--- title: "CMOS NAND gate; totem-pole output" subject: "Digital Logic" chapter: 10 key: "semester_3/digital_logic:cmos-nand-totem-pole" source: https://www.bctengineer.com/q/semester_3/digital_logic/cmos-nand-totem-pole site: BCT Engineering --- # CMOS NAND gate; totem-pole output Digital Logic · Chapter 10: Digital Integrated Circuits - Asked in: 2069 Chaitra, 4+4 marks - Video: [CMOS NAND Gate Explained: Circuit, Working, Implementation, and Truth Table](https://www.youtube.com/watch?v=YAQbPbI-2XI) — 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/cmos-nand-totem-pole - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-10 ## Question Draw the schematic circuit for CMOS NAND gates. What do you mean by totem-pole output? ## Short answer A CMOS NAND gate uses two parallel PMOS and two series NMOS transistors; output Y = (AB)'. Totem-pole output is a TTL stage with stacked pull-up (Q3) and pull-down (Q4) transistors, driven in opposite phase for low output resistance and fast switching. ## Answer #### CMOS NAND gate ```text VDD VDD | | A --o| P1 B --o| P2 | | +--------+--------+ | +------------ Y = (AB)' | A --| N1 | B --| N2 | GND ``` (P = PMOS, ON when gate = 0; N = NMOS, ON when gate = 1.) The two PMOS transistors are in **parallel** between $V_{DD}$ and the output; the two NMOS transistors are in **series** between the output and ground. - **A = 0 or B = 0:** at least one PMOS is ON, connecting Y to $V_{DD}$, and the series NMOS path is broken → **Y = 1**. - **A = 1 and B = 1:** both NMOS are ON, connecting Y to ground; both PMOS are OFF → **Y = 0**. | A | B | P1 | P2 | N1 | N2 | Y | |---|---|---|---|---|---|---| | 0 | 0 | ON | ON | OFF | OFF | 1 | | 0 | 1 | ON | OFF | OFF | ON | 1 | | 1 | 0 | OFF | ON | ON | OFF | 1 | | 1 | 1 | OFF | OFF | ON | ON | 0 | So $Y = \overline{AB}$. In no input state are both networks ON, so no DC current flows from $V_{DD}$ to ground. #### Totem-pole output **Totem-pole output** is the TTL output stage in which two transistors are stacked one above the other between $V_{CC}$ and ground, like a totem pole: **Q3 (active pull-up)** on top with R4 = 130 Ω and diode D1, and **Q4 (pull-down)** below. The output Y is taken from their junction. The phase splitter Q2 drives them in opposite phase, so only one is ON at a time. ```text +5 V | R4 130 | c Q2c-b Q3 (ON for HIGH output) e | D1 +------- Y c Q2e-b Q4 (ON for LOW output) e | GND ``` | Output | Q3 | Q4 | Path | |---|---|---|---| | HIGH | ON | OFF | load connected to $V_{CC}$ via R4, Q3, D1 | | LOW | OFF | ON | load connected to ground via Q4 | **Significance:** - **Low output resistance in both states:** Q3 sources current and Q4 sinks current, so load capacitance is charged and discharged quickly → short rise and fall times, higher speed than a resistor pull-up. - **Low power:** in the LOW state Q3 is OFF, so no current flows from $V_{CC}$ through a pull-up into the ON transistor, unlike a passive (resistor) pull-up. - **D1** adds an extra 0.7 V drop so that Q3 stays OFF when Q4 is saturated. - **R4** limits the current spike that flows for a few ns when both transistors are briefly ON during switching. - **Limitation:** totem-pole outputs must not be wired together (wired-AND); if one output is HIGH and another LOW, a large current flows and may damage the gates. Open-collector or tristate outputs are used for bus connections. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How are the transistors arranged in a 2-input CMOS NAND gate? A. PMOS in series, NMOS in parallel B. PMOS in parallel, NMOS in series C. All four in series D. All four in parallel Answer: B. The parallel PMOS pull-up network gives Y = 1 if any input is 0, and the series NMOS network pulls Y low only if both inputs are 1. (Key point: CMOS NAND arrangement) 2. For A = 1 and B = 1 in the CMOS NAND, what happens? A. Both PMOS ON, output HIGH B. Both NMOS ON, both PMOS OFF, output connected to ground (Y = 0) C. One NMOS ON, output floating D. Both networks ON, large DC current Answer: B. A HIGH gate turns NMOS ON and PMOS OFF, so Y is connected to ground through the series NMOS pair. (Key point: A = B = 1 case) 3. What is a totem-pole output? A. A TTL output stage with Q3 (pull-up, with R4 and D1) stacked above Q4 (pull-down), driven in opposite phase B. A single transistor with a resistor pull-up C. An output with two transistors always ON together D. An open-collector output Answer: A. Two transistors are stacked between VCC and ground, with the output at their junction, and only one is ON at a time. (Key point: Totem-pole definition) 4. Why must totem-pole outputs not be wired together? A. The gates would run slower B. If one output is HIGH and another LOW, a large current flows and may damage the gates C. The noise margin becomes zero D. D1 would reverse bias Answer: B. A HIGH output sourcing into a LOW output sinking creates a low-resistance path from VCC to ground. Open-collector or tristate outputs are used for shared lines. (Key point: Wiring totem-poles together) ## Related in Digital Logic - [3-input TTL NAND; totem-pole output](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-nand-3input-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 - [XOR gate using minimum NAND gates](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Axor-using-nand-only) — Digital Logic, chapter 2, asked 2× - [De Morgan; NOR gate from NAND gates](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ademorgan-importance-nor-from-nand) — Digital Logic, chapter 2 - [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 - [CMOS parameters; 2-input CMOS NAND](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acmos-parameters-nand-2input) — 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 - [CMOS NAND, NOR (and NOT) circuits](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Acmos-nand-nor-circuits) — Embedded System, chapter 8, asked 2× - [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× - [Why NOR and NAND are universal gates](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Auniversal-gates-nor-nand) — Basic Electronics Engineering, chapter 6