Dear Dennis... analogZONE's Dennis Feucht answers your design queries in his new Circuit Design Clinic! September 2006

analogZONE presents a new, interactive analog design service to readers! Send us your design questions (with relevant data; schematics in JPEG or GIF, please) for some free engineering advice from analogZONE's circuit consultant, Dennis Feucht, on how you might solve a design problem or improve circuit performance. Submissions may be edited for clarity or brevity, and submitters and their email addresses will remain anonymous (unless otherwise indicated). Please send your questions to Dennis here.

Mixed-Technology Laser System, Part 2

The June 2006 installment covered some problems with a medical laser design. The nonlinear flashlamp-laser combination had a parabolic transfer function which was gain-compensated with a square-root circuit. This resulted in constant loop gain over the output power range. In the first part, attention was on the forward path of the loop and the linearization of the square-law laser-flashlamp subsystem. The laser output reflected off a mirror, and a photodiode behind the mirror sensed it. A small but fixed fraction of laser light would transmit through the mirror. The fraction was different for different polarizations of light, and that caused an apparent drift in output power as polarization changed. Before this fact was discovered, it was thought that power drift was due to drift in the photodiode amplifier (PDA).

In this sequel, more of the feedback loop design is addressed, notably the photodiode amplifier in the feedback path.

Problem:

Redesign the PDA, shown below, to make it less noisy, simpler, less costly, and extended at the low end of the linear dynamic range to zero volts.

Read the entire solution here (63 kb Microsoft Word .doc file)