Norton Amplifiers


Chris Bartram G4DGU
 

Having, courtesy of Dom, seen information on the Norton feedback amplifier circuit, can I just make a couple of comments from rather a lot of direct experience of 'noiseless' feedback linearised amplifiers, and related circuitry, make a few observations and comments?

I'm not suggesting that the basic negative feedback topology doesn't work. It does, but really only in its original format at VHF. The problem is that both the active device and the feedback transformer, like most circuit elements at high frequencies, have phase shifts which can easily combine at frequencies above the operating frequency to produce positive feedback. If the gain of the amplifier is greater than unity at that frequency, a spurious oscillation WILL result! I have a tee-shirt collection ...

Those of us who were using the technique 40 years ago were able to make stable amplifiers using now historical devices like the BFT66, BFR91, BFR96 etc. They worked - with a bit of fiddling, such as a ferrite bead in series with the collector lead simply because the fT of the devices was quite small. I even managed to design a reliable NFB LNA using a C-band GaASFET at VHF, albeit using a different circuit. A lot of these designs found the way into production - indeed the FET amplifier circuit was ripped-off by another designer for an IF preamp design for a military radar!

The Ft of the next generation of bipolar devices was rather greater than that of the previous parts, and the simple Norton approach became untameable. Simply replacing the transistor in DJ7VY's design with something more recent is very likely indeed to result in an 'informal oscillator'!

There are other -ve feedback circuit topologies which can be used to make feedback linearised low-noise amplifiers (and interestingly transmitter power amplifiers) but they suffer from the need to have a hybrid in the input signal path. I have looked at linearising modern broad-band MMICs, but there isn't a simple reproducible topology which I've yet found despite a lot of playing both on the bench and by using modelling. I have achieved good performance from a couple of prototypes at 144, though, with IPI3 figures in the +30dBm region and NFs ~1.2dB, but they haven't proved reproducible.

My approach, FWIW, to the design of a second stage of a 'bomb-proof' receiver at 432 or 1296MHz would be to use a group of more modern MMIC devices such as the PGA-105 combined together with hybrids. Two groups of two combined using 0degree hybrids and then combined into a push-pull amplifier would give very good performance, but beware of mixer damage if you have strong local signals!!

73

Chris

G4DGU


 

On Wed, 19 May 2021 at 18:46, Chris Bartram G4DGU <chris@...> wrote:
My approach, FWIW, to the design of a second stage of a 'bomb-proof'
receiver at 432 or 1296MHz would be to use a group of more modern MMIC
devices such as the PGA-105 combined together with hybrids. Two groups
of two combined using 0degree hybrids and then combined into a push-pull
amplifier would give very good performance, but beware of mixer damage
if you have strong local signals!!

73

Chris G4DGU

I think we need an Eimac 4CX10,000A  as a mixer in contests, with a 4CX250B in the front end. This reminds me, I need to send you some data sheets. I had forgotten about that but will do it asap.

Dave


Neil Smith G4DBN
 

I seem to recall seeing the layout of a Minicircuits amp that used hybrids and paired amplifier chains.  One of the benefits siuggested was that if one leg of the amp died, the other would carry one at only 6dB down, as well as being very linear and robust with a high output IP3.  I thought it was the ZRL-700+ but can't find the notes about the internal circuit of that one, so I might be wrong. NF is 2dB at 70cm and the gain is very high, so not much benefit unless it is close to the masthead. It wasn't the TAMP-72LN+ drop-in module either. Perhaps I was dreaming.

Neil G4DBN


Conrad, PA5Y
 

Could you be referring to the HELA10B dual monolithic amplifier? I have a pair of these on my 2-channel adaptive EME system. They are there to overcome splitter losses and give a bit of extra gain for the IQ+. The path for one channel is HELA10B (11db gain 2.5dB NF on 144) then a Band pass filter, then a 3dB hybrid splitter. One side feeds the IQ+ (used for the adaptive RX) and the other my transverter which is used a linear polarisation (either +/- 45 deg) channel with the K3S. The whole system seems to work very well.

I will try one on 432 but the noise figure is significantly higher on the 4 that I built.

https://www.minicircuits.com/app/AN60-009.pdf

You need a heatsink!

Conrad PA5Y

-----Original Message-----
From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of Neil Smith G4DBN via groups.io
Sent: 19 May 2021 20:07
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Norton Amplifiers

I seem to recall seeing the layout of a Minicircuits amp that used hybrids and paired amplifier chains.  One of the benefits siuggested was that if one leg of the amp died, the other would carry one at only 6dB down, as well as being very linear and robust with a high output IP3.  I thought it was the ZRL-700+ but can't find the notes about the internal circuit of that one, so I might be wrong. NF is 2dB at 70cm and the gain is very high, so not much benefit unless it is close to the masthead. It wasn't the TAMP-72LN+ drop-in module either. Perhaps I was dreaming.

Neil G4DBN


Neil Smith G4DBN
 

Note dreaming after all. Phew. This is the Appnote I vaguely recalled. Looks like it is about the ZRL-700+ (and the other models)

https://www.minicircuits.com/appdoc/AN60-039.html

Neil G4DBN

On 19/05/2021 19:23, Conrad, PA5Y wrote:
Could you be referring to the HELA10B dual monolithic amplifier? I have a pair of these on my 2-channel adaptive EME system. They are there to overcome splitter losses and give a bit of extra gain for the IQ+. The path for one channel is HELA10B (11db gain 2.5dB NF on 144) then a Band pass filter, then a 3dB hybrid splitter. One side feeds the IQ+ (used for the adaptive RX) and the other my transverter which is used a linear polarisation (either +/- 45 deg) channel with the K3S. The whole system seems to work very well.

I will try one on 432 but the noise figure is significantly higher on the 4 that I built.

https://www.minicircuits.com/app/AN60-009.pdf

You need a heatsink!


Conrad, PA5Y
 

That looks very useful Neil but a bit too much gain.

I am glad that you weren't dreaming because to dream of such things would be a little, dare I say, odd?

73

Conrad


From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> on behalf of Neil Smith G4DBN via groups.io <neil@...>
Sent: 19 May 2021 20:43
To: UKMicrowaves@groups.io <UKMicrowaves@groups.io>
Subject: Re: [UKMicrowaves] Norton Amplifiers
 
Note dreaming after all. Phew. This is the Appnote I vaguely recalled.
Looks like it is about the ZRL-700+ (and the other models)

https://www.minicircuits.com/appdoc/AN60-039.html

Neil G4DBN

On 19/05/2021 19:23, Conrad, PA5Y wrote:
> Could you be referring to the HELA10B dual monolithic amplifier? I have a pair of these on my 2-channel adaptive EME system. They are there to overcome splitter losses and give a bit of extra gain for the IQ+. The path for one channel is HELA10B (11db gain 2.5dB NF on 144) then a Band pass filter, then a 3dB hybrid splitter. One side feeds the IQ+ (used for the adaptive RX) and the other my transverter which is used a linear polarisation (either +/- 45 deg) channel with the K3S.  The whole system seems to work very well.
>
> I will try one on 432 but the noise figure is significantly higher on the 4 that I built.
>
> https://www.minicircuits.com/app/AN60-009.pdf
>
> You need a heatsink!
>