end (multi-emitter Q1 and phase splitter Q2) followed by an inverter (Q3) and totem-pole output; Y = A·B. When both inputs are high, Q2 conducts, turning Q3 off and giving Y=1; otherwise Q2 is off and Y=0. ## Answer #### Characteristics of CMOS logic families **CMOS** (complementary MOS) logic uses a PMOS pull-up and an NMOS pull-down network in each gate. Its main characteristics: | Characteristic | CMOS (4000B / 74HC, 5 V) | |---|---| | Supply voltage | 3–15 V (4000B), 2–6 V (74HC) | | Static power | almost zero (nW per gate): one network is always OFF | | Dynamic power | $P_D = C_L V_{DD}^2 f$, grows with frequency | | Noise margin | high, about 30% of $V_{DD}$ (≈ 1.4 V at 5 V) | | Propagation delay | 25–100 ns (4000B), 8–10 ns (74HC) | | Fan-out | about 50 (inputs draw almost no current) | | Input impedance | very high (≈ $10^{12}$ Ω) | | Weakness | damaged by static charge; unused inputs must not float | #### TTL 2-input AND gate A **TTL AND gate** (7408) is a NAND input stage followed by one more inverting stage and the totem-pole output, so the output is the complement of NAND. ```text Input section (NAND stage): +5 V +----------+ R1 4k R2 4k | | b +------- X A--e Q1 | B--e c--+ c +--b Q2 e | R4 1k | GND ``` ```text Output section (common to AND and OR): +5 V +------+------+ R3 1.6k R5 130 | | +------+ c | +----b Q4 c e X -----b Q3 | e D1 | | +------+ +----- Y | | c R6 1k +----b Q5 | e GND -----+-------------+ ``` Node X (collector of Q2) drives the base of Q3. Q1 = multi-emitter input, Q2 = NAND stage, Q3 = phase splitter, Q4–D1–Q5 = totem-pole output. **Operation:** - **Any input LOW:** Q1's base is held at about 0.9 V, so **Q2 is OFF**. Node X is pulled up by R2 and supplies base current to Q3, so **Q3 turns ON**, which turns **Q5 ON** and Q4 OFF → **Y = 0**. - **A = B = 1:** Q1's base current goes into Q2, so **Q2 saturates** and pulls node X (Q3's base) LOW → **Q3 OFF** → Q5 OFF, Q4 ON → **Y = 1**. | A | B | Q2 | Q3 | Q5 | Y | |---|---|---|---|---|---| | 0 | 0 | OFF | ON | ON | 0 | | 0 | 1 | OFF | ON | ON | 0 | | 1 | 0 | OFF | ON | ON | 0 | | 1 | 1 | ON | OFF | OFF | 1 | So $Y = AB$. Because of the extra stage, a TTL AND gate has a slightly longer delay than a TTL NAND. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is CMOS static power almost zero? A. Its supply voltage is tiny B. One of the pull-up/pull-down networks is always OFF C. It uses bipolar transistors in saturation D. Its inputs are shorted to ground Answer: B. No DC path exists between VDD and ground in steady state. (Key point: CMOS static power) 2. What is the input impedance of CMOS? A. Very high, about 10^12 ohm B. About 1 kilo-ohm C. About 130 ohm D. About 4 kilo-ohm Answer: A. The insulated MOS gate draws almost no current. (Key point: Input impedance) 3. How is a TTL AND gate (7408) built from the NAND? A. Two NAND gates in parallel B. NOR stage followed by a diode C. NAND input stage followed by an extra inverting stage and then a totem-pole output D. Open-collector NAND with pull-up resistor Answer: C. An extra inversion converts NAND to AND. (Key point: AND gate structure) 4. With any input LOW in the AND gate, why is Y = 0? A. Q2 saturates and turns Q4 ON B. Q3 is OFF so Q5 is OFF C. Q1's base is at VCC D. Q2 is OFF, node X turns Q3 ON, which turns Q5 ON and Q4 OFF Answer: D. Q2 OFF lets R2 drive Q3, so the pull-down Q5 conducts. (Key point: AND operation, inputs low) 5. With A = B = 1, why is Y = 1? A. Q2 saturates, pulling X low, so Q3 and Q5 are OFF and Q4 is ON B. Q2 is OFF so Q3 is ON C. Q3 saturates and Q5 conducts D. Q1 conducts through its emitters Answer: A. With Q3 OFF, Q5 is OFF and Q4 pulls Y HIGH. (Key point: AND operation, all high) 6. Why does a TTL AND gate have a slightly longer delay than a NAND? A. It has no totem pole B. It has an extra inverting stage C. It uses larger supply voltage D. It has more input emitters Answer: B. One more stage adds delay. (Key point: Delay) ## Related in Digital Logic - [2-input TTL NAND; CMOS characteristics](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nand-cmos-characteristics) — Digital Logic, chapter 10 - [TTL NOR gate; CMOS characteristics](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nor-cmos-characteristics) — Digital Logic, chapter 10 - [2-input TTL NOR gate](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nor-2input) — Digital Logic, chapter 10 - [TTL NOR gate; 74XX characteristics](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nor-74xx-characteristics) — Digital Logic, chapter 10 - [3-input CMOS gate](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acmos-three-input-gate) — Digital Logic, chapter 10 - [2-input TTL NAND gate](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nand-2input) — Digital Logic, chapter 10 ## Related in other subjects - [CMOS inverter / NOT gate operation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acmos-inverter-operation) — Basic Electronics Engineering, chapter 3, asked 4× - [Schematic diagram of rural gravity scheme](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arural-gravity-scheme-diagram) — Water Supply Engineering, chapter 1, asked 5×