35

For linearity and speed To load the photodiode less To provide a buffered output The op-amp forces the voltage across the photodiode to be constant which means there is no charging/discharging of the capacitance which speeds up the response. Since the voltage across the photodiode is being held constant, the only signal of value from it is the current which ...


28

It's all down to speed. What your circuit doesn't show is the self-capacitance of the photodiode: - Given that the signal produced by the photodiode is current (Iph shown above), if this is rapidly changing like in an optical data receiver, the junction capacitance will have a significant effect on rise and fall times. However, with a transimpedance ...


23

My advice is to not bother doing what you're doing, and spend the $3.50US to get the part designed for the task, like https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/SFH-7060/475-3174-2-ND/6137022


19

Different semiconductor junctions have different forward voltages (and reverse leakage currents, and reverse breakdown voltages, etc.) The forward drop of a typical small-signal silicon diode is around 0.7 volts. Same thing only germanium, around 0.3V. The forward drop of a PIN (p-type, intrinsic, n-type) power diode like a 1N4004 is more like a volt or ...


18

Photoresistors were the predecessors of photodiodes. Instead of acting as current source they were light dependent resistors (LDRs). Their main disadvantage is that they react very slowly to light changes.


16

In addition to or to expand on DKNguyen's comment, the capacitance of the photodiode and the resistance of the load resistor will form a filter and slow down the response. The TIA keeps a virtual ground across the capacitor allowing the bandwidth to be higher (within the limits of the op-amp's loop gain). It does cause noise peaking though as the noise ...


15

Not really used in a similar way for stopping machines like in your question, but before there were phototransistors/diodes we had photomultiplier tubes. Under influence of a high voltage a single photon that collides with the light sensitive cathode will release several electrons. Then these electrons are attracted to the anode, on their way there collide ...


14

Fundamentals All materials in the chemical table and molecules of different combinations have unique electrical properties. But there are only 3 basic electrical categories; conductor, insulator( = dielectric) and semiconductor. The orbital radius of an electron is a measure of its energy, but each of many electron orbits formed in bands can be: spread ...


13

A photomultiplier tube is one such device. They are still used in some applications and as per a photodiode comparison with photomultipliers looking at the some disadvantages of a photodiode some areas where a photomultiplier has advantages: Larger detection area Has internal gain Much higher sensitivity Photon counting possible without special cooling and ...


13

Semiconductor laser effect is described by two coupled partial differential equations of carrier density and photon density, the rate equations. The solution of these equations result in a non linear current-intensity relationship causing relaxation oscillation when the diode is turned on. See here or following image: (image source: p. 45 of this document)...


12

Simpler than the photomultiplier tube is the basic vacuum tube photodiode: The curved plate is the photocathode, and the wire post in the center is the anode. Photons knock electrons free from the surfaces of both elements, but since the area of the cathode is so much larger than the anode, there's a net flow of them from the cathode to the anode — ...


12

You are confusing 'inverting' with 'negative feedback'. Open loop simulate this circuit – Schematic created using CircuitLab Figure 1: op-amp with open-loop inverting mode. In Figure 1 the op-amp will amplify the difference between its inputs by the open loop gain. Let's say the open loop gain is 1,000,000 and we apply +1 mV at the '-' input. Since ...


11

It could be motion. Probably at the laser source, but it could also be at the photodiode end, even a fan somewhere can cause sensitivity. However, if you have no aperture, or the wrong aperture, it is also possible to get stray laser paths to the sensor as the laser crosses the metal shroud of the sensor or reflects around inside the arrangement. If you ...


11

It's a particular classification of nonlinear behavior that has important applications. In the same way as you can consider, say, a BJT as linear over a limited range, you can consider something like a diode as square law over a limited range. That simplification allows you to analyze functions such as RF detectors analytically. See, for example, this ...


10

The simplest way to connect a photodiode (even an LED can be used in this way as a photosensor) is as below: simulate this circuit – Schematic created using CircuitLab Note that the photodiode is reverse biased through the 1 Meg resistor R1. The photocurrent generated by the diode opposes the flow of leakage current through this reverse biased diode, ...


