[time-nuts] RF Isolation Amplifier ...

Gerhard Hoffmann dk4xp at arcor.de
Sun Oct 14 16:14:59 EDT 2018



Am 14.10.2018 um 18:47 schrieb Dana Whitlow:
> Corby,
>
> Now that I learn that the BFT66 is a "wild one", I'd remark that stability
> is
> indeed the likely issue.  I was puzzled by the presence of that capacitor in
> the first place, as this circuit is clearly neither tuned nor of especially
> high frequency capability.
If the circuit really has a stability problem, one could try first to
harvest the low-hanging fruit and change the base resistors from
2R5 to 22R, maybe even more.

That won't damage the noise behavior, that has already been done by
the 180R in the input. The only possible reason I see for the 12 pF is
to kill the gain of the first stage at VHF+.  It may be a disservice to
the 2nd stage, depending on the isolation performance of the R
between the upper emitter and the lower Collector.

Also, the 100nF+10nF caps in the base divider will parallel resonate
at +/-10 MHz, depending on layout; and that means "no real decoupling" 
there.

All in all, the circuit is somewhat weird; the BFT66 was a boutique part
even in it's heyday: low noise, high gain, high ft, high price and a 
package
that was not really RF-ish. The second stage is abt. as low cost as you 
could get.
With all-3904/3906 that would be a text book AF amplifier.

Have a nice Sunday evening,

Gerhard, DK4XP







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