Arduino Forum

Using Arduino => Displays => Topic started by: MatCat on Sep 28, 2012, 02:09 pm

Title: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 28, 2012, 02:09 pm
I build this circuit here:
(http://mysite.verizon.net/res0fab4/files/Schem1.gif)

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. 
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: retrolefty on Sep 28, 2012, 02:24 pm
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

Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 28, 2012, 02:35 pm
should I do any other form of amplification or do you feel this is solid enough to split the signal out 4 ways?
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: retrolefty on Sep 28, 2012, 02:55 pm

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
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 28, 2012, 02:59 pm
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. 
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: retrolefty on Sep 28, 2012, 03:06 pm

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
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 28, 2012, 11:25 pm
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.
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: retrolefty on Sep 28, 2012, 11:59 pm
I don't know how better to state it. That video amp is designed to drive just one 75 ohm video load (receiver).

Lefty
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 29, 2012, 12:31 am
So should I build 4 exact copies of the amp for each output with a single driving input?
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: retrolefty on Sep 29, 2012, 12:58 am

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
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 29, 2012, 01:01 am
That defeats my purpose, what are the commercial units doing that I can't do?
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: retrolefty on Sep 29, 2012, 01:21 am

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
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 29, 2012, 01:59 am
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.
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: dhenry on Sep 29, 2012, 03:04 am
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.
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: Docedison on Sep 29, 2012, 08:33 am
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
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: Grumpy_Mike on Sep 29, 2012, 09:26 am
Yes but those chips have only one output and several inputs exactly the opposite of what is needed.

A standard video signal is 1V park to peak. That amplifier circuit is a very poor circuit as you can tell from the trace. All those sync pulses should be flat at the same zero volts level. This is because there is no black level clamping in the circuit and the circuit is DC coupled.
You need an AC coupled input and some clamping. Then you need four of them to drive a seprate output each. You can't chain video signals.
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: kg4wsv on Sep 29, 2012, 01:32 pm
Also take a look at Maxim's web site.  They have a variety of video amplifiers and buffers, many of which operate off a single supply (no negative voltage supply required).

-j
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Sep 29, 2012, 01:34 pm
Do you know any part #'s off the top of your head that would be ideal?
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: dhenry on Sep 29, 2012, 04:53 pm
Quote
Do you know any part #'s off the top of your head that would be ideal?


Any jfet would be a great video buffer: easy to bias, and good for hundreds of Mhz easily, and natively single rail supply.

Video opamps, until you are very skilled in their applications / layout, are a beast to tame.
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: Pavilion1984 on Oct 01, 2012, 10:33 pm
MAX4213, EL5050 for you to start with and you will need 1 for every video output. http://www.maximintegrated.com/ (http://www.maximintegrated.com/) have lots of good info about designing what you want to do in the product video section http://www.maximintegrated.com/products/video/ (http://www.maximintegrated.com/products/video/) and also try google for video distribution amplifiers (VDA).
Title: Re: Video Amplifier Circuit Doing the Opposite!
Post by: MatCat on Oct 03, 2012, 03:07 am
Would the LT1193 work well?