The receiver will accurately
condition a wide range of light inputs at up to 10 Mhz data rates. The optical signal is
detected by the PIN photodiode and amplified by a broadband fed-back stage, Q1-Q3. A
second, similar, stage gives further amplification. The output of this stage (Q5's
collector) biases a 2-way peak detector (Q6-Q7). The maximum peak is stored in Q6's
emitter capacitor while the minimum excursion is retained in Q7's emitter capacitor. The
dc value of Q5's output signal's mid-point appears at the junction of the 0.005uF
capacitor and the 22M ohm unit. This point will always sit midway between the signal
excursions, regardless of absolute amplitude. This signal-adaptive voltage is buffered by
the low bias LT1012 to set the trigger voltage at the LT1016's positive input. The
LT1016's negative input is biased directly from Q5's collector. |