Hello All,
I have decided to start playing around with LiDAR for range finding with the eventual goal of measuring wind speed using either the Doppler shift of the laser light or using particulate in the air. In order to get to that point though I need a LiDAR system first so I thought I'd build one, and here I am. I found an app note that has a pretty cool description of power needs of pulsed laser diodes (PLD) and found this circuit for a power output stage that i'd like to build but am a bit confused and it has given rise to some questions, here is my current guess at circuit operation.
When Vcc is applied, C2, C3, L4, and L3 all charge while L1 and L2 remain discharged. When avalanche mode is triggered, current flows from collector to base creating a forward bias on the emitter side diode essentially turning on the transistor. Now that there is a path to ground a current spike can be generated by C2 and the node between C2 and D1 is now negative. This makes D1 forward biased instead of reverse and is now able to flick on.
My questions are now primarily about the inductors. I realize that L1 and L2 are probably there for current limiting but why not just use low value resistors to keep the amps drawn high? Additionally, what is the purpose of L4, a series tank? I have the same question for C3 and L3 because there doesn't appear to be a need for a positive voltage at that node when discharging. Lastly, the only avalanche mode circuits I have seen have been setup in astable mode and don't have triggering at the base, rather the collector voltage is just cranked up, is the trigger there primarily to just "reset" the avalanche conditions or is it actually what causes it?
I've attached an image of the circuit, along with its originally app note, as well as a link to Linear Technology's app Note 47 that has an astable setup for testing o-scope rise time on p.93. Thanks for any help you can provide in advance, I just can't wrap my head around it fully this time.
http://cds.linear.com/docs/en/application-note/an47fa.pdf
I have decided to start playing around with LiDAR for range finding with the eventual goal of measuring wind speed using either the Doppler shift of the laser light or using particulate in the air. In order to get to that point though I need a LiDAR system first so I thought I'd build one, and here I am. I found an app note that has a pretty cool description of power needs of pulsed laser diodes (PLD) and found this circuit for a power output stage that i'd like to build but am a bit confused and it has given rise to some questions, here is my current guess at circuit operation.
When Vcc is applied, C2, C3, L4, and L3 all charge while L1 and L2 remain discharged. When avalanche mode is triggered, current flows from collector to base creating a forward bias on the emitter side diode essentially turning on the transistor. Now that there is a path to ground a current spike can be generated by C2 and the node between C2 and D1 is now negative. This makes D1 forward biased instead of reverse and is now able to flick on.
My questions are now primarily about the inductors. I realize that L1 and L2 are probably there for current limiting but why not just use low value resistors to keep the amps drawn high? Additionally, what is the purpose of L4, a series tank? I have the same question for C3 and L3 because there doesn't appear to be a need for a positive voltage at that node when discharging. Lastly, the only avalanche mode circuits I have seen have been setup in astable mode and don't have triggering at the base, rather the collector voltage is just cranked up, is the trigger there primarily to just "reset" the avalanche conditions or is it actually what causes it?
I've attached an image of the circuit, along with its originally app note, as well as a link to Linear Technology's app Note 47 that has an astable setup for testing o-scope rise time on p.93. Thanks for any help you can provide in advance, I just can't wrap my head around it fully this time.
http://cds.linear.com/docs/en/application-note/an47fa.pdf
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