[time-nuts] RF Isolation Amplifier ...

Dr. Ulrich L. Rohde ka2weu at aol.com
Sun Oct 14 17:05:14 EDT 2018


Well said 

Sent from my iPhone

> On Oct 14, 2018, at 4:14 PM, Gerhard Hoffmann <dk4xp at arcor.de> wrote:
> 
> 
> 
>> 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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