[time-nuts] RF Isolation Amplifier ...
Dr. Ulrich L. Rohde
ka2weu at aol.com
Sun Oct 14 17:04:21 EDT 2018
Well some of the 7 GHz and higher Ft transistors have internal matching networks to prevent such oscillations, but yes it is an important issue.
73 de N1UL
Sent from my iPhone
> On Oct 14, 2018, at 4:14 PM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
>> On 10/14/2018 11:20 AM, Dr. Ulrich L. Rohde via time-nuts wrote:
>> Actually the BFT is out of production since quite a while there are more stable and higher Ft devices on the market.
>> 73 de N1UL
> How is a higher Ft device more stable? Those attributes
> would seem to be mutually exclusive.
> For time nuts purposes, I would submit this is a bad trend.
> What we want is higher DC beta, not higher Ft. The higher
> Ft just makes the device want to oscillate. For any designs
> I do, I put a 100 ohm resistor in series with the collector
> as an oscillation killer.
> There is a similar problem with gain block amplifiers having
> bandwidths into the double digit GHz. I routinely put a
> 10 pF capacitor directly from input to ground to kill high
> A related problem is that newer devices have lower base
> spreading resistance. This does help with noise figure
> but again risks HF oscillations.
> Rick N6RK
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