[hpsdr] Norton Amplifiers and Noise Performance

[Ray Anderson]

>>     Can anyone comment on what attributes the Makhinson (N6NWP)
circuit

>> (which is essentially a pair of Norton amps in push-pull) brings to
the

>> table that would make it preferable over the Norton amp in this

>> application.

 

>The main advantage of a push-pull amplifier is that you'll get vastly
improved 

>second-order inter-mod. performance. The third-orders will improve by
about 

>3dB as well.

 

>I have to be a little cautious about what I due to professional
confidences. 

 

>It's certainly possible to get around +50dBm IPI3 from a push-pull
Norton 

>amplifier, and the BFG591 which has been mentioned, is capable of
approaching 

>this in the right circuit. There are better devices... One of the
problems 

>with the 'traditional' Norton amplifier is that the load line for the
active 

>devices doesn't coincide with the conditions for a 50ohm match at the
sort of 

>currents and voltages necessary for the highest IM performance. There's
also 

>the problem that the transistor bias current flows through the feedback


>transformer, which, of course, causes magnetisation of the core, and
limits 

>the dynamic range. That usually means that the ferrites specified in a 

>'traditional' Norton amp. have to be larger than would be necessary
from just 

>a signal handling viewpoint. 

 

>There are ways around both of these problems, but I can't discuss them 

>further...

 

>Incidentally, measuring +50dBm plus IPI3 is not a trivial matter...

 

>The noise figure required of a receiver is ultimately limited by the
noise 

>which comes down the antenna cable. At HF and VHF this is a well known 

>quantity, and there are graphs in most of the standard text books from
which 

>it can be calculated. If you work on the basis that the receiver system
will 

>not degrade the available S/N by (say) 1dB, then receiver noise figures
of 

>10dB at 10m and 6dB on 6m are more than adequate. I know that there's a


>market for (allegedly) 0.3dB NF pre-amps for 6m, but any system
performance 

>improvement they bring is more likely to be psychological than real...

 

>Vy 73

 

>Chris

>GW4DGU

 

-- 

 

Chris-

 

Thanks for your insightful comments regarding Norton amps and noise
performance. I fully understand your inability to get into specifics due
to proprietary considerations.

 

It is looking like we may be able to obtain very good performance on the
HF bands without the need for a preamp, however on 6 meters some noise
figure improvement may be beneficial to the system performance. I may
look into the feasibility of a switchable preamp so that the circuit may
be used when necessary, but may removed when not.

 

 

73    Ray   WB6TPU

 

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