# Video buffer amp

D

#### Data888

Jan 1, 1970
0
I'm not sure if any of you remember but I've posted some questions about
building a video switcher. I found this schematic for a buffer amplifier
to put on the output:
http://www.mopacmedia.com/tips/pxlmod3.html
Is this suitable? I don't understand much about transistors but if the
amplifier is running off 9v, won't the output be peaking at 9v? I
thought a video signal was 1V p-p.
Can anyone answer there questions for me? Thanks.

Nick

P

#### Peter Bennett

Jan 1, 1970
0
I'm not sure if any of you remember but I've posted some questions about
building a video switcher. I found this schematic for a buffer amplifier
to put on the output:
http://www.mopacmedia.com/tips/pxlmod3.html
Is this suitable? I don't understand much about transistors but if the
amplifier is running off 9v, won't the output be peaking at 9v? I
thought a video signal was 1V p-p.
Can anyone answer there questions for me? Thanks.

Nick

That is a simple emitter follower, which will have a gain of 1 (pretty
closely) - if you feed a 1V P-P video signal into it, you will get 1V
P-P out.

G

#### Glenn Gundlach

Jan 1, 1970
0
Peter Bennett said:
That is a simple emitter follower, which will have a gain of 1 (pretty
closely) - if you feed a 1V P-P video signal into it, you will get 1V
P-P out.

The OP doesn't say if he wants to drive a cable. The buffer doesn't
account for the driving impedance of the cable and the gain of 2
needed to accomplish it. Also, most video has sync negative (back
porch at 0 v) and the buffer with its ground reference can't go
negative and throws in a negative offset to boot. If the buffer is
used internally on a circuit board it could be usable.
GG

B

#### Baphomet

Jan 1, 1970
0
snip... snip...
Also, most video has sync negative (back
porch at 0 v) and the buffer with its ground reference can't go
negative and throws in a negative offset to boot. If the buffer is
used internally on a circuit board it could be usable.

http://www.mopacmedia.com/tips/pxlmod3.html

Putting a clamping diode (cathode to transistor base - anode to ground)
should solve the problem (I think) of the negative sync.

G

#### Glenn Gundlach

Jan 1, 1970
0
Baphomet said:
snip... snip...

http://www.mopacmedia.com/tips/pxlmod3.html

Putting a clamping diode (cathode to transistor base - anode to ground)
should solve the problem (I think) of the negative sync.

You mean as a 'catch' diode to clamp the sync tip?. That helps with
the clipping but does not deal with the necessary DC offset. The real
problem is to provide current boost, preferably with no DC offset,
possible gain of 2, possible 75 ohm line drive (very low source
impedance with proper resistor). My personal favorite is to use
current feedback amps with bipolar supplies and DC coupling. DC
restoration if that parameter is important. Yeah, I know it costs more
but I'm using several hundred dollars of computer and an extra few
bucks doesn't seem that prohibitive to me-- particularly if it works
great and I can forget about it and just use it. Sorry to ramble.
GG

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