[time-nuts] Norton amplifiers

[Bruce Griffiths]

Simulation indicates that the required C value depends on the transformer turns ratios and the RC product sets the notch frequency. 

Large signal modulation of the transistor parameters results in a distorted signal at the base when a large signal is applied to the output.

the RC product is around 4ns for a simulated 2N3904 for a reverse isolation notch at 10MHz. The value if C is a multiple (dependent on transformer turns ratio) of Ccb.

Bruce

> On 14 January 2020 at 09:01 Claudio Girardi via time-nuts <time-nuts at lists.febo.com mailto:time-nuts at lists.febo.com > wrote:
> 
> 
>     Some years ago I played a little with that buffer circuit and found out that the isolation up to a few MHz can be improved a little with an RC feedback network between base and collector.
>     I cannot find my notes right now, IIRC from the output the circuit can be seen as a sort of bridge where one can balance the various impedances for minimum feedback.
>     Enclosed is a graph for the measured gain and isolation for that circuit, using a 2N3904 (!), for several configurations -
>     without the RC feedback, with an RC combination giving some overall improvement in S12 and with some values that gave a sharp null at 10 MHz.
> 
>     Claudio
> 
>         > > Il 13 gennaio 2020 alle 18.27 Jeffrey Pawlan <pawlan at runbox.com mailto:pawlan at runbox.com > ha scritto:
> > 
> > 
> >         Many years ago I did a study of Norton amplifiers and optimized for IP3
> >         using non-linear circuit simulation tools. I published a two part
> >         article in RF Design Magazine which covered the amplifier itself as well
> >         as the non-linear model for the BJT. My use for the Norton amplifier did
> >         not require high isolation so I spent little tile on that aspect. I am
> >         friends with the co-inventor of the original and the author of the
> >         subsequent patents. His name is Allen Podell.  The webpage you included
> >         speculated that the reverse isolation degradation at high frequencies
> >         was owing to the layout or the transformer. Although those are
> >         contributors, the simulation showed high frequencies had poorer s12 so
> >         it is expected.
> > 
> >         If high isolation is what you need, then as written on this list, there
> >         are ICs which can provide this much better than a single stage
> >         amplifier. They do suffer from more residual noise however.
> > 
> >         Jeffrey Pawlan
> > 
> > 
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