SNOAA75A June   2021  – April 2022 LMP7704-SP

 

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Design Simulations

Simulation

The following figure shows the TINA-TI™ simulation schematic of the photodiode amplifier. The current source IG1 and the capacitor C24 serve to form a simple model that represents the reverse current, IPD, of the photodiode and junction capacitance CPD, respectively.

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DC Transfer Function

A DC transfer characteristic analysis is used to verify the gain and output voltages of the circuit. The output current of IG1 is swept from 0 to 20 μA and the output voltage is plotted.


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For zero input current, the simulated output voltage is 200.032 mV, and at 20-μA input current, the simulated output voltage is 2.500 V and the gain is 115 kV/A.

AC Transfer Function

The AC transfer function was measured using an AC transfer characteristic analysis in TINA-TI™. The simulated –3-dB point was 188.21 kHz.


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Stability Analysis

For simulation of the loop stability, the feedback path of the amplifier is broken at the output using a large inductor (Ltest). This inductor allows the circuit to converge to the proper DC bias point but acts as an open circuit in an AC transfer characteristic simulation. Voltage source V_AC is AC coupled into the feedback loop by capacitor (Ctest) and the loop gain is measured by voltage probe Vout.


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The loop gain plot shows the loop closure point (loop gain is 0 dB) at 269.22 kHz with 63.70 degrees of phase margin.