[time-nuts] RF Isolation Amplifier ...

[Richard (Rick) Karlquist]

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