As part of a phosphorescence detector, [lcamtuf] has been working with photodiodes. The components, like all diodes, have some capacitance at the junction, and this can limit performance. That’s why [lcamtuf] turned to bootstrapping to make that parasitic capacitance almost disappear.
The technique appears in several Analog Devices datasheets that presents a mystery. An op amp circuit that would normally limit changes to about 52 kHz has an unusually-placed JFET and claims to boost the bandwidth to 350 kHz.
The JFET turns out to be in a voltage-follower configuration. The photodiode sees approximately the same voltage on both terminals, so the internal capacitor can’t charge and, therefore, doesn’t impose any limits on rate of change.
Of course, a better way to think of it is that tiny changes cause an immediate response to counteract them, and so the capacitor’s charging and discharging are kept to a minimum.
It really isn’t important that the capacitor is not charged, but rather that the capacitor doesn’t increase or decrease charge. This leads to a second design, which imposes a DC bias voltage on the diode but prevents any signal from causing the capacitor to change from its precharged value.
Photodiodes seem exotic, but honestly, all semiconductor diodes are photodiodes if you let the light get to them. It seems that capacitors and op amps are always at loggerheads.
A similar issue applies to charge amplifiers used for measuring piezoelectric sensors. The piezo element generates a small charge, which leaks back through the device almost instantly. The other issue is the capacitance of the cable that connects the sensor to the amplifier, which eats up the small amount of charge you’re trying to measure. As a solution, the charge amplifier is built in a similar fashion to keep the voltage across the piezo at zero (trimmed for op-amp input offset etc.) so any charge generated is removed and put into the integrating capacitor (Cf in diagram). Of course there’s still leakages, so it cannot do steady state (DC) but when well adjusted the circuit can work down to milli-Hertz without too much distortion. I once made an anti-alias filter for such a setup and found the -3 dB point at around 0.1 Hz out of the charge amp.
For photodiodes, the scheme is slightly different because there’s a threshold voltage that has to be overcome for the diode to push out current, so that’s why the Vref in the diagram and the “pre-charge” over the diode can’t be zero. It can be, but then it won’t respond very quickly and the voltage follower output has to dip below ground.
All semiconductor diodes are photodiodes if you shine enough light on them, and all diodes are LED’s if you give them enough current…
Surely you mean incandescent light bulbs.
light emitting resistors
No, it’s a silicon-carbon-metal vapor arc lamp.
It’s those tantalum capacitors that turn into incandescent light bulbs.
As a kid I discovered that if you leave the dropping resistor off of a green LED, it will shine orange. That might have been blackbody radiation.
And PCB tracks are fuses.
And the PCB is the case (if you know how to make it pretty) and the case is PCB (when you get zapped ‘coz you didn’t ground it properly).
All components are smoke generators if you get enough current through them.
get promoted to “Smoke facilitator”
i don’t think it takes any particularly large current to make, say, a silicon rectifier diode emit light? i think the only reason there aren’t a bunch of youtube videos of like 1n4001 with the plastic filed down to show the light is that it’s the wrong frequency…infrared. seems like the frequency of the light is proportional to the Vf?
Precisely. The forward voltage is the amount of energy that the electron loses when it crosses the band gap, which defines the energy or color of the photons that get emitted. Likewise, when a photon hits a band gap in a semiconductor, it pushes an electron for the distance of the band gap against a voltage potential. Force over distance is energy. If the photon has more energy than required to jump the gap, the extra is converted to heat. If less, it the photon doesn’t get captured.
But you also need a direct bandgap. Silicon has an indirect bandgap, so if you see light coming off of it, expect some smell too
As long as electronics exist the capacitor will live for ever.The capacitor in a electronics circuit does a job very important, transfer a signal to the next circuit,and lot of other important jobs in the electronics curcuit
A capacitor, a resistor and an inductor walk into a bar…
But the transistor ducked in time!
A capacitor, a resistor and a trans sister wal8nto a bar. The trans sister walks up to the bar keep and says, “i think I’m a typo…”
Interesting circuit. I would have expected it to make very little difference, considering TIA already keeps the voltage almost constant.
Another photodiode effect I have run into is capacitive coupling from surroundings. I got false readings when objects moved near the photodiode, even though light level was not changing. Adding a transparent conductive film in front of the diode and connecting it to ground fixed that.
I don’t get why it’d be better to have a RC on the TIA op-amp than the photodiode capacitor. Wouldn’t the TIA “limits” performance as well?
Suppress oscillations?
That’s what makes it into a TIA. The op-amp is just a normal op-amp in an integrator configuration.
“TIA” is just a fancy name given for the particular application.
Without the resistor, the leakage of the opamps and diode would charge the cap at constant current, railing it almost immediately.
Pretty ingenious. It’s a voltage follower, but not exactly wired as one since the output is fed back to the diode. It keeps the voltage across the diode close to 0 (or voltage change if a capacitor is used). So it’s more like a voltage eliminator. If you think about it the photodiode produces a current signal so you want a low impedance input instead of a high impedance input. Normally a voltage follower is used in the opposite way to have a high impedance input for a voltage signal.
Years ago I was programming eprom boards and one day I saw every location on the screen light up like little fuses spark in side of the eprom. I did several boards on purpose just for the heck of the show.
Light Emitting Memory?
I wonder if you can turn it into a game. Pong with EEPROM as a display.