[time-nuts] Distribution amp - Use a video amp unit ?

[Charles P. Steinmetz]

Bruce wrote:

>A circuit schematic for a current feedback triple with reasonably 
>low noise and distortion is attached.

Quite a good performer for such a simple circuit.  I found, both in 
modeling and on the bench, that there is the usual noise bump at 
200-300 MHz and non-monotonic behavior out in the 900 MHz 
region.  The latter can be solved by using an MPSH10 for Q1, which 
also brings the in-band noise and phase noise down a little.  The 
former can be addressed by adding 8-10 pF across R2, at the expense 
of lowering the 3 dB point from around 150 MHz to around 80 MHz.  For 
use as a 5 or 10 MHz distribution amp, I'd include the cap.

The input impedance stays decently high everywhere the amp has useful 
gain -- there should be no problem paralleling 10 of them on a 50 ohm 
source.  You can raise R2 just a tad to get back to unity gain, if 
needed.  The reverse isolation is about 35 dB.  This can be improved 
to around 50 dB by adding an emitter follower at the input, adjusting 
R7 and R8 to maintain Q1's base voltage.  The noise increase is negligible.

It is fairly sensitive to power supply noise, so you want a nice 
quiet supply.  I used a regulator built with an LM399 and LT1028.

Since the transformer is 1:1, one might be tempted to omit it.  For a 
distribution amp that will be connected to a number of different 
instruments, however, one is well advised to include it to isolate 
the various returns.  6 bifilar turns on a T43-37 toroid core and 14 
bifilar turns on a T61-37 both worked fine for me.  If you have 1:1 
transformers from a spare Ethernet card, those should, too.

For a Q&D distribution amp, this would be a pretty good candidate.

Best regards,

Charles