[time-nuts] Re: Isolation amp transistors

[ghf at hoffmann-hochfrequenz.de]

Am 2022-07-07 7:22, schrieb Bob kb8tq via time-nuts:
> Hi
> 
>> On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nuts 
>> <time-nuts at lists.febo.com> wrote:
>> 
>> The 2N5179 has high base spreading resistance (decreases isolation).
> 
> As does sticking a resistor (even a small one) in series with the base
> …. Yes, inductance is even worse.

and at frequencies where beads work, they also generate thermal noise,
like any other dissipative thingie. See the sim of a random ferrite bead
from the LTspice library. V1 is only there as a compiler pleaser to
enable the proper syntax. It really has no influence.

> For “best isolation” in a cascode you very much want the base of
> the common base stage nailed to ground. Typically “lower” Ft 
> transistors
> with a decent base structure are the best choice for the common base 
> stage.
> Both stages benefit from low 1/F noise > in the audio range if this is 
> for
> a phase noise test set.  This is why people use what would normally be
> considered “audio” transistors ….

Cascodes do not add much noise when they have a decent beta. Zin at the
Emitter is a few Ohms and Zout of the driving stage is maybe KOhm. That
makes the driving stage DICTATE the collector current. Also, 1/f noise
is not THAT bad since the load resistance is near 0 in the 1/f region.
Thus, at least no gain at 1/f frequencies. In a linear amplifier, it
would not get mixed up anyway. Makes me like the Driscoll oscillator.

I could not find a transformer (Macom, Pulse Eng.) that provided an
acceptable S22. The resistive 50 Ohm in par with (4.7u + bead) worked 
best.
At least the momentary collector voltage can exceed the supply.
Appreciated in the light of 12V operation. But without the transformer,
one pays with a lot of bias current, therefore sot-89.
The circuit is not exact, it was in the middle of what/if experiments.
Thus, some funny values.
The cascode is mostly harmless (TM). What hurts, that is Q2 stability.

@Florian:  150 mA, not too much Vce required, 1-1.5 GHz ft.

Cheers, Gerhard
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