10

If you look at the data sheet for the LED it says that at 20 mA the forward voltage may be between 2.8 V and 3.6 V. Importantly it states that this is at an ambient temperature of 25 degC. So, how well regulated is the ambient temperature in your experiment? I ask this because you seem to be relying on generating a constant current by using a resistor and a ...


10

The HW5P-1 is in fact a phototransistor. This is the diagram of the example circuit from the datasheet: The circuit symbol is clearly a phototransistor, regardless of what the poorly translated text says. A phototransistor and a photoresistor (like a cadmium sulfide cell) both react to light, but they are fundamentally different in how they operate. A ...


10

at a given time, the same LED is either on or off, and to virtually get different intensities, the duty-cycle of being turned on and off has to change. So, the response is like ones and zeros. No, that's incorrect. The amount of photons emitted from an LED is directly proportional to the current through it. It's a common method of varying the perceived ...


9

Actually, that sounds about right. I don't know where you got the 800 µA figure from — both the table and the graph indicate much less. "Bright office lighting" is about 500 lux. The sensitivity of the SFH 203 P is specified as minimum 5 µA, typical 9.5 µA @ 1000 lux. The graph in the middle of page 4 agrees with this. The rise and fall times were ...


8

The easiest way to get a photodiode, and to control it, is to use one of the built-in optocouplers. The base/collector junction of a phototransistor is a photodiode (just ignore the emitter): Or use the 4N2x.sub/CNY17.sub as a model to build your own subcircuit.


8

Depending on the speed you need, you may well be out of luck. You state that your current diodes "with a picosecond level rise and fall time", while you link to a PD with 12 nsec rise and fall times. Furthermore, you say that your PDs are "expensive ", but the linked units are only about 6 bucks in onesies, and if you think 6 bucks is expensive for a ...


8

The op-amp you have chosen (LF412) is unsuitable for the power supply regime you show in your schematic. The input common-mode voltage range is a couple of volts inside the power rails hence putting the non-inverting input to ground (also your negative supply rail) is a mistake - you need bipolar supplies for this op-amp. In addition, the minimum supply ...


7

Your source, of course, is http://www.electrooptical.net/www/frontends/frontends.pdf. A direct adaptation is fairly simple. Rbias should be connected to -15, as shown, but the 20K resistor must be tied to a separate bias voltage, Vb. For a given diode bias voltage Vd, Vb = 1.67 Vd. In your case, this would call for a Vb of about -100 volts. All of this ...


7

The signal is picked up with a high-speed photodiode array of several sensors to allow the tracking circuitry and focusing to work. Using closed-loop control allows the mechanical parts to have relatively loose tolerances compared to even an LP record turntable. As one Japanese Engineer said you could make the player chassis out of chopsticks and it would ...


7

You could modulate your LED by driving it at a particular frequency (20kHz for example). Then instead of just looking for a simple on/off signal from your photodiode, you look for that particular frequency instead.


7

You can connect the photodiode in either direction (anode to GND or cathode to GND). Both variants work; the difference between both variants is just the polarity of the signal. (You need to look up in the datasheet of whatever diode type you are actually using to find out which of the leads is the anode/cathode; or just measure direction of conductivity of ...


7

There's what's called "analog switches and multiplexers", which exactly fulfill the role of your pushbuttons here. Basically, all large silicon chip manufacturers have them (for example, Texas Instruments). However, the currents from photodiodes are really small and sensitive. You don't want to switch these, if it can be avoided. These switches can easily "...


7

How small can the obstruction be? (I guess it depends on the surface area of the LDR, itself.) Yes, LDR tend to collect light from a larger area than photodiodes or phototransistors. A photodiode or phototransistor may include a lens that will focus incoming light to the tiny internal silicon sensor. You can roughly estimate its light-collecting area by the ...


7

Read the following data sheet extract from the TL074 and see why your circuit is ineffective: - So, if you fixed the supply rails the next problem will be that your output signal will be negative relative to 0 volts; this is because photocurrent flows into the cathode and out from the anode thus forcing the op-amp output to go negative to maintain -Vin at 0 ...


6

The first electrical light sensors were selenium cells. Selenium was used for resistances at the receiving station on the transatlantic telegraph cable in the 1860s, and it was noticed that it gave erratic results in daylight. Selenium can generate a small photovoltaic current so it was used in pre-war lightmeters and (I think) "Magic Eye" demonstrations at ...


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