AD834 multiplier application - Page 1
Forum section: electronics / projects

author: dzseki
author_rank: Frequent Contributor
timestamp: October 04, 2015, 10:18:09 am
content: There is a project I'm working on. The goal is to mate a CRT amplifier from an other branded CRT projector (Electrohome) in to my NEC CRT projector, because that board offers much more video bandwidth. For this I need to make small PCB to add contrast control to the Electrohome amplifier board, because this function would be needed in my NEC.Attached the circuit diagram of the planned circuit. The base (The AD834+transistors+opamp) of this circuit was simply cut out from the Electrohome's design. however I want to improve this circuit even more. They originally used CLC449 as U2, but I intend to use the THS3201 as it looks to be a better opamp for the purpose. Also the 2N3906s seem to be quite low end for the task, and I intend to replace them with BFT95s.I've ran simulations with different transistors in the common base stage, and it showed that with the BFT95s the bandwidth would be worse than with the 2N3906, can someone explain me why?

author: Icarus
author_rank: Regular Contributor
timestamp: October 04, 2015, 12:05:09 pm
content: Probably BFT95s are saturated. Change resistor values.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2015, 12:12:10 pm
content: If it's just a video VGA, does it really need to use a $33 four quadrant multiplier? Surely a crappy JFET would do?Don't know why the bandwidth would be worse, but simulation models can be all over the place. Trusting them out-of-the-box is careless at best...Tim

author: dzseki
author_rank: Frequent Contributor
timestamp: October 04, 2015, 05:37:21 pm
content: Yes, it's "just" video VGA, but the projector eats 1080p with ease and at 1080p-60Hz the pixel clock is around 150MHz, resulting a 75MHz sine wave worst case. For proper display you'd want to reserve as much of this bandwidth until the light reaches the screen. And this costs bandwidth a lot.Quite similar to what bandwidth is needed for the oscilloscope to measure an X frequency signal.Honestly I also thought about JFETs initially, but they don't work out as you'd want.1. You can't put then into CF opamp's feedback2. You can't construct simple voltage divider as this is a 75Ohm terminated system.Believe or not this contrast circuit around AD834 was introduced in 1992 for CRT projectors...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2015, 09:18:27 pm
content: Ok, so not just NTSC...Also, then that must be VGA, so it's not just a $33 multiplier, but $99 worth of them?1. Obviously didn't try hard enough!2. Sure, just buffer after. Or build a tee attenuator with tracking control (admittedly rather tedious to do with video). Or not and just assume the load will terminate accordingly (but, this puts some demands on the transmission line following the attenuator, which probably isn't a safe assumption).There are purpose-made VGAs (variable gain amplifiers) specifically for video (~1V signal level, three in a chip) that are cheap and plentiful, or, there should be. The gain factor may be more sensitive (e.g., proportional to absolute temperature, due to a cheaper circuit), but with a feedback loop (i.e., person sees it's too dim, person adjusts level..), that's not necessarily a problem.Also, what kind of projector is this that doesn't have brightness/contrast controls internally? I haven't seen a CRT without such controls in... literally ever. They've always had them, since they were invented back in the 30s. Is it just not enough range? Ah, but do you need to go up or down? If it's not bright enough, you may need to replace capacitors in the unit, or it's just too tired (if it's a CRT type, the cathodes eventually age and go dim). If it's an LCD or DLP type and simply doesn't have that control... WTF? You got shafted or something.So if it's VGA port style video, I should expect it's a 75 ohm system with 1.0Vpk or 0.3Vpk levels. With the output being +/-4mA, you need 4mA into 1V, or 250 ohms transimpedance. You get no current gain from the folded cascode, pretty good voltage gain, and a little added current noise. Output then is 600 ohms (Rfb) * +/-4mA = 2.4V, rather high (or it's being used to convert from 0.3 to 1.0V video, I suppose).As for different transistors, 2N3906's fT is around 300MHz, so it'll have beta ~ 2 at your maximum bandwidth. Not necessarily as little alpha (which is the current loss in cascode), but definitely a poor choice. MMBTH81 would seem to be pretty minimal. Some gain peaking is probably desirable (base or collector inductance?).Hmm, ya know, that op-amp claims the input range and supply ratings are enough that, if you went with V- = -5V and V+ = +9V, you could drop the cascode entirely.Tim

author: dzseki
author_rank: Frequent Contributor
timestamp: October 05, 2015, 04:01:59 pm
content: You don't have to worry about the price of the AD834s as I already have the three and it came with 6 HFA1100 opamps, the whole batch did not cost me more than 50€, and no it didn't came from ChinaThe projector sure have brightness controll, actually not even just one...I describe what I am working on in more detail, so you'll have more insight. I have a NEC XG135 type CRT projector, that is working fine except for some odd reason the neckboards of this machine -despite the seemingly good construction, and fine components- badly cuts the bandwidth of the signal. The advertised (of course overblown) bandwidth is 150MHz, but in reality it cuts off at about 30MHz, which is ridiculously low.First I thought I had a bad luck with this actual machine so bought some backup boards, they were the same bad... Then I checked an other machine that was for sale near to me but that was just as bad as mine. So there are some major problem with the original neckboards.Actually I went very deep into this and it looks like the problem both lies in bad PCB design (or some odd aging of the PCB material) and components used which do not meet their promised performance (namely the Sony CXA1779P -250MHz video preamplifier ic which made the contrast controll originally for each color...)I've arrived to these assumptions after a lot of experimenting and using my HP 1720A (275MHz) oscilloscope and a matching HP 1120A (500MHz) FET probe.The Electrohome Marquee series CRT projectors using neckboards which are known to deliver the best possible bandwidth for CRT projectors. And by chance the driving signals those serving out this board are mostly compatible between the NEC and Electrohome. Except that they using different supply voltages, and the Electrohome board lacks contrast controll (realized on the neckboard at least).You could ask why not buy then a complete Electrohome projector then? The answer for this is that while Electrohomes are thrilling in bandwidth they are lacking on other areas where the NEC do not, and also where I live they are quite hard to come by... And that wouldn't be fun of course!So the plan is to mate the Electrohome boards with the NEC machine for the sake of performance. So far I have the Electrohome boards to put in the NEC, I have the auxiliary power supply for the needed voltages for the Electrohome board. I did a lot experimenting if my concept is viable, and so far it seems to be.So one of the last step is to construct a small daughter board to give an individual contrast controll for each color, and this small board's schematic is added in the first post.The NEC has an input switching preamplifier board which has a global contrast/brightness controll, but because it is possible to adjust color balance through a feedback loop contrast controll for each color is also needed, and this was originally done on the neckboard, this is why I also have to realize this individually for each color.This AD834 contrast controll circuit stuff was introduced by (also) Electrohome, in 1992 so actually this was how they did the contrast controll for the neckboards but it was populated on an other board, they originally used the MMBT3906s as common base and was amplified initially CLC409, then they soon changed it to HFA1100 (and finally to CLC449) opamp, I want to improve this circuit even further just for the fun of it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2015, 10:45:35 pm
content: Hmmmm, so the original NEC terminally sucks, and you wish to replace at least part of its circuit from a completely different machine, except for some notable changes that require these adjustments...An interesting project, Mr. Frankenstein!As for improvements, yeah, moving away from 2N3906 sounds like a good idea.Any of those op-amps seems to be good enough, I wouldn't worry about it too much.This is the kind of thing I was thinking about:http://www.digikey.com/product-detail/en/VCA824ID/296-22898-5-ND/1766827it's basically the whole circuit rolled into one package, with a cheaper two quadrant mixer.Tim