---
title: "Short note: EEG recorder"
subject: "Biomedical Instrumentation"
chapter: 4
key: "semester_7/biomedical_instrumentation:short-note-eeg-recorder"
source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/short-note-eeg-recorder
site: BCT Engineering
---

# Short note: EEG recorder

Biomedical Instrumentation · Chapter 4: Bioelectric Signals Measurement and Recording System

- Asked in: 2075 Chaitra, 4 marks
- Video: [Electroencephalography (EEG Machine) | Part 1 | Biomedical Engineers TV](https://www.youtube.com/watch?v=WENh6_JaJQk) — Biomedical Engineers TV
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/short-note-eeg-recorder
- Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-4

## Question

Write a short note on EEG Recorder.

## Short answer

An EEG recorder captures brain electrical activity (10-100 µV, 0.5-70 Hz) via 21 Ag/AgCl electrodes placed per the 10-20 system. The signal passes through a differential preamplifier (high gain, CMRR >100 dB), filters (0.5-70 Hz bandpass, 50 Hz notch), main amplifier, and is recorded on paper at 30 mm/s or digitally. Brain waves include delta (0.5-4 Hz), theta (4-8 Hz), alpha (8-13 Hz), and beta (>13 Hz). Used for epilepsy, tumors, sleep disorders, coma, and brain death diagnosis.

## Answer

An **EEG (electroencephalograph)** records the electrical activity of the brain from electrodes placed on the scalp. The amplitude is very small, 10 to 100 µV, with a frequency range of 0.5 to 70 Hz.

#### Block diagram
```text
 Scalp electrodes (10-20 system)
        |
 [Electrode selector / montage switch]
        |
 [Pre-amplifier (differential)] -- [Calibrator]
        |
 [Filters: LPF 70 Hz, HPF 0.5 Hz, 50 Hz notch]
        |
 [Main amplifier / driver] --> [Pen recorder / ADC + display]
```
#### Parts
- **Electrodes:** Ag/AgCl cup or disc electrodes, 21 in number, placed by the international **10–20 system** and attached with conductive paste or held in a cap; the ear lobes are used as reference, and there is a ground electrode.
- **Electrode selector:** chooses a bipolar or referential montage.
- **Differential preamplifier:** high gain (about 1000), high CMRR (>100 dB), high input impedance (>10 MΩ).
- **Filters:** remove noise, and a notch filter removes the 50 Hz mains interference.
- **Recorder:** a pen recorder on paper moving at 30 mm/s, or digital storage with a computer display.

#### Brain waves
| Wave | Frequency | State |
|---|---|---|
| Delta | 0.5–4 Hz | deep sleep |
| Theta | 4–8 Hz | drowsiness |
| Alpha | 8–13 Hz | relaxed, eyes closed |
| Beta | >13 Hz | alert, mental activity |

**Uses:** diagnosis of epilepsy, brain tumours, sleep disorders, coma, and brain death.

## Check yourself

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

1. What are the typical amplitude and frequency range of the EEG?
   A. 10 to 100 µV; 0.5 to 70 Hz
   B. 1 to 5 mV; 20 Hz to 10 kHz
   C. 1 mV; 0.05 to 100 Hz
   D. 10 mV; 100 Hz to 1 kHz
   Answer: A. EEG is a very small signal, 10 to 100 µV, within 0.5 to 70 Hz. (Key point: EEG signal)

2. Which preamplifier properties are needed for EEG?
   A. Single-ended, low gain, low input impedance
   B. Differential, high gain, high CMRR and high input impedance
   C. Differential, unity gain, low CMRR
   D. Single-ended, high gain, low CMRR
   Answer: B. The tiny signal in noise needs a differential amplifier with high gain (about 1000), CMRR above 100 dB and input impedance above 10 MΩ. (Key point: Preamplifier requirement)

3. Which brain wave is seen when a person is relaxed with eyes closed?
   A. Delta
   B. Theta
   C. Alpha (8 to 13 Hz)
   D. Beta
   Answer: C. Alpha (8 to 13 Hz) appears in relaxed, eyes-closed state; delta is deep sleep and beta is alertness. (Key point: Brain waves)