---
title: "BJT vs diode log amplifier; applications"
subject: "Advanced Electronics"
chapter: 5
key: "semester_4/advanced_electronics:bjt-vs-diode-log-amp"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/bjt-vs-diode-log-amp
site: BCT Engineering
---

# BJT vs diode log amplifier; applications

Advanced Electronics · Chapter 5: Op-Amp Bipolar Transistor Logarithmic Amplifier

- Asked in: 2069 Chaitra, 1+2 marks
- Video: [Logarithmic Amplifier by using BJT | GATE & ESE Electrical, Electronics (EE, ECE) | Sanjay Rathi](https://www.youtube.com/watch?v=jsoTFP-TeMI) — GATE & ESE Electrical, Electronics in 10
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/bjt-vs-diode-log-amp
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-5

## Question

What is the advantage of BJT log amplifier over diode log amplifier? List out the applications of log-antilog amplifier.

## Short answer

A BJT log amplifier offers wider dynamic range (7–9 decades) and better accuracy than a diode version (3–4 decades) because the collector current follows the ideal exponential law. Applications include analog multiplication/division, RMS detection, signal compression, dB meters, and sensor linearization.

## Answer

#### Advantage of the BJT log amplifier over the diode log amplifier
In a BJT (transdiode) log amplifier, only the **collector current** flows through the feedback path. The collector current follows the ideal exponential law IC = Is·exp(VBE/VT) very closely over a wide range, about 7 to 9 decades of current. A diode current includes recombination and leakage components. Its ideality factor η varies between 1 and 2, so the diode follows the log law accurately over only about 3 to 4 decades. The BJT version therefore has a **wider dynamic range and better log accuracy**.

#### Applications of log–antilog amplifiers
- Analog multiplication and division (log, add or subtract, then antilog)
- Raising to a power, square and square root, and RMS computation
- Compressing signals with a wide dynamic range (audio, sonar, radar)
- dB meters, light (photodiode) measurement and pH measurement
- Analog computation, function generation and true-RMS meters

## Check yourself

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

1. What is the main advantage of a BJT log amp over a diode log amp?
   A. It has no VT dependence
   B. It needs no op-amp
   C. Wider dynamic range and better log accuracy
   D. It works with negative inputs
   Answer: C. Only the ideal collector current flows, giving about 7 to 9 decades. (Key point: Main advantage)

2. Why is the diode log amp accurate over only 3 to 4 decades?
   A. Diodes cannot be put in feedback
   B. Diode current includes recombination and leakage, and its ideality factor varies between 1 and 2
   C. Diodes have no exponential law
   D. A diode has too low a VT
   Answer: B. The varying ideality factor spoils the pure exponential. (Key point: Why diode is limited)

3. Which operation is done using log-antilog amplifiers?
   A. Analog multiplication and division
   B. Digital-to-analog conversion
   C. Clock generation
   D. Rectification only
   Answer: A. Logs convert multiplication into addition. (Key point: Application)

## Related in Advanced Electronics

- [Short note: Applications of log-antilog amplifiers](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ashort-note-log-antilog-applications) — Advanced Electronics, chapter 6, asked 2×
- [Need for log-antilog circuits; analog multiplier](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Awhy-log-antilog-multiplier) — Advanced Electronics, chapter 6, asked 2×
- [Multiplier and divider using log-antilog](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Amultiplier-divider-log-antilog) — Advanced Electronics, chapter 6
- [Short note: RMS of sine wave using log-antilog](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ashort-note-rms-log-antilog) — Advanced Electronics, chapter 6, asked 5×
- [Design Vo = (Vx/Vy)^α·Vz with log-antilog](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adesign-vx-vy-power-alpha) — Advanced Electronics, chapter 6, asked 2×
- [Multifunction converter using log-antilog](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Amultifunction-converter) — Advanced Electronics, chapter 6

## Related in other subjects

- [Differential amplifier using BJT](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adiff-amp-bjt-circuit) — Basic Electronics Engineering, chapter 4, asked 2×
- [BJT differential amplifier collector waveforms](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adiff-amp-bjt-waveform) — Basic Electronics Engineering, chapter 4
- [BJT as amplifier: gm, rπ, re](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abjt-amplifier-gm-rpi-re) — Electronics Devices and Circuit, chapter 2
- [Advantage of C++ over C](https://www.bctengineer.com/question.md?key=semester_3%2Fobject_oriented_programming%3Acpp-advantage-over-c) — Object Oriented Programming, chapter 2