I'm running through some test of a phototransistor signal conditioner, and I'm facing with wild noise issues. The conditioning is a basic TIA stage:
Feedback resistor is 5K, feedback capacitor is 470pF, negative bias is -12V, and photocurrent is in the range 8uA-800uA. The bandwidth should be about 50kHz. Opamp is OPA2277UA.
Problem is that phototransistor if far away from TIA, with four meters of cable running along with power cables (valves, actuators, motors in the 5kW range). This causes tenth of uA of noise well inside the signal bandwith.
The same phototransistor, with same cabling, works great with the simple resistor between collector and supply rail:
TIA is better for me because it will keep phototransistor far away from saturation, but maybe the finite opamp bandwidth is actualy amplifying noise. Hints?
Doesn't have to be single ended. The OP could run the emitter to another TI A and then the two TIA outputs to a difference circuit. Need to split the b ias across the TIAs. Bringing the TIA out to the photo-transistor is usual ly the better solution for noise but it's not always practical or necessary . Differential TIAs is a lot more circuitry and runs into calibration issue s with balancing the two TIAs.
Interesting question. The collector side of a cap multiplier looks like a b ig inductor, so if you built a pair of them that shared the same RC on thei r bases and had similar betas, that ought to work--the differential signal would get through without needing to move the RC.
Il giorno domenica 17 luglio 2016 09:56:50 UTC+2, SilverLeo ha scritto:
I'm replying to myself: I apologize, this seems the only action Google grou ps let me to do.
So: interesting the cap multiplier approach; I'll do some math, maybe it'll be handy in the future. I cannot move the TIA close to the phototransistor: it's a retrofit for an old device for estimate the density of filled paper tubes (filters for very specific application) through the detection of diffused light. The more fi lled is the tube, the less is the light diffused. Phototransistor is buried inside a cast iron frame, far away from any condi tioning electronic.
The main reason that lead to the TIA approach was the fact that pototransis tor with classic resistor on collector will fall into saturation (Vce near or below Vcesat), actually messing up density estimation. A TIA, as far as the opamp keeps doing its magic, will force Vce=Vbias, r unning the phototransistor deep in active region.
This in theory: in real world application the TIA output shows 50mV of nois e voltage, that does not correspond to a 10uA of current noise, because wit h the simple resistor there is no evidence of it. This noise comes out when some valves or motors are running.
TIA input impedance should be zero (as stated in books) because of virtual ground at inverting input. But, because emitter is at -12V, the phototransi tor will see a variable resistor matched to drop exactly 12V for any curren t. So, for "small" currents this resistor becomes big", and for big current this resistor turns "small". Is this right,somehow?
I could add a resistor from inverting input to ground (10K range) to set a limit to input impedance, and a capacitor in parallel to that resistor (nF range) to bypass everything the opamp will not be able to follow due to ban dwidth limitation. Useless or not?
If I cannot overcome this issue I'll switch back to old, classi resistor on collector; but, it would be nice to understand what's going on.
n old device for estimate the density of filled paper tubes (filters for ve ry specific application) through the detection of diffused light. The more filled is the tube, the less is the light diffused.
istor with classic resistor on collector will fall into saturation (Vce nea r or below Vcesat), actually messing up density estimation.
running the phototransistor deep in active region.
ise voltage, that does not correspond to a 10uA of current noise, because w ith the simple resistor there is no evidence of it. This noise comes out wh en some valves or motors are running.
l ground at inverting input. But, because emitter is at -12V, the phototran sitor will see a variable resistor matched to drop exactly 12V for any curr ent. So, for "small" currents this resistor becomes big", and for big curre nt this resistor turns "small". Is this right,somehow?
a limit to input impedance, and a capacitor in parallel to that resistor (n F range) to bypass everything the opamp will not be able to follow due to b andwidth limitation. Useless or not?
on collector; but, it would be nice to understand what's going on.
As Phil said, it's most likely the capacitance to ground on the inverting i nput that is causing the noise. (The noise at the non-inverting input see's a the feedback resistor and cap to ground.... that gives gain that grows as t he frequency increases... (cap impedance decreases.) if it gets too bad you've got an oscillator.)
What's wrong with the resistor? Can you make the bias (collector) voltage higher?
Except that with the same size resistor and power supply, both will saturate at about the same light level--the TIA doesn't help that at all. Phototransistors have horrible linearity anyway, so the swing at the collector is the least of your worries.
If you don't understand what I said about not hanging cables on summing junctions, just trust me. Your load resistor trick will work a lot better.
No. It's the transistor that adjusts, not the load.
That won't make any difference except to increase the noise a little.
You really don't want to hang a capacitor to ground on a summing junction. It's very likely to make the whole thing oscillate.
I recommend "The Art of Electronics", third edition, sections 2.5 (negative feedback) and Chapter 4 (op amps).
It's all a matter of field strength. I learned that the hard way as a teenager. In high school physics class we learned about dielectric materials and isolators but not about loss tangents and the like. So I had the marvelous idea of using a large high voltage cap from a discarded TV set for the Pi-filter in my kilowatt-class ham radio amp. It had the required voltage ratimng alright. Free stuff! Woohoo! It worked for a while and then ... tzzzt ... *BAM* ... THWOCK ... splatter. The previously white ceramic of the capacitor had turned into green molten glass pebbles. Luckily I didn't get any in my eyes.
Sure, like you gotta keep the phototransistor clean.
The big fat cheap plastic fiber, like toslink, has a big aperature to let light in, but it has a lot of attenuation and wouldn't like high temperatures. There is fat glass fiber, but it tends to be expensive.
John Larkin Highland Technology, Inc
lunatic fringe electronics