[time-nuts] Re: Isolation amp transistors

[Magnus Danielson]

Hi,

On 2022-07-07 07:22, Bob kb8tq via time-nuts wrote:
> 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 ….

The NIST isolation amplifiers does exactly this. Looking for Fred Walls 
in the NIST T&F archive usually makes me find the article quick.

Cheers,
Magnus