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