[time-nuts] RF Isolation Amplifier ...

Gerhard Hoffmann dk4xp at arcor.de
Sun Oct 14 17:18:15 EDT 2018


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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