Fiberoptic Interface

An op amp is used to interface between a fiberoptic system and the MOS SCR to multi-cycle, half-wave control of a load. This receiver has two complementary outputs, one at a quiescent level of about 0.6V and the second at 3V. By adding a 4.7V zener in series with the return bus, the effective Vcc becomes 5.3V and also the 0.6V output level is translated up to about 5.3V. This level is compatible with the reference input (5.9V) of the single-ended powered op-amp acting as a comparator.

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Fiber Optic Receiver, High Sensitivity, 2uW

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Fiber Optic Receiver, High Sensitivity, 30NW

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Fiber Optic Link

Fiber optic applications require analog drivers and receivers operating in the megahertz region. This complete analog transmission system is suitable for optical communication applications up to 3.5 MHz. The transmitter LED is normally biased at 50 mA operating current. The input is capacitively coupled and ranges from 0 to 5 V, modulating the LED current from 0 to 100 mA. The receiver circuit is configured as a transimpedance amplifier. The photodiode with 0.5 amp per watt responsivity generates a 50 mV signal at the receiver output for 1 uW of light input.

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50Mb/s Fiber Optic Led Driver

The pull-up transistor of the totem-pole output is used to turn on the LED and the pull-down transistor is used to turn off the LED. The lower impedance and higher current handling capability of the saturated pull-down transistor is used as an effective method of transferring the charge from the LED's anode to ground as its dynamic resistance increases during turn-off. The slightly higher output impedance of the pull-up stage ensures that the LED is not over peaked during the less difficult turn-on transition. This asymmetric current handling capability of the output stage with its variable impedance substantially reduces the pulse-width distortion and long-tailed response. As the signal propagates through two NAND gates, each transition passes through the high-to-low and low-to-high transition once, normalizing the total propagation delay through the circuit.

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10Mhz Fiberoptic Receiver

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.

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