[time-nuts] Norton amplifiers
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Jan 13 22:08:42 UTC 2020
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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