PhotodiodeReceiver-ElectricalandComputerEngineering.docVIP

Photodiode Receiver Andrew Dorsey ENEE417 I. Introduction The purpose of this subsection was to pick up and process a signal that was sent via light emitting diode. The main challenge in design and photodiode selection was frequency response. This receiver had to be able to pick up a 1Mhz carrier signal with enough accuracy to retain the pulse with modulation or frequency modulation. II. Parts Used This is a general list of parts used in this subsection. More detailed listings are shown in the circuit layout and simulation sections as well as the parts list in the budget section. Pacific Silicon Sensor Inc. PS3.6-5 High Speed P-I-N Photodiode THS4631 Operational Amplifier LM7171 Operational Amplifier Various Resistors for biasing and feedback resistances .1uF Capacitors for AC coupling III. Circuit Breakdown A ) Photodiode: The purpose of a photodiode is to transfer light energy into current. The current produced by a photodiode is usually on the range of μA or less. Below shows the responsivity curve based on the frequency of the light. The LED sending the signal is infrared with a frequency of 820 nm, right around the peak of this curve. For more photodiode information see the Appendix: Data Sheets. Figure 1: Responsivity curve of led based on light frequency (Pacific Silicon Sensor Inc.) B) Transimpedance Amplfier: The first part of the photodiode receiver is a transimpedance amplifier. The purpose of this is to take the small current (μA) supplied by the photodiode and amplify the impedance of this signal. This gives the signal a voltage that is useful for further processing and decoding. It is simply an operational amplifier with a feedback resistance and a feedback capacitance. The operational amplifier used was the Texas Instruments THS4631 wide-band FET input operational amplifier. A FET input op amp was necessary because it can operate with a low input current bias, usually in the nA range. A BJT input op amp requires an input