--- title: "TTL inverter schematic and working" subject: "Digital Logic" chapter: 10 key: "semester_3/digital_logic:ttl-inverter" source: https://www.bctengineer.com/q/semester_3/digital_logic/ttl-inverter site: BCT Engineering --- # TTL inverter schematic and working Digital Logic · Chapter 10: Digital Integrated Circuits - Asked in: 2073 Shrawan, 3+4 marks - Video: [TTL Inverter with Totem Pole Output](https://www.youtube.com/watch?v=GZRmmyrCF7M) — EE Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/ttl-inverter - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-10 ## Question Draw the schematic diagram of TTL Inverter. Explain the working principle of circuit. ## Short answer With A=0, Q1 emitter conducts, base ~0.9V, so Q2 and Q4 are off, Q3 conducts, output high (~3.4V). With A=1, Q1's base-collector junction drives Q2 on, saturating Q2 and Q4, Q3 off, output low (~0.2V). The totem-pole output gives fast switching and low output resistance. ## Answer A **TTL inverter** (7404) is the TTL NAND circuit with a single-emitter input transistor. #### Schematic diagram ```text +5 V +-----------+----------+ R1 4k R2 1.6k R4 130 | | | b +-----+ c A--e Q1 | +--b Q3 c--+ c e +--b Q2 | e D1 +-----+ +---- Y | | c R3 1k +--b Q4 | e GND --------+----------+ ``` - **Q1:** input transistor (base through R1 = 4 kΩ to $V_{CC}$). - **Q2:** phase splitter (R2 = 1.6 kΩ collector, R3 = 1 kΩ emitter). - **Q3, D1, Q4:** totem-pole output (R4 = 130 Ω). #### Working principle - **A = 0 (≈ 0.2 V):** Q1's base–emitter junction is forward biased; its base is at about 0.2 + 0.7 = 0.9 V. Turning on Q2 and Q4 needs about 2.1 V at Q1's base (Q1's base–collector junction plus the base–emitter junctions of Q2 and Q4), so **Q2 and Q4 stay OFF**. Q2's collector is pulled up by R2, so **Q3 is ON** (emitter follower) and current flows from $V_{CC}$ through R4, Q3 and D1 to the load. Output **Y ≈ 3.4 V (HIGH)**. - **A = 1 (≥ 2.0 V):** Q1's emitter junction is reverse biased; R1 current flows through Q1's base–collector junction into Q2's base, and Q1's base rises to about 2.1 V. **Q2 saturates**, its emitter current through R3 drives **Q4 into saturation**, and the output is pulled to $V_{CE(sat)} \approx 0.2$ V (**LOW**). Q2's collector is at about 0.9 V, which cannot forward bias both Q3's base–emitter junction and D1 (needs about 1.4 V above the 0.2 V output), so **Q3 is OFF**. | A | Q1 | Q2 | Q3 | Q4 | Y | |---|---|---|---|---|---| | 0 | emitter conducts | OFF | ON | OFF | 1 | | 1 | inverse active | ON | OFF | ON | 0 | So $Y = \overline{A}$. The totem pole gives low output resistance in both states, so the gate switches quickly (≈ 10 ns) and can drive 10 standard TTL loads. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the TTL inverter, when A = 0 (about 0.2 V), what are the states of Q2, Q3 and Q4? A. Q2 ON, Q3 OFF, Q4 ON B. Q2 OFF, Q3 ON, Q4 OFF C. Q2 OFF, Q3 OFF, Q4 ON D. Q2 ON, Q3 ON, Q4 OFF Answer: B. Q1's base sits at about 0.9 V, too low to turn on Q2 and Q4. Q2's collector is pulled up by R2, so Q3 conducts and the output is HIGH. (Key point: Input LOW state) 2. Why do Q2 and Q4 stay OFF when A = 0? A. Q1's base is only about 0.9 V, below the roughly 1.4 V needed to turn on Q2 and Q4 B. Q1's base-collector junction is reverse biased by 5 V C. R3 shorts the base of Q4 to the supply D. D1 blocks current into Q2 Answer: A. With the emitter junction conducting, Q1's base is held at 0.2 + 0.7 = 0.9 V. Turning on Q2 and Q4 needs about 1.4 V. (Key point: Why Q2 stays off) 3. When A = 1, how does Q2 receive base current? A. Through Q1's forward-biased emitter junction B. Through R1 and Q1's base-collector junction, as the emitter junction is reverse biased C. Directly from R4 D. From the output via D1 Answer: B. A HIGH input reverse biases Q1's emitter junction, so R1 current flows through the base-collector junction into Q2's base, saturating Q2 and then Q4. (Key point: Input HIGH state) 4. What is the main advantage of the totem-pole output stage? A. It lets outputs be wired together safely B. It gives low output resistance in both states, so switching is fast C. It removes the need for a phase splitter D. It makes the gate draw no current at all Answer: B. Q3 sources and Q4 sinks current, so the output resistance is low in both HIGH and LOW states, giving fast switching and good drive. (Key point: Role of totem pole) ## Related in Digital Logic - [TTL voltage profile; tristate TTL inverter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-voltage-profile-tristate) — 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 - [TTL NOR gate; CMOS characteristics](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nor-cmos-characteristics) — Digital Logic, chapter 10 - [CMOS characteristics; TTL 2-input AND](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acmos-characteristics-ttl-and) — Digital Logic, chapter 10 - [TTL OR gate; propagation delay and power](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-or-propagation-power) — 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 ## Related in other subjects - [Working principle of dc generator with diagram](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-diagram) — Electrical Machines, chapter 3 - [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× - [TTL to RS-232 interfacing circuit](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Attl-rs232-interface) — Instrumentation II, chapter 3 - [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4×