How do I select transistors for logarithmic amplifier?

Question

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.

Look for transistor pairs on the same silicon : CA3046 comes to mind as a starting point; no doubt there are better choices. — user_1818839, Apr 10, 2015 at 13:18
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. — George Herold, Apr 10, 2015 at 13:18
It's a lot easier if you don't care much about DC stability. — Spehro Pefhany, Apr 10, 2015 at 20:04
I asked about this back when such questions were allowed on the site: electronics.stackexchange.com/questions/11263 — markrages, Jul 24, 2015 at 20:44

WhatRoughBeast · Accepted Answer · 2015-04-10 18:21:35Z

2

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.

answered Apr 10, 2015 at 18:21

WhatRoughBeast

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\$\begingroup\$ Thanks for an absolutely great link. Unfortunately at that Electronics Design link the diagrams (schematics!) are no longer displayed. I was able to find a scan of the article somewhere else, pgs.18-20 of this rather large PDF introni.it/pdf/Bob%20Pease%20Lab%20Notes%20Part%204.pdf \$\endgroup\$
– Daniel Chisholm
Sep 14, 2020 at 11:29
\$\begingroup\$ @DanielChisholm - Thanks for the kind words. While I'm at it, I need to stress the need for tight thermal coupling between the transistors - they MUST be kept at the same temperature. Which is, of course, why log amps are usually made as ICs. Two transistors next to each other on a die are usually very well matched, AND their temperatures will be be almost identical. It's a lot more bother to do this with discretes. \$\endgroup\$
– WhatRoughBeast
Sep 20, 2020 at 16:30

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