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Author Topic: Solar Cell Preamplifier Design Problem Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 11-28-2005 03:30 PM                     I work on preamp design and now i got something new, but don't working like i was planned :-) , maybe somebody know answer. This is schematic of it: OK, When photodiode is decoupled from preamp with capacitor everything is ok, but when i conected it directly i don't get sound like it should be, i got it but like the wire losing contact or have poor contact, but everything is OK with contact, but is sounding like that (don't know how different i described that sound), and when i put capacitor in serial with photodiode i got ok sound and everything working fine. I use some polish photodiode 1PP75, because i have it in mine projector sound head in the moment, but that's not problem i think!! Can you figure what could be problem of this? Did you ever have problem like this? I also try to change polarity of photodiode but then i got no sound at all, or some poor sound but with same effect. Tnx!  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 11-28-2005 06:09 PM                     Is the photodiode a solar cell, that is, it produces current when light hits it? If so, then this DC voltage will miss bias the op amp, and the output will sit at one of the power supply rails.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 11-28-2005 07:11 PM                        Bruce is prob. exactly right. Why not use a suitable capacitor or transformer? Louis  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-28-2005 10:29 PM                     What is the gain of your system? The TL072 will gladly amplify DC too. Whereas your diode will produce volts or current...the op amp will dutifully amplify that to its voltage rail... and that is all she wrote. Once you block the DC offset from the input...you probably don't need the cap on the output but it is always safer to decouple. Another thing to consider is that the TL072 may not be the best choice of optical preamp...look for a chip that is more geared towards "microphone preamp". Also, strongly consider going for a differential input design. Unbalanced preamps such as yours are very prone to picking up all sorts of noises and pops. Steve  |  IP: Logged Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 11-29-2005 02:06 AM                     Yes differential input is mine next step, but that's no problem to do with OP amp. So do you think to put capacitor on input or directly connect cell/photodiode on input? Is there reason using cell/photodiode? Also i considering use NE5532 as OP amp  |  IP: Logged Gilbert Travin Expert Film Handler Posts: 101 From: Villeurbanne / France Registered: Jan 2004 posted 11-29-2005 10:05 AM                        Hello, all! The preamp based on the Tl072 (low cost and low noise op amp) is a current to voltage converter. The solar cell is a current generator. The cell produces always a current because the optical tracks are not entirely black even when nothing is recorded (there is a continuity line to avoid transcient noise (cloc !)). This cell-current is converted to a voltage by the op amp (voltage output is cell-current value times value of the two feedback resistors) : so you have always a DC voltage at output. That's normal you saturate your op amp if resistors value is too high. With capacitor, this phenomena disappears. The better solution has been suggested supra : using shielded transformer or differential amplifier based on very low noise op amp ! Good luck  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 11-29-2005 10:49 AM                        Some mention should be made of the value of a L.F.rolloff. Very low frequencies will allow the shutter (or its modern equivalent) to thump each time it opens. Suggest you dump everying below 60 hz or so. Most transformers have some losses about there. Louis  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 11-29-2005 05:40 PM                  I would never use an input/coupling transformer in that application. You'd still have the same DC bias problem that could saturate the transformer and still need the isolating/coupling cap. Besides, they usually have crappy freq response unless you spend big $ for one. I would leave it as single-ended, unbalanced input with the cap unless you have a hum problem. I'd add a 2-3k load resistor across the PC. I do agree that you need some LF ***AND*** HF roll-off RC networks in the OPAMP feedback loop. While I haven't looked at the manual section, you might want to look there at some schematics to get some ideas. Good luck.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 11-29-2005 09:43 PM                     With an op amp the gain is determined by R-1/R-2. For a gain of 40 db which is typical for a mic preamp. The pot would be R-1 and there would also be a resistor from the - input (with one end of the pot) of the opamp to ground called R-2. Divide R-1 by R-2 to get the gain of the circuit. Say that R-2 is 10k and R-1 (the pot) is 100K... 100/10 = a gain of 10. Try 75K for R-1 and 2.2K for R-2, a gain of about 37. Also you want to have the photo diode feed the + input, not the - input... Feeding the photo diode in the - input will invert the signal... it will be out of phase with the input.... Use the cheap TL-072 chips to experiment with and when you get it right pick a high quality Burr-Brown chip with high snr and good slew rate. Burr Brown and Analog Devces make some of the best op-amps out there. If anyone here finds that my math or theory is wrong please let us know.... Math is not my forte...... Its also been a long time since I messed withh op-amps... I've since converted over to using vacuum tubes..... Mark  |  IP: Logged Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 11-30-2005 03:18 AM                     ¸Mark, i know what you thinking to do but, in mine circuiot we have virtulay zerro efect on input, that means input impendance is close to 0 because +input of OP amp is conected directly to earth, this is because photodiode or cell must be threated as curent source to get lovest distorzion, is this wright? But after this comes HF EQ circuit which also got input on -in of OP amp so finaly we get signal in phase! Here we also could use Mark relation for Gain calculation if we put resistor in serial with photodiode input, but small resistor of about 100ohm Can eneyone got schematic of CP45 preamp or something newerlike that, i would like to saw it how is based? Because in manual section there only circuit with transformer.  |  IP: Logged Gilbert Travin Expert Film Handler Posts: 101 From: Villeurbanne / France Registered: Jan 2004 posted 12-01-2005 02:38 AM                        Marin Your circuit is a current to voltage converter and its input impedance is very low : that's a good solution with a solar cell (a current generator needs a very low loading impedance). The circuit discribed by Mark is (I think ) an inverter : you can obtain a low input-impedance with R1 = 100 Ohms (and R2 = Gain x R1). But you have always the off-set problem (and saturation if gain is too high ....) A very good transformer (such as CP 50) is not cheap. This subject has been discussed on this forum and Gordon had given an answer If you use a photodiode, you must bias it (reverse current obtained with a DC source and a resistor).  |  IP: Logged Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 12-01-2005 04:17 AM                     I was thinking that photo diode and solar cell got similar characteristic, ok if i use solar cell you think this preamp would work ok? With photo diode i already use bias voltage, when i get home i draw schematic of that solution and upload it. Does de coupling capacitor on input effect on input impedance much?  |  IP: Logged Gilbert Travin Expert Film Handler Posts: 101 From: Villeurbanne / France Registered: Jan 2004 posted 12-01-2005 04:40 AM                        Marin ; I think it is better to use solar cell (a solar cell don't need biasing and gives a higher and noiseless signal than photodiode). With your scheme, you risk saturation because of DC current : you must use a coupling capacitor (i.e. 1 uF) or compensate DC off-set with a DC source and a trimmer. The preamp-diagram becomes then more complicated !  |  IP: Logged Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 12-01-2005 10:59 AM                     Then capacitor is simpler choice if result is same :-)! When i get home i will hook-up some solar cell in projector sound head and try same schematic with it, can't wait to hear difference! Can i use some other solar cell except cell for soundhead?  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 12-01-2005 07:06 PM                        Use an op-amp labs 350p optical preamp. Low cost and mountable in octal based "tube." This is a mike level preamp widely used for amplifying a solar cell output; especially in the pre-Dolby era. Louis  |  IP: Logged

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