PhotoDiode Amplifier Design with Howland Current Inverter

A PhotoDiode Amplifier Circuit with Howland Current source and current inverter is analyzed in this circuit design video. This circuit has applications in optical receiver, optical signal amplification and optical current source design. This circuit is constructed with a pair of operational amplifiers, a Zener Diode, one photodiode and a potentiometer (variable resistor). The first op amp stage has two responsibilities. First, with properly biased Zener diode it provides a good reverse DC bias for the photodiode to operate and generate electric current proportional to the optical energy (signal) it receives. And two, using the adjustable or programmable resistor (possibly RDAC) the first stage provides a variable gain for its output voltage proportional to the current of photodiode (proportional to the received optical signal energy). The second stage is a modified Howland current source current inverter that that receives two input voltages V1 and V2 from the first stage and then produces an inverted-direction output current that is shown and computed to be proportional to the current of photodiode. With proper design, a practical approximation is simply the ratio of potentiometer resistor to the small internal resistor as computed in this analysis using combination of Kirchhoff's circuit voltage and current laws (KCL and KVL) and op amp virtual short. More video examples of sensor amplifiers and signal conditioning are posted in the Sensors & Instrumentation Amplifiers Playlist.

[@STEMprof]

A PhotoDiode Receiver Amplifier Circuit with Howland Current source/inverter is analyzed in this video.. For more analog circuit videos see: PhotoDiode Amplifier with Data Compression Explained ua-cam.com/video/hqrRx2ufAwg/v-deo.html
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VCA Electronic Gain Control (Part 1): Voltage-Controlled Attenuator Overview ua-cam.com/video/cFzYZsPEtP0/v-deo.html
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Push-Pull Power Amplifier Design with Op Amp, Sziklai Darlington Transistors ua-cam.com/video/8BFzsi7-Vbs/v-deo.html
Analog Vector Summer Circuit Design with Op Amp and BJT Transistors ua-cam.com/video/PIAsa0QNVns/v-deo.html
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And the Analog Circuits Video playlist: ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html I hope these Circuit design and analysis videos are interesting.

[@ChiefBridgeFuser]

I recall that bias on photodiodes is used to reduce the junction capacitance and thus increase speed/bandwidth. I also recall that the downside of biasing is increased noise. So, in high sensitivity, low speed (like a photometer for chemical measurements) zero bias is desirable.

[@STEMprof]

Thanks for sharing your thoughts and insights.

[@ernestb.2377]

i learn a lot from this. I miss a little bit of reasoning why and when should one use such a circuit. Facts about the advantages and disadvantages.

[@STEMprof]

Glad that this video is helpful. The first Op Amp operates as a transimpedance Amplifier by converting weak current of photodiode to a practical voltage at its output. The 2nd Op Amp uses that voltage to generate a reversed direction current source proportional to the original current value. I hope this is helpful.

[@ernestb.2377]

@STEMprof Yes, I really enjoy following your explanations. But I meant in general, for other videos too. For example, this video: why would one want that reversed current source, as we just could use the first voltage as the voltage output with low output impedance. In other words, it would be beneficial (in my opinion) to explain beforehand a video why should one use specific configuration, typical use cases, why not use maybe another obvious configuration, pitfalls and so on. Don't get me wrong, your circuit analysis is very interesting to follow along 😀

[@STEMprof]

Thabk you for sharing your thoughts & comment. Glad that you like my videos. I understand your point. In some design problems (including this one) the desired output is not a low impedance voltage and instead a high impedance current source (not sink) is required proportional to the input. This is a design for such use case. I hope this is helpful.

[@ernestb.2377]

Great!

[@STEMprof]

Thank you! Glad that you like this photodiode amplifier video. For more sensor videos pls see the Sensors Amplifiers playlist ua-cam.com/play/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj.html

[@Chupacabras222]

Very interesting, as always :) but I see one problem there. If the first opamp is powered by 5V, and Ipd can go to 1mA, than Rp cannot be 10k pot. Because V2 would not be able to go to 13V.

[@STEMprof]

Glad that you found this video interesting. Thank you for sharing your observations. You have a good point. 👍 Either max 2k Ohm potentiometer should be used or op amp supply has to be say +15v. For more sensor videos pls see the Sensors Amplifiers playlist ua-cam.com/play/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj.html
Thank you