---
title: "CMOS logic level profile"
subject: "Digital Logic"
chapter: 10
key: "semester_3/digital_logic:cmos-logic-level-profile"
source: https://www.bctengineer.com/q/semester_3/digital_logic/cmos-logic-level-profile
site: BCT Engineering
---

# CMOS logic level profile

Digital Logic · Chapter 10: Digital Integrated Circuits

- Asked in: 2078 Kartik, 3 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-logic-level-profile
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-10

## Question

Draw the CMOS logic level profile for both input and output.

## Short answer

For VDD=5V, output levels: VOH(min)=4.9V, VOL(max)=0.1V; input thresholds: VIH(min)=3.5V, VIL(max)=1.5V, giving noise margins of 1.4V. The output swings nearly rail-to-rail, and input thresholds are at 30% and 70% of VDD.

## Answer

The **CMOS logic level profile** shows the voltage ranges for logic 0 and 1 at the output and at the input of a CMOS gate. Typical values for $V_{DD} = 5$ V:

```text
  Output                 Input
 5.0 +--------+         5.0 +--------+
     | HIGH   |             | HIGH   |
 4.9 +--------+ VOH(min)    |        |
     |        |  NM_H=1.4   |        |
     |invalid |         3.5 +--------+ VIH(min)
     |        |             | indet. |
     |        |         1.5 +--------+ VIL(max)
     |        |  NM_L=1.4   | LOW    |
 0.1 +--------+ VOL(max)    |        |
     | LOW    |             |        |
 0.0 +--------+         0.0 +--------+
```

- Output: $V_{OH(min)} = 4.9$ V, $V_{OL(max)} = 0.1$ V (nearly $V_{DD}$ and 0 V, since MOS outputs drive almost no DC current).
- Input: $V_{IH(min)} = 3.5$ V (70% of $V_{DD}$), $V_{IL(max)} = 1.5$ V (30% of $V_{DD}$).
- Noise margins: $NM_H = 4.9 - 3.5 = 1.4$ V and $NM_L = 1.5 - 0.1 = 1.4$ V, much larger than TTL's 0.4 V.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What are the typical CMOS output levels at VDD = 5 V?
   A. VOH(min) = 2.4 V and VOL(max) = 0.4 V
   B. VOH(min) = 4.9 V and VOL(max) = 0.1 V
   C. VOH(min) = 3.5 V and VOL(max) = 1.5 V
   D. VOH(min) = 5.0 V and VOL(max) = 1.5 V
   Answer: B. MOS outputs drive almost no DC current, so they swing nearly rail to rail. (Key point: CMOS output levels)

2. What are the CMOS input thresholds as fractions of VDD?
   A. VIH(min) about 70% of VDD and VIL(max) about 30% of VDD
   B. VIH(min) 40% and VIL(max) 20%
   C. VIH(min) 90% and VIL(max) 10%
   D. Both are 50%
   Answer: A. 3.5 V and 1.5 V at 5 V correspond to 70% and 30%. (Key point: CMOS input levels)

3. What are CMOS noise margins and how do they compare with TTL?
   A. 0.4 V each, same as TTL
   B. 1.4 V, smaller than TTL
   C. 1.4 V each, much larger than TTL's 0.4 V
   D. 0.1 V and 0.4 V
   Answer: C. NM_H = 4.9 - 3.5 and NM_L = 1.5 - 0.1, both 1.4 V. (Key point: Noise margins)

4. In the CMOS input profile, voltages between 1.5 V and 3.5 V are:
   A. Valid logic 0
   B. Valid logic 1
   C. Output-only voltages
   D. Indeterminate
   Answer: D. They lie between VIL(max) and VIH(min) and should be avoided. (Key point: Indeterminate region)

## Related in Digital Logic

- [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
- [Propagation delay; 3-input CMOS NOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apropagation-delay-cmos-nor-3input) — Digital Logic, chapter 10
- [CMOS characteristics; 2-input CMOS NOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acmos-characteristics-nor-2input) — 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
- [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
- [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

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