Thanks for those ideas. I was considering putting a low pass filter after
the amp. The 2 shunt caps work out to be 560pF each.
JEFF
"Mac" wrote in message
news
On Thu, 22 Sep 2005 09:14:55 +1200, Jeff wrote:

Hi,
Ive been trying to locate a suitable unity gain buffer amplifier IC that
can
drive a 50 load.
Application is to buffer a VCO output at 5Mhz.
Needs to be 8pin DIP and operate on a single-ended supply.
I had found what I wanted in the CLC109, but cant seem to find a
supplier.
Any suggestions greatly appreciated regarding type and supplier that is
user
friendly to international countries.
JEFF
ZL3JK
New Zealand

What you want is a video buffer or video op-amp. Linear Technologies,
National Semiconductor, and Analog Devices are among the many companies
that make such things.

Note that some of these devices get a bit squirrelly if they see a
capacitive load. So I recommend that you do something like this: (use
courier or other constant-width font for ASCII art schematic)



|\ 50
in--|+\__+____/\/\____/ to 50 Ohm load
+--|-/ |
| |/ |
| |
+--/\/\-+
| Rf
\
/Ra
\
/
|
-----
GND

Rf and Ra would be the same value. Use a value recommended by the
datasheet, or 1k if the datasheet doesn't recommend a specific value.
Follow any other suggestions in the datasheet as well. For example, they
may suggest a small capacitor directly from the op-amp output to inverting
input.

You probably want to terminate the input with a 50 Ohm resistor, too, but
I don't know, so I'll leave it out.

It sounds like you may need AC-coupling, so you can put blocking
capacitors on the input and/or output. Oh, if you put a blocking cap on
the input, be sure to also provide a DC (resistive) path to GND or to the
mid-rail point if you are going single-supply.

Just chose a capacitor big enough so that 1/(2*pi*R*C) is much lower than
5 MHz, your signal of interest. (R in this case is 50 Ohms) There is no
problem putting a capacitor like this in the signal path: Since it looks
like a short circuit at higher frequencies, it does not capacitively load
the amp at frequencies where the amp might be unstable. Also, the series
50 Ohm resistor keeps the overall impedance resistive.

HTH

--Mac

Try a SL560

"Jeff" schreef in bericht
...
Hi,
Ive been trying to locate a suitable unity gain buffer amplifier IC that
can
drive a 50 load.
Application is to buffer a VCO output at 5Mhz.
Needs to be 8pin DIP and operate on a single-ended supply.
I had found what I wanted in the CLC109, but cant seem to find a supplier.
Any suggestions greatly appreciated regarding type and supplier that is
user
friendly to international countries.
JEFF
ZL3JK
New Zealand

Winfield Hill wrote:

Jeff wrote...

Needs to drive into 50 ohms. input level approx 150mV at 5Mhz

Winfield Hill wrote ...
Jeff wrote...

Ive been trying to locate a suitable unity gain buffer amplifier
IC that can drive a 50 load.
Application is to buffer a VCO output at 5Mhz.
Needs to be 8pin DIP

LTC's LT1206 comes in TO220, miniDIP and soic packages,
and Digi-Key has them in stock for $6.25 each...
http://rocky.digikey.com/WebLib/Line...ata/LT1206.pdf
http://www.digikey.com/scripts/DkSea...ail?Ref=329191

and operate on a single-ended supply.

Oops, +/-5V supply. What's your signal voltage? And did
you want your output to have a 50-ohm source impedance?

Unless you source with 50 ohms (2x signal plus 50-ohm resistor),
and terminate with 50-ohms, the issue becomes how much cable
capacitance? Either the load looks resistive (100 ohms in the
former case) or capacitive.


I would say it depends on the cable you choose. If there is a 50 Ohm load
and you connect it to your buffer amplifier with 50 Ohm coaxial cable, then
the buffer amplifier will see a 50 Ohm resistive load, not capacitive at
all.

It is probably still wise to use the gain-of-two amplifier and 50 Ohm source
resistor in case the load happens to be a poor match. If you really don't
need this feature then I would be tempted to use an emitter follower with
just one transistor. (Do they make transistors in DIP-8 packages?)

Chris