[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
> frequencies.
> 
> 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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