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Author Topic: expected bandwith of Norton rf amplifiers.  (Read 12936 times)
WB6BYU
Member

Posts: 13481




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« Reply #15 on: December 25, 2012, 10:04:36 AM »

Hmmm...  5 MHz = 60m

Then a good starting guess for 200 ohm reactances would be
L = 60m / 10 = 6uH
C = 60m * 3 = 180pf

That's for 200 ohms.  For a loaded Q of 10 in a 50 ohm system,
each would need to be 500 ohms instead, or larger by a factor
of 2.5.

6uH * 2.5 = 15uH
180pf / 2.5 = 72pf

So 6400pf would be too large by about 2 orders of magnitude.

These aren't exact calculations, but are adequate for estimates
when trying to work problems in the bath or while mowing the lawn.
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VK2TIL
Member

Posts: 340




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« Reply #16 on: December 25, 2012, 12:01:17 PM »

I was lazy; I used a calculator for a bridged-T diplexer;

http://i48.tinypic.com/2h2h3ps.jpg

It seems to work OK; it's both a filter;

http://i50.tinypic.com/2relds4.jpg

and a termination;

http://i46.tinypic.com/2zgvo6c.jpg

at the same time.
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ZL1BDA
Member

Posts: 33




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« Reply #17 on: December 25, 2012, 07:55:13 PM »

Many thanks to everyone and Happy Holidays and 2013.
I took your suggestions on board and have 6 2N5109's and s selection of the recommended
toroids on order from W8DIZ. They seem to give VERY good service BTW, even over the Xmas
break. With an Ft of ~ 1.2GHz the 2n5109 should be good up to over 100 MHz and give me
 everything I need to drive that small but power hungry mixer.
When this is finished I am thinking seriously of building it into a transceiver. It would be good to
 be back on the air with an all home brew station again.
I'm still a bit horrified by the poor efficiency of the finals in my old Kenwood 820s. My old home
 brew TT21 final ran at the optimum 67% efficiency on 14 MHz!
73 Cliff Wright
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ZL1BDA
Member

Posts: 33




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« Reply #18 on: January 07, 2013, 06:16:23 PM »

Well I just got delivery today of the 2N5109's from W8DIZ. Very good service BTW, they beat some transistors coming 50 Km from Auckland!!!
So I put it into the same amp as the old 908's. But Ft now 1.2Ghz rather than 500 Mhz!
No more bandwidth problem, it seems fine up to at least 80 MHz.
BUT I got a lower than expected output from the sysnthesiser.
Checked the synth output and it was 0.5 volts into 50 Ohms just as advertised.
However the Norton amps input impedance is now obviously a LOT
less than 50 Ohms. At a rough guess, I would say its about 10 to 15 Ohms!!!!!
Very odd indeed. I am using a standard 2 hole balun core at the moment and have designed for about 12dB
of gain (50 Ohms in and out).
Anyone else out there had a similar problem?
So far my thought is to replace the balun core with some of the suggested toroids that
I got from W8DIZ along with the transistors. But the input circuit is the usual 22uH choke
in series with 68 Ohms, unbypassed and with the single turn "Norton" feedback winding
as usual. The amp alone worked fine on the bench, but I did notice that it was loading the
signal generator more than before especially at higher frequencies.
So anyone got any ideas,which will be very gratefully received.
Cliff Wright ZL1BDA ex G3NIA
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ZL1BDA
Member

Posts: 33




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« Reply #19 on: January 07, 2013, 06:56:15 PM »

Just tried out the amp with the FT50-61 toroid instead.
EXACTLY the same result!!!!!
I must admit that I am getting really at a loss as to what is going on.
The stage runs at about 30 mA Ic and all the DC voltages are OK.
My old Advance sig gen (the later one) is a reasonable 50 Ohm source
and trying a series 51 Ohm resistor in the input merely proved that
Zin is a lot less than 50 Ohms.

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VK2TIL
Member

Posts: 340




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« Reply #20 on: January 07, 2013, 07:31:23 PM »

Good to hear that you are making progress!

Assuming that there are no wiring mistakes (it's not difficult with those little transformers, that's why I put many of my home-made transformers on a little PCB) the blame now shifts to the transformer core.

Ferrite ain't ferrite; I never use ferrites of unknown material.

You didn't say what material you are using; if it's the common Jaycar "balun core" it is M7 material which has a ui of 160 compared to a ui of 850 for 43 material.

Try winding the transformer on a 43-material core; 2302 size will give good performance in the mid-upper HF range and into VHF.

If gain drops-off below about 10 MHz and you want more there, try stacking two 2302s or use a 2402 which is larger and easier to wind as a bonus for we oldies (I'm only a couple of years behind you!).

As you can see from my earlier photos, my 2 x 9dB amplifier gives good performance up to 300 MHz with 43-material transformers.

ADDED; you posted again while I was typing in my painful one-finger style.

61 material has a ui of 125; it may not give sufficient winding L down low.

What frequency are you testing at?

If it's low, try testing at, say 30 or 50 MHz and see what happens; if things are better up higher, try 43 material.


