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BJT vs diode log amplifier; applications

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.

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

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

✓ Answer checked by Claude Opus 5.5 (high effort)

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