Francesco
Yes, just keep the gain of each stage low and cascade 2 or3 if you need
higher reverse isolation.
Since the input impedance is around 1k (predominantly capacitive) at
10MHz you can drive several in parallel from a 50 ohm source.
If needed you can add a bridged T-coil network at the input to improve
the broad band matching.
If you need 100MHz operation just substitute a 4GHz ft transistor for
the 2N5109/2N5943.
In this case you may need to reduce the supply volatage dependent on the
transistor ratings.
I've simulated cascades of 3 such amplifiers and found that the phase
noise remains low.
The only thing to watch when cascading for improved isolation is reverse
coupling via the common power supply.
However with well planned supply decoupling this won't be a problem.
The ft of the BC548/558 is only 300MHz compared to ~ 1GHz or so for the
2N5109/2N5943, so reverse isolation at high frequencies will be worse
when you use the BC548/558.
Bruce
francesco messineo wrote:
Hi all,
indeed this is very interesting, can this buffer amplifier be used as
a building block for a distribution amplifier for the 10 MHz signal of
a thunderbolt? I remember having seen on the list a similar version
but with european transistors (like the ubiquitous bc548/bc558?) that
are very common here, but I can't remember when it was to surf the
list archives in the right time-period. Anyone?
best regards
Francesco IZ8DWF
On 12/9/09, Ed Palmer<ed_pal...@sasktel.net> wrote:
I was thinking that I might be reinventing the wheel by designing a buffer
amp. You guys have almost built the entire car! Thanks again for the
ideas. It'll take me a while to get it built and tested.
Ed
Bruce Griffiths wrote:
John Miles wrote:
If it helps I can send you some LTSpice schematics so that you can
simulate the circuit for yourself.
The breadboards behave as predicted by the simulations at 10MHz.
John Miles has done some preliminary phase noise measurements on his
version.
The transformers are wound on binocular ferrite cores.
I used some 14mm (long) cores intended for 40MHz to 220MHz (I had
some)
operation in my breadboard which works well at 5MHz and 10Mhz.
You can also use an off-the-shelf Mini-Circuits transformer for
low-power
applications. The T13-1 was the one I tried. I'll stick some of
the plots
up on the web later tonight if possible.
See http://www.ke5fx.com/norton.htm for measurements
and connection details
of the copy of Bruce's amp that I added to my 5061A.
-- john, KE5FX
Note the LED I used in the schematic was merely for simulation purposes
(ie LTSpice had a model for it).
A standard red or amber LED is just fine.
Another point is the LTSpice LED model isn't particularly accurate for
simulating the effects of temperature variations.
Does anyone knows of more accurate LTSpice compatible LED models?
The LED model voltage drop increases with temperature even at low current,
whilst the voltage drop across a real LED at low currents decreases with
temperature.
In practice the variation in the LED forward drop tracks the variation in
the pnp Vbe quite well.
Bruce
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