--- title: "TTL voltage profile; tristate TTL inverter" subject: "Digital Logic" chapter: 10 key: "semester_3/digital_logic:ttl-voltage-profile-tristate" source: https://www.bctengineer.com/q/semester_3/digital_logic/ttl-voltage-profile-tristate site: BCT Engineering --- # TTL voltage profile; tristate TTL inverter Digital Logic · Chapter 10: Digital Integrated Circuits - Asked in: 2075 Chaitra, 2+6 marks - Video: [U4 L5 | Tri State TTL | TTL Tri- state inverter | Logic family | Transistor Transistor logic |TTL](https://www.youtube.com/watch?v=KK3-A-lkKlo) — Techno Tutorials ( e-Learning) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/ttl-voltage-profile-tristate - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-10 ## Question Describe the voltage profile of TTL. Explain the working principle of tristate TTL inverter. ## Short answer TTL voltage profile: VOH(min)=2.4 V, VOL(max)=0.4 V; VIH(min)=2.0 V, VIL(max)=0.8 V, giving noise margins of 0.4 V. A tristate TTL inverter adds an enable input E: when E=1 the circuit acts as a normal inverter; when E=0 both output transistors (Q3 and Q4) are turned off, placing the output in a high-impedance (Hi-Z) state. ## Answer #### Voltage profile of TTL The **voltage profile** of TTL ($V_{CC} = 5$ V) shows the voltage ranges that represent logic 0 and logic 1 at the output and at the input of a gate: ```text Output (driving) Input (receiving) 5.0 +--------+ 5.0 +--------+ | HIGH | | HIGH | 2.4 +--------+ VOH(min) 2.0 +--------+ VIH(min) |invalid | NM_H=0.4 | indet. | 0.4 +--------+ VOL(max) 0.8 +--------+ VIL(max) | LOW | NM_L=0.4 | LOW | 0.0 +--------+ 0.0 +--------+ ``` - Output HIGH ≥ $V_{OH(min)} = 2.4$ V; output LOW ≤ $V_{OL(max)} = 0.4$ V. - Input read as HIGH if ≥ $V_{IH(min)} = 2.0$ V; as LOW if ≤ $V_{IL(max)} = 0.8$ V; 0.8–2.0 V is indeterminate. - Noise margins: $NM_H = 2.4 - 2.0 = 0.4$ V and $NM_L = 0.8 - 0.4 = 0.4$ V. #### Tristate TTL inverter A **tristate (three-state) inverter** has, besides the normal HIGH and LOW outputs, a third **high-impedance (Hi-Z)** state in which the output is effectively disconnected. It is used to connect many outputs to a common bus, with only one enabled at a time. ```text +5 V +-----------+----------+ R1 4k R2 1.6k R4 130 | | | b +-----+ c A--e Q1 | +--b Q3 E--e c--+ c | e | +--b Q2 | D1 | e | | | +--+ | +---- Y | | | | c | R3 1k +--)--b Q4 | | | e | GND D2 | | | GND +----- E line ----+ ``` Q1 has a second emitter tied to the enable line E; diode D2 has its anode at Q3's base (Q2 collector) and its cathode on E. **Working:** - **E = 1 (enabled):** the second emitter of Q1 and diode D2 are reverse biased and have no effect. The circuit works as a normal TTL inverter: A = 0 → Q2, Q4 OFF, Q3 ON → Y = 1; A = 1 → Q2, Q4 ON, Q3 OFF → Y = 0. - **E = 0 (disabled):** the E emitter of Q1 conducts, so Q2 and Q4 are **OFF** whatever A is. D2 also conducts and clamps Q3's base to about 0.9 V, which is not enough for Q3 and D1, so **Q3 is OFF** too. With both totem-pole transistors OFF the output floats: **Hi-Z**. | E | A | Q3 | Q4 | Y | |---|---|---|---|---| | 1 | 0 | ON | OFF | 1 | | 1 | 1 | OFF | ON | 0 | | 0 | x | OFF | OFF | Hi-Z | In ICs such as the 74125/74126 buffers the external enable pin may be active-LOW, with an internal inverter driving this E line. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which TTL levels are correct? A. VOH(min) 2.4 V, VOL(max) 0.4 V, VIH(min) 2.0 V, VIL(max) 0.8 V B. VOH(min) 2.0 V, VOL(max) 0.8 V, VIH(min) 2.4 V, VIL(max) 0.4 V C. VOH(min) 3.5 V, VIL(max) 1.5 V only D. VOH(min) 4.9 V, VOL(max) 0.1 V Answer: A. Output limits are tighter than input limits; the difference is the noise margin. (Key point: TTL profile) 2. What is the TTL noise margin and how is it found? A. 1.4 V, from VCC minus 3.6 V B. 0.4 V for both levels, from output limits minus input limits C. 0.8 V, from VIL minus 0 V D. 2.0 V, from VIH minus 0 V Answer: B. 2.4-2.0 = 0.4 V and 0.8-0.4 = 0.4 V. (Key point: Noise margin) 3. What is the third state of a tristate inverter and why is it used? A. A weak HIGH, to save power B. An indeterminate level between 0.8 and 2.0 V C. High impedance, so many outputs can share a bus with only one enabled D. A short-circuit state for faster switching Answer: C. In Hi-Z the output is effectively disconnected. (Key point: Tristate purpose) 4. When E = 0, how does the circuit reach Hi-Z? A. Q4 is saturated and Q3 is ON B. Only Q3 is turned OFF; Q4 follows A C. The E emitter is reverse biased D. Q1's E emitter turns Q2, Q4 OFF and D2 clamps Q3's base, so Q3 is OFF too Answer: D. Both totem-pole transistors OFF means the output floats. (Key point: Disabled operation) 5. When E = 1, how does the circuit behave? A. Output is Hi-Z B. Like a normal TTL inverter, since the E emitter and D2 are reverse biased C. Output is always HIGH D. Output is always LOW Answer: B. A = 0 gives Y = 1 and A = 1 gives Y = 0. (Key point: Enabled operation) ## Related in Digital Logic - [TTL inverter schematic and working](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-inverter) — Digital Logic, chapter 10 - [TTL voltage profile; TTL-to-CMOS interface](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-voltage-profile-ttl-cmos-interface) — Digital Logic, chapter 10 - [TTL voltage profile and noise margin; Gray code](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-voltage-profile-noise-margin) — 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 - [Single-phase inverter fundamental output voltage](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Asingle-phase-inverter-r-fundamental) — Power Electronics, chapter 5 - [180° three-phase inverter and fundamental phase voltage](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ainverter-180-fundamental-per-phase) — Power Electronics, chapter 5 - [Zener voltage regulator working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Azener-voltage-regulator) — Basic Electronics Engineering, chapter 2 - [GPS, components and working principle](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-components-working-principle) — Surveying II, chapter 8, asked 19×