Am 14.10.2018 um 22:14 schrieb 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.
Every mm has a nH all for itself. With 1.6mm for the board thickness,
2 mm for the transistor leads, 0.5 mm for the bond wires, we have
> 3 nH in base, emitter and collector, and they are coupled.
That can annoy the most well behaved RF transistor. [1]
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.
I do love those Zetex BJTs with extra low base spreading resistance.
We just discussed this in the usenet sci.electronics.design newsgroup.
In "Art of Electronics edition 3 " by Horowitz? and Winfield Hill, there
is a base
band amplifier featuring 70 pV/rtHz voltage noise.
I have verified that. One of the Zetexes was not enough, it took 16 in par
to get Rbb low enough, just like demonstrated by Horowitz&Hill
<
https://www.flickr.com/photos/137684711@N07/31348447748/in/album-72157662535945536/
>
anf the pictures to the right.
( the pics to the left are off-topic, but some of them nevertheless nice.)
regards,
Gerhard
[1] analog to <
http://gunkies.org/wiki/Vonada%27s_Engineering_Maxims >
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