[time-nuts] Re: Isolation amp transistors

[Bob kb8tq]

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. 

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 ….

Bob


> 
> ---
> Rick Karlquist
> N6RK 
> 
> On 2022-07-06 12:18, ed breya via time-nuts wrote:
> 
>> My favorite VHF Q is the good old 2N5179 or similar, but it appears you want something in surface mount, and not obsolete. I'm not familiar with the modern SMT stuff. If your present transistors are working, but just need a bit more stability, it seems it should be OK with the right scheme, and not the transistors' fault.
>> 
>> Ed
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