Photodiode AmplifiersPaul RakoThe Photodiode:The Photodiode:The Photodiode:The Photodiode:Inside the Photodiode:Inside the Photodiode:Photodiode Amplifier Types:Photodiode Amplifier Types:Photodiode Amplifier Types:Photodiode Amplifier Types:Photodiode Amplifier Types:Biasing the Photodiode:The Photodiode Amplifier:The Photodiode Amplifier:The Photodiode Amplifier:Amplifier Stability:Amplifier Stability:Mechanical Analogy:Mechanical Analogy:Mechanical Analogy:Mechanical Analogy:Compensated Amplifier:Biasing the Amplifier:Biasing the Amplifier:Biasing the Amplifier:Biasing the Amplifier:Amplifier Input Stage:Amplifier Input Stage:Amplifier Input Stage:Correcting DC BiasCorrecting DC BiasAmplifier noise:Amplifier noise:Some Potential Parts:Conclusions:Resources:Thank You!1Photodiode AmplifiersChanging Light to ElectricityPaul RakoStrategic Applications EngineerAmplifier Group23© 2004 National Semiconductor CorporationThe Photodiode:Simple?2 VoltsLightBig Resistor (1 Meg)Tiny current flows here (10 nanoAmperes)Makes about a 10 millivolts here4© 2004 National Semiconductor CorporationThe Photodiode:No, not reallysimple:2 VoltsLightBig resistors make noiseDark Current (diode leakage) flows too and is worse with temp.10 millivolts is not very useful.5© 2004 National Semiconductor CorporationThe Photodiode:Worse yet:2 VoltsLightDiodes are capacitors too, so fast signals are difficult.High impedance point difficult to interface with.And the capacitance changes with voltage across the diode.6© 2004 National Semiconductor CorporationThe Photodiode:Still Worse:2 VoltsLightTo make the diode more sensitive to light you make the P-N junction big.And that big junction has even more capacitance.7© 2004 National Semiconductor CorporationInside the Photodiode:A cap and a current source:The bigger the voltage across the diode the further the junction boundaries are pushed apart and the lower the capacitance.8© 2004 National Semiconductor CorporationInside the Photodiode:(And a really big resistor)There is also a bulk resisistivity to the diode but it is usually very high (100 MΩ). This represents the “Dark Current”.9© 2004 National Semiconductor CorporationPhotodiode Amplifier Types:Two ways to use the diode:1) The Photovoltaic Mode:Note ground– no voltage across diode.Light+-10© 2004 National Semiconductor CorporationPhotodiode Amplifier Types:The Photovoltaic Mode:No voltage across diode means no current though the big resistor ~ • No dark current.Also:• Linear output• Low Noise11© 2004 National Semiconductor CorporationPhotodiode Amplifier Types:Use Photovoltaic Mode:• Where precision is more important then speed.The lack of dark current removes an entire error term. The lower noise makes smaller measurements possible. The linear output makes calculations trivial.12© 2004 National Semiconductor CorporationPhotodiode Amplifier Types:The Photoconductive Mode:- 10V, there is voltage across the diode.+-Light13© 2004 National Semiconductor CorporationPhotodiode Amplifier Types:Use Photoconductive Mode:• Where speed is more important then precision.The voltage across the diode lowers it’s capacitance. This allows faster amplifiers:• Less capacitance allows a faster amplifier while maintaining stability.14© 2004 National Semiconductor CorporationBiasing the Photodiode:• Apply a big voltage(that doesn’t change):We want a low capacitance so put a big voltage across the diode. We want fast response so don’t let the voltage ever change. How?15© 2004 National Semiconductor CorporationThe Photodiode Amplifier:• Connect the diode to a virtual ground:If this pin is at ground so must this pin be at ground.-10 VoltsLight+-As much reverse voltage as the diode can stand.16© 2004 National Semiconductor CorporationThe Photodiode Amplifier:• Oh yeah, addsome feedback:-10 VoltsLight+-+-This pin stays at ground so output goes more positive with more light. This current makes positive voltage here17© 2004 National Semiconductor CorporationThe Photodiode Amplifier:• So it oscillatesand/or clips,what is wrong?-10 VoltsLight+-18© 2004 National Semiconductor CorporationAmplifier Stability:• Oscillations causedby capacitive diodeon input.-10 Volts+-Photodiode looks like cap to amp19© 2004 National Semiconductor CorporationAmplifier Stability:• Input pole (freq domain) or feedback lag (time domain) is bad.-10 Volts+-Photodiode current source causes output to change.But, photodiode capacitor means feedback signal will lag the actual output change.20© 2004 National Semiconductor CorporationMechanical Analogy:• A gear and rack mechanical servo.This gear is the amp output stage.This gear is the feedback.This rack is the output voltage.You are the amplifier front-end trying to keep the pointers the same.21© 2004 National Semiconductor CorporationMechanical Analogy:• Input cap is like backlash in feedback mechanism.Backlash here is a lag in the feedback.The lag in your feedback pointer will cause you to oscillate the rack.22© 2004 National Semiconductor CorporationMechanical Analogy:• Input cap is like backlash in feedback mechanism.Backlash=-10 Volts+-23© 2004 National Semiconductor CorporationMechanical Analogy:• Interesting note: Driver backlash is like output capacitance.Backlash=Without compensation either cap will cause oscillations.+-24© 2004 National Semiconductor CorporationCompensated Amplifier:• Add a feedback capto compensate.-10 VoltsLight+-25© 2004 National Semiconductor CorporationBiasing the Amplifier:• The output is stablebut there is a big DCoffset. Why?-10 VoltsLight+-Output never goes below here even with no light.26© 2004 National Semiconductor CorporationBiasing the Amplifier:• Or maybe there is nooutput at all. Why?-10 VoltsLight+-Output stuck at zero even with maximum light.27© 2004 National Semiconductor CorporationBiasing the Amplifier:• Answer: Input biascurrent.-10 VoltsLight+-Input pins will have small currents in or out of the pin (bias current).28© 2004 National Semiconductor CorporationBiasing the Amplifier:• Bias current mayexceed photodiodecurrent.-10 VoltsLight+-15uA out of this pin reacts against 1M feedback to try and put -15 volts on output.1M29© 2004 National Semiconductor CorporationAmplifier Input Stage:• Input transistors have base current.Input bias current may be 15 µA, but won’t vary much over PNP Input StageNPN Input Stagetemperature.30© 2004 National Semiconductor CorporationAmplifier Input