Kerry VK2TIL.
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ZL1BDA
Member

Posts: 33




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« Reply #21 on: January 07, 2013, 08:44:01 PM »

Hi Kerry!
The amp in question amplifies the output of a DSS synthesiser
That covers from 45.3 to 75 Mhz to upconvert to a 45 MHz 1st
IF using a high level diode mixer needing 17dBm of drive!
That's why I used type 61 material first. I have some higher u
stuff also. It is very odd though, every published circuit emphasises
that the Norton has a 50 Ohm input and output impedance.
Output, yes, but I now measure the input Z of mine at very close
to 17 ohms. Of course that means that it is loading the DSS output
far to much.
BTW otherwise it amplifies just as designed over a bandwidth of ~100Mhz.
(Sudden thought. Could it be a question of the input capacitance of the
2N5109?) that would mean the emitter capacitance, of course.
The choke I am using is one of those little 22uH potted ones.
Tomorrow I might try using something else, as that could have some sort of
effect all on its own.
Cliff Wright.

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VK2TIL
Member

Posts: 340




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« Reply #22 on: January 07, 2013, 08:57:48 PM »

61 material sounds fine for that service.

You may be correct about the choke.  I think that most circuits I've seen use a lower value; I used "bought" 4u7 ones (probably Jaycar) in mine.

I often wind my own chokes for critical applications and check them for SRF; perhaps, at your fairly-high frequencies, there is resonance going-on.

Otherwise I'm puzzled; you seem to have covered all bases.

Kerry.
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VK2TIL
Member

Posts: 340




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« Reply #23 on: January 08, 2013, 12:57:47 PM »

Just looked at the input VSWR/RL of my amplifier;

http://i47.tinypic.com/1zxy0rm.jpg

Good over about 10 MHz; not so good below that.

"50 ohms" is a bit flexible when talking about amplifier impedance; I usually regard better than 2:1 VSWR as good.

Kerry VK2TIL.
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ZL1BDA
Member

Posts: 33




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« Reply #24 on: January 08, 2013, 01:17:48 PM »

Thanks for the reply Kerry.
Yes, I would reckon that a 2:1 VSWR would be OK.
But here the impedance has got so low that it is having
a very bad effect on the previous stage.
I have some jobs to do around my son's place this morning,
but will try changing the choke and one other thing that I
came across. Apparently as the negative feedback increases
the input impedance should rise. So I will try a 2 turn feedback
winding in the emitter. Fortunately it is easy to take out this
 sub assembly so I can do it on the bench.
BTW it doesn't look like a resonant choke problem as it is not very
frequency dependent.
It should still have just about enough gain even with more feedback.
I'll post my results so others might be helped who, just possibly,
run into similar problems.
Cliff Wright ZL1BDA
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VK2TIL
Member

Posts: 340




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« Reply #25 on: January 08, 2013, 02:39:44 PM »

You have a lot of room on that FT50 toroid; it might be worth trying an increase in all the turns while keeping the "Norton" ratios the same.  61 material has ui of 125 which is fairly low and requires more turns for the same inductance than 43 material with its ui of 850.

Keeping the ratios the same with two turns instead of one seems possible using the Norton equation but I'm no mathematician.

Chris Trask, in his paper on Norton amplifiers, commented that "... the performance is less dependent on the transistor and more dependent on the transformer, particularly the coupling co-efficient between the three windings.".

The advantage of the two-hole balun core is that good coupling is obtained almost automatically.

Kerry.
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ZL1BDA
Member

Posts: 33




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« Reply #26 on: January 08, 2013, 07:50:54 PM »

How right you are about the toroids Kerry!
I think that in future I will stick to the good old 2 hole
baluns, I have a good stock of those.
You know thinking back I have had problems with toroid transformers before
I built one of the VK5 noise cancellers, and it took a lot of fiddling to get good cancellation.

With the Norton I tried turns modification at a basic level.
A 2 turn feedback winding brings up the input impedance fine, to around 70 Ohms
but of course it "kicks the guts" out of the gain.
However the position of the emitter turn relative to the output windings is critical.
It really must be at the supply end, even then it is still a bit under 30 Ohms with a single turn.
So after going to the dentist tomorrow morning I will wind a new 2 hole transformer using all
the suggestions and will let you know what I get.
BTW the synthesiser is on of the SDR kits. Goes fine but mine never gave the claimed output level.
I have just emailed SDR with a query about the Si570 chip which mine uses as their figures and other
published data don't seem to match.
73 Cliff Wright
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VK2TIL
Member

Posts: 340




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« Reply #27 on: January 08, 2013, 08:43:14 PM »

Perhaps the relatively-small number of turns in a Norton transformer is getting "lost" on that quite large toroid and the coupling is not "tight".

This is a very good article on the Norton amplifier with a complete description of the construction of the transformer;

www.ece.vt.edu/swe/lwa/memo/lwa0071.pdf

I did some thinking about design of Norton transformers with two or more turns on the emitter winding; the maths is quite beyond me but I have asked some more "mathematical" people to consider it.

I don't think it's simply a matter of "scaling"; there is the "square-law" action of the impedance transformation and the maths to account for that is way beyond me.

I'd like to know the equation and also the gain equation in the two-turn case; it might be interesting to experiment.

Good luck at the dentist; never fun!

Kerry.
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VK2TIL
Member

Posts: 340




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« Reply #28 on: January 09, 2013, 12:57:57 PM »

Something has occurred to me; I hope you won't be offended if I ask whether you are terminating the amplifier output in 50 ohms for your tests.

A Norton amplifier "reflects" its output load as its input impedance so the output impedance must be well-defined to give a similar input impedance.  An attenuator on the output helps if the load impedance is not well-defined (as, eg, a mixer or a filter) but there must be some spare gain for this.

Kerry VK2TIL.

 
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