[time-nuts] Re: Isolation amp transistors

[Bob kb8tq]

Hi

> On Jul 7, 2022, at 12:09 AM, Gerhard Hoffmann via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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.

A cascode buffer is a very common thing in an oscillator. Over the decades
folks have made a *lot* of them. Having them turn into oscillators at UHF /
microwave frequencies is not at all unusual. Having the loose isolation for 
various reasons ( somebody used the wrong bypass cap maybe …) is also 
not at all unusual. 

Coming up with a model for a spice analysis that will always catch these 
things is non-trivial. A transistor intended to be used to < 50 MHz rarely has
a model that includes everything that’s relevant at 1.8 GHz. 

> 
>> 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.

In this case, the amplifier is driving into a double balanced mixer that has
a *very* similar transformer. Whatever you are seeing with the part you 
buy off the shelf is already there in the next stage. 

If you are trying for S22 from DC to light then yes, transformers have issues.
They also have issues if your “acceptable” number something past 40 db. 
Mixers (used as phase detectors) need to see termination at fairly specific
frequencies. This helps quite a bit.  

Bob

> 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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