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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 26 21:40:23 UTC 2012


Charles P. Steinmetz wrote:
> 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 stability of Sziklai pair like configurations (in this case a 
triple) is enhanced if the input device has a significantly higher ft 
than the output device(s).
Stable operation at unity gain is necessary if a feedback capacitor is used.

> 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.
The above reverse isolation is about 25dB lower than I would expect.
The impedance of the power supply rail at the test frequency should be 
low so using bypass caps with low inductance and esr is desirable.
The reverse isolation of an emitter follower is determined by the 
current gain at the test frequency.
Thus with an ft of 300MHz it should be around 30dB at 10MHz.

>
> 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.
>
An LM329 has similar noise without the dissipation of the internal 
heater in the LM399.
The 10nF cap between the output transistor bases is the primary source 
of  this sensistivity.

> 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
>
Bruce




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