--- title: "3-input TTL NAND; totem-pole output" subject: "Digital Logic" chapter: 10 key: "semester_3/digital_logic:ttl-nand-3input-totem-pole" source: https://www.bctengineer.com/q/semester_3/digital_logic/ttl-nand-3input-totem-pole site: BCT Engineering --- # 3-input TTL NAND; totem-pole output Digital Logic · Chapter 10: Digital Integrated Circuits - Asked in: 2079 Bhadra, 6 marks - Video: [TTL NAND Gate with Totem Pole Output: Circuit and Working](https://www.youtube.com/watch?v=ye_wOuWRUtI) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/ttl-nand-3input-totem-pole - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-10 ## Question Explain the operation of three input TTL NAND gate. What is the significance of totem-pole output in it? ## Short answer A 3-input TTL NAND uses a multi-emitter transistor with three emitters; any input low gives high output, all inputs high give low output. The totem-pole output provides active pull-up (Q3) and pull-down (Q4), low output resistance, fast switching, and good fan-out, but cannot be wired-AND. ## Answer A **3-input TTL NAND gate** (7410) is the same as the standard TTL NAND circuit, but the input transistor Q1 has **three emitters**, one for each input A, B, C. ```text +5 V +-----------+----------+ R1 4k R2 1.6k R4 130 | | | b +-----+ c A--e Q1 | +--b Q3 B--e c--+ c e C--e +--b Q2 | e D1 +-----+ +---- Y | | c R3 1k +--b Q4 | e GND --------+----------+ ``` #### Operation - **Any input LOW:** that emitter junction of Q1 is forward biased and Q1's base is held at about 0.9 V. No current reaches Q2's base, so **Q2 and Q4 are OFF**. Q2's collector is high (through R2), so **Q3 is ON** and the output is **HIGH** (≈ 3.4 V, ≥ 2.4 V guaranteed). - **All inputs HIGH:** all three emitter junctions are reverse biased. R1 current flows through Q1's base–collector junction into Q2, so **Q2 and Q4 saturate** and the output is **LOW** (≈ 0.2 V). Q2's collector is about 0.9 V, too low to turn on Q3 and D1, so **Q3 is OFF**. | A, B, C | Q2, Q4 | Q3 | Y | |---|---|---|---| | at least one input 0 | OFF | ON | 1 | | all inputs 1 | ON | OFF | 0 | So $Y = \overline{ABC}$. #### Significance of the totem-pole output - Q3 (pull-up) and Q4 (pull-down) are stacked; only one is ON at a time. Q3 actively **sources** current for a HIGH output, Q4 **sinks** current for a LOW output. - Both states have **low output resistance**, so the load capacitance charges and discharges quickly: fast switching and good fan-out (10). - **Lower power** than a resistor pull-up, since no resistor current flows in the LOW state. - **D1** keeps Q3 OFF when Q4 is ON; **R4 (130 Ω)** limits the current spike when both conduct for an instant during switching. - Drawback: totem-pole outputs cannot be tied together (wired-AND), as a HIGH output shorted to a LOW one draws heavy current. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the 3-input TTL NAND (7410) differ from the 2-input one? A. Q1 has three emitters, one per input B. It uses three phase splitters in parallel C. It uses three totem poles D. It uses three PMOS transistors Answer: A. Only the input transistor changes: a three-emitter Q1. (Key point: 3-input NAND structure) 2. When A = B = C = 1, what happens in the 7410? A. Q2 and Q4 are OFF and Y is HIGH B. Q3 saturates and Y is HIGH C. Emitter junctions are reverse biased, R1 current drives Q2 and Q4 into saturation, Q3 is OFF, Y is LOW D. Only Q4 conducts but Q2 is OFF Answer: C. Current through Q1's base-collector junction saturates Q2 and Q4, giving Y = 0 (about 0.2 V). (Key point: All inputs HIGH) 3. Why is the output HIGH when any input is LOW? A. Q4 is ON and pulls the output to VCC B. Q1 base is held near 0.9 V, so Q2 and Q4 are OFF and Q3 conducts C. Q2 saturates and turns on Q3 D. D1 is reverse biased and shorts the output Answer: B. No current reaches Q2's base; its collector rises through R2 and turns Q3 ON. (Key point: Any input LOW) 4. What is the main advantage of a totem-pole output? A. It allows outputs to be wired together B. It removes the need for a phase splitter C. It gives a high-impedance state D. Low output resistance in both states, giving fast switching and good fan-out Answer: D. Q3 sources and Q4 sinks current actively, so load capacitance charges and discharges quickly. (Key point: Totem-pole significance) 5. What is the purpose of D1 and R4 in the totem pole? A. D1 keeps Q3 OFF when Q4 is ON; R4 limits the current spike during switching B. D1 limits current; R4 keeps Q3 OFF C. Both set the input threshold D. Both provide the high-impedance state Answer: A. D1 adds a 0.7 V drop; R4 (130 ohm) limits the transient current when both conduct briefly. (Key point: D1 and R4) 6. Why can't totem-pole outputs be wired together? A. They would produce a Hi-Z state B. They would increase noise margin C. A HIGH output shorted to a LOW one draws heavy current and may damage the gates D. They would double the propagation delay Answer: C. Wired-AND needs open-collector outputs; totem poles would fight each other. (Key point: Drawback) ## Related in Digital Logic - [CMOS NAND gate; totem-pole output](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acmos-nand-totem-pole) — Digital Logic, chapter 10 - [TTL NOR gate; totem pole](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nor-totem-pole) — 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 - [Propagation delay; 2-input TTL NAND](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apropagation-delay-ttl-nand) — Digital Logic, chapter 10 - [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 NAND gate; transistor switch](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nand-transistor-switch) — Digital Logic, chapter 10 ## Related in other subjects - [X-OR gate using NAND gates only](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Axor-using-nand) — Basic Electronics Engineering, chapter 6