[time-nuts] RF Isolation Amplifier ...
Richard (Rick) Karlquist
richard at karlquist.com
Sun Oct 14 16:14:17 EDT 2018
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.
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