Video Amplification and/or baluns

[MrAl]

Hello,<p>I'm looking for information regarding
the amplification of video signals from
approx 15KHz to 5MHz and providing
impedance matching between 75 ohm
coax cable to 100 ohm twisted pair cable
(typically used in CCTV installations).<p>The video amp would provide both
amplification and impedance matching, while
the balun would only have to provide impedance
matching.
Both devices have to have good common mode
rejection.<p>Any knowledge in either of these two areas
would help, including web sites, chip
recommendations, theory, problem areas, etc.<p>The bandwidth is 15KHz to 5MHz, but preferrably
DC to 5MHz for the amplifier.<p>Im looking to build up the circuit(s) from
discreet parts or chips, not to purchase ready
made equipment.<p>Thanks much!<p>--Al
p.s. Please include part numbers along with any
part recommendations.

[russlk]

I expect a balun over such a wide range would be a problem. The LM833 has 10mHz GBW but the 5 volts/usec slew rate would limit the signal level.

[L. Daniel Rosa]

The MC33070 series (33071, 33072, 33074) have a gain bandwidth product of 4.5 MHz and a slew rate of 13V/uS. If the phase margin doesn't kill your signal, you may get a volt or two.<p>My limited understanding of baluns leads me to believe that they can only be used with narrow bandwidths.

[L. Daniel Rosa]

Oops, sorry 'bout that. I looked some terms and found that at 4.5MHz in the gain-bandwidth product, the device stops amplifying. Boy do I feel like a dork! This is partially due to parts that deliberately defeat the amplifier at high frequencies to prevent oscillation. There are opamps made without this, however care and extra components must be spent to guarantee that the amp never sees frequencies at or above the point where the phase margin is 180 degrees.<p>Opamps are usually three stage amplifiers, the input (differential), intermediate (also differential) and the output stage (push-pull). If you don't need such high open loop gain, you could build it from discretes and eliminate the middle stage. Some bandwidth could be gained here.<p>You may want to look up "surface mount" at http://www.electronicsinfoline.com/ . There're a few enlightening links there.

[Ron H]

Al, there are hundreds of op amps that will handle baseband video. Try these web sites:<p>http://www.national.com/parametric/0,18 ... -0,00.html<p>http://www.linear.com/prod/parametric.h ... mily=opamp<p>http://para.maxim-ic.com:80/compare.asp ... mpComp.cfm<p>I'm no balun expert, but you can buy baluns that do exactly what you want, and some have bandwidth down to DC. For example:<p>http://www.almexltd.com/balun.htm
http://www.etslan.com/Download/pv950.pdf<p>I have no idea how well these work. Do a Google search for "video balun".<p>BTW, you need to go much lower than 15kHz for baseband video. The field rate is around 60 Hz (50 Hz for some formats), and DC restoration is difficult if you don't have the low-end bandwidth.<p>Have fun!
Ron H

[MrAl]

Hi again,<p>Thanks for the interesting replies !<p>Russ:
Yes, i think i might need higher then 10MHz GBW, because i might need a
gain of at least 4 or 5 even at 5 MHz. I'm thinking min 20MHz op amp GBW
product, prefer 100MHz. But i'll have to check that part out anyway.<p>L. Daniel:
Yes, that too will be too low for any amplification at 5MHz, but i'll have to
check that out anyway. Thanks for the data sheet too, which makes it a lot
easier to view the specs!<p>RonH:
Thanks for the many great links! I'm sure there is something in that
lot i can use. I saw one op amp that went as high as 200MHz. I must
have missed that one when i was looking for high freq op amps several
months ago. The links on the baluns look good too.
The reason i was quoting "15KHz to 5MHz" was because i had seen that
some balun manufacturers had spec'd that range on their video baluns.
I'm not sure why, but i would prefer to go down to DC anyway as you
suggested. Cant figure out why the video baluns are so expensive though,
being about $30 each and you need at least two for one camera installation
which of course runs $60 US per camera just for that! All they have to
do is match 75 ohm coax to 100 ohm twisted pair, and then back again at
the other end of the line with good CMR. Can this be all that hard?
Standard TV matching transformers (for matching 300 ohms to 75 ohms)
are only about $1 US, and they incorporate one small 'balun' type magnetic
core with a few turns of wire in a special arrangement. Any ideas here?<p>
Well thanks again to all that replied!<p>Take care for now,
Al

[Chris Smith]

One stop shop at MAXIM everything you'll ever need. On the Web.<p>Filters, switching, amps, multiplexers, etc, way past 250 megs. Even test board kits.