Re: HF Bands QRP Linear Amplifier

by **Ivan** Sat Oct 09, 2021 7:41 pm

neutral1 wrote:Hi Harry and Ivan,

Thank you for the information, really useful.

There is a 5uH inductor in the circuit, what do you recommend for that?
Harry has already described it.

Regarding the ferrite rings for the T1 and T2, I have got some, I think they are FT37-2 red and some FT37-6 yellow toroids.
I would use the PA for mainly 30M but with the option of it covering 20M, 40M and possibly 80M, what colour toroid would be more suitable for that frequency range?
Both the mix no.2 and no.6 will do, no.6 being better. No2 is best at 0,25 - 10 MHz, no.6 at 3 - 40 MHz. See e.g. toroids.info

What wattage/tolerance should the resistors be and what type of capacitors would you recommend, voltage/tolerance?
P = U.I = R.I^2
10% tolerance is O.K. in homebrewing.
Be aware that the capacitor between T2 and output must pass all the output power without getting hot.

It states on the diagram 12 volts would it be ok to go a bit higher to 13.8 volts which I think is what some PSU’s have?
Yes. Set the bias under this condition.

Also what current should the PSU be able to supply at a minimum for the PA.
1A is more than enough.

Thank you again.

Andrew.

2E1HSU.

73 Ivan OK1SIP

by **admin** Fri Oct 08, 2021 9:17 pm

Hi Andrew,

5uH, that would be about 4 turns on a small ferrite bead.

The idea is to get a reactance of about 300Ω at 10 MHz, as it is a part of the neutralising circuit. You could use 50pf capacitor there, although that will affect the DC bias to the transistor, but you can adjust RV1 to compensate.

If it self-oscillates then you will need the ferrite and not a cap. I used a berrite bead because I had a lot of them. Beads and rings keep the magnetic field inside the loop so there is less chance of mutual coupling to any other coils or wires carrying RF.

BR Harry

by neutral1 Fri Oct 08, 2021 6:13 pm

Hi Harry and Ivan,

Thank you for the information, really useful.

There is a 5uH inductor in the circuit, what do you recommend for that?

Regarding the ferrite rings for the T1 and T2, I have got some, I think they are FT37-2 red and some FT37-6 yellow toroids.
I would use the PA for mainly 30M but with the option of it covering 20M, 40M and possibly 80M, what colour toroid would be more suitable for that frequency range?

What wattage/tolerance should the resistors be and what type of capacitors would you recommend, voltage/tolerance?

It states on the diagram 12 volts would it be ok to go a bit higher to 13.8 volts which I think is what some PSU’s have?

Also what current should the PSU be able to supply at a minimum for the PA.

Thank you again.

Andrew.

2E1HSU.

by **Ivan** Fri Oct 08, 2021 5:22 pm

neutral1 wrote:Hi Harry and Ivan,

Thank you both for your reply’s, they are very useful.

Ivan you mentioned for the ferrite binoculars I could glue 6 to 8 small toroids together instead, did you mean ferrite beads and if so any particular type/size?
I mean ferrite rings. Beads are longer, but they usually have small holes. The rings should be like 8x4x3 mm or similar, what you can get for cheap. The mix should be suitable for HF of course.

What I want to do initially is to use a PA like this to give a WSPR signal, 200mV or less a bit more power. Would a WSPR signal with frequency shift keying be ok with this PA and if required run at 4 watts or what ever the maximum power that can be produced with out overheating or signal distortion, running continuously?
This a class A amplifier, so it is suitable for all modes.

What type/size heat sink would be suitable for running 4 watts?
Depends on the transistor type and case. Use common formulae for heatsink calculations. You need to dissipate more than 4W of heat.

Just to confirm, T1 and T2 are really just one transformer is that correct?
NOOOO!! They are two identical autotransformers on two separate cores.

What is the number of turns required?
According to the picture.

I’m not to sure if I understand the winding diagram although it is probably straight forward.
Look once more please. It is one bifilar winding passing through the holes so that the transformer is wideband.

What is the PA input and output impedance?
I expect 50 ohm.

Sorry for all the questions.
Do not apologize. And sorry for the changing font, it happens somehow automatically.

Thank you again.

Andrew.

73 from Ivan

by **admin** Fri Oct 08, 2021 4:59 pm

Hello again Andrew,

1 - Ferrite size (T2)

For only four watts the ferrites can be very small. I wouldbe tempted to use 2x 10mm Dia x 5mm thick RF ferrites, and perhaps for good measure I would use 6 ferrites super-glued to make the binoculars using three ferrites in each side - total 6 ferrites. I have used this size ferrites for up to 10 watts with only four ferrites and it worked perfectly. If you need more ferrite then the former will start to get warm with a bit of RF power.

T1 can be one pair of ferrite rings, 10mm Diameter by 5mm thick with a 5mm Dia hole. It looks less like binoculars and more like spectacles.

2 - Heatsink size

This is really easy. I could not find formulas on the web in 1999 so I did some experiments myself.

You must accept a rise in temperature of X degrees Celcius per watt. I will assume that your radio shack is really warm and you like the heat. You have your room at 25°C. You want to have your power transistor no hotter than 50°C (somewhat below the threshold of pain for an adult), so you can afford a temperature rise of 25°C.

Formula: Area ( K /TempPerWatt)² = cm²

Where K is a constant, depending on heatsink material. Aluminium = 50

You want a Class A amplifier for 4-Watts so the total dissipation will be about 8 Watts peaks. That is 3°C per watt.

HF Bands QRP Linear Amplifier Heat-1

Area = ( 50 / 3°C )² = 16² = 256 cm²

Metal against a PCB or anything else does not count in the budget. Metal should NOT be painted. If more than one pair of fins are used then the heatsing must be mounted vertically so there is good air-flow.

A bit of metal 8cm x 16cm will be sufficíent since the metal plate has two sides with 128 cm² on each side.

160mm langest side, x 1.5% = 2.4mm thick, or thinner if you use a copper plate to spread the heat.

3 - "T1 and T2 are really just one transformer is that correct?"

NO!! That is false. T1 is the input transformer. T2 is the output transformer.

Have I answered your questions?

BR Harry - SM0VPO

