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Author Topic: Video Amplifier Circuit Doing the Opposite!  (Read 2408 times)
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I build this circuit here:


However the results I get are attached below.  The one with higher voltages is un-amplified, the lower voltage signal is the one that is ran through the amplifier, anyone have any ideas?

Full circuit setup is RCA jack with ground to ground, signal to breadboard which then goes to another perf board with 4 RCA jacks and the amplifier circuit, this will be a diversity controller when it's done with 2 inputs and 4 total outputs. 


* IMAGE005.BMP (150.05 KB, 320x240 - viewed 20 times.)

* IMAGE004.BMP (150.05 KB, 320x240 - viewed 12 times.)
« Last Edit: September 28, 2012, 07:14:11 am by MatCat » Logged

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Looks to me that is a "emitter follower" amplifier stage. Such a amp cannot have a higher output voltage then the input voltage, however it can have increased current drive for what is wired to the output connectors, so it's still classified as an amplifier circuit, it just supplies current amplification not voltage amplification. Some might call it a 'buffer stage'. Driving a 75 ohm circuit load is not a trivial matter and such emitter output circuits are common in the video world.

Lefty

« Last Edit: September 28, 2012, 07:26:37 am by retrolefty » Logged

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should I do any other form of amplification or do you feel this is solid enough to split the signal out 4 ways?
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should I do any other form of amplification or do you feel this is solid enough to split the signal out 4 ways?

Depends on the 4 video input channels you are driving. If they have the means to 'turn off' their 75 ohm input matching impedance resistor and have a 'daisy chain' video input and output connectors then you can loop into and out of the first 3 video receivers and only have the last receiver have it's 75 ohm termination resistor turned on. If however you want to drive 4, 75 ohm video input stages in parallel that represents a 18.75 ohm load and your circuit cannot provide enough current to drive them all wired in parallel.

Lefty
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Not quite sure what you mean completely...  The idea is to take the single video feed and be able to display it on up to 4 separate video devices. 
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Not quite sure what you mean completely...  The idea is to take the single video feed and be able to display it on up to 4 separate video devices. 

yes, I realize that, but if the video devices are not designed to operate with a switchable 75 ohm termination resistor (on or off) then your single amp circuit will not be able to drive 4 75 ohm video loads at the same time. Perhaps if you could link to a technical specs of your video devices we could see if they are compatible to 'daisy chain' serial video hookups.

Lefty
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Let's assume they do not, because honestly I have no idea what all display devices will be hooked up at the same time.  Probably atleast 2 displays, a DVR, and perhaps a set of video goggles.
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I don't know how better to state it. That video amp is designed to drive just one 75 ohm video load (receiver).

Lefty
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So should I build 4 exact copies of the amp for each output with a single driving input?
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So should I build 4 exact copies of the amp for each output with a single driving input?

That might work, but I have no way of knowing that the existing video input signal shown of that schematic can drive four such amp stages. Perhaps there might be commercial amplified video splitter devices available better suited then trying to roll our own?
http://www.cablewholesale.com/specs/composite-splitter/41r4-13500.htm?utm_source=GoogleShopping&utm_medium=organic&zmam=54972865&zmas=1&zmac=1&zmap=41R4-13500&gclid=CJX_l4mw2bICFQpgMgodgn4AxA

Lefty
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That defeats my purpose, what are the commercial units doing that I can't do?
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That defeats my purpose, what are the commercial units doing that I can't do?

No offence, but I haven't a clue what you can do or not do in the way of video design and construction projects, as I can only judge you on the questions you have asked so far. But I do see a price effective proven commercial solution there that you might want to consider. I buy lots of prebuilt modules and shields that I could probably build myself but looking at it figure I couldn't match the price + time consumed Vs other things I could be designing and building.

Good luck;

Lefty
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I shall rephrase my question, what would be the best circuitry to drive all 4 outputs properly?  I realize there are time considerations to factor but that isn't a factor for me, thanks.
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Quote
what would be the best circuitry to drive all 4 outputs properly?

Beauty is in the eyes of the beholder.

Before there was arduino, there is a company called 4d systems that marketed (and still markets) similar stuff (aka shields). One of the more impressive items they marketed is called picaso, a vga signal generator on a shield.

Google it and take a look at its video buffer stages - one of the more elegantly designed buffers in my view.

To me, a proper solution here are four independently buffered outputs, aka picaso.
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Take a look at this... I have several hundred of them and I can sample you 3 or 4 with a SASE... No Charge.

Bob

* EL4443CS.pdf (339.23 KB - downloaded 13 times.)
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