I need a logarithmic amplifier and found this equivalent schematic in the datasheet for the LOG104:

enter image description here

The LOG104 is rather expensive, so I want to build this myself. What are the requirements for the transistors? (I guess they have to be matched.)

I want to use this with a phototransistor to measure light levels.

  • 1
    \$\begingroup\$ Look for transistor pairs on the same silicon : CA3046 comes to mind as a starting point; no doubt there are better choices. \$\endgroup\$ – Brian Drummond Apr 10 '15 at 13:18
  • \$\begingroup\$ The voltage generated by a photodiode is logarithmic in the light intensity. (see fig 1a here.) ti.com/lit/an/sboa035/sboa035.pdf I know that's not what you asked. \$\endgroup\$ – George Herold Apr 10 '15 at 13:18
  • \$\begingroup\$ It's a lot easier if you don't care much about DC stability. \$\endgroup\$ – Spehro Pefhany Apr 10 '15 at 20:04
  • \$\begingroup\$ I asked about this back when such questions were allowed on the site: electronics.stackexchange.com/questions/11263 \$\endgroup\$ – markrages Jul 24 '15 at 20:44

Bob Pease addressed this question in one of his inimitable "What's All This ..." articles http://electronicdesign.com/analog/whats-all-logarithmic-stuff-anyhow. See particularly figure 7.

Basically, you set up two transistors to draw the same current, and match for Vbe.

While you don't say exactly what dynamic range you're looking for, I'd guess that almost any good small-signal transistor will do.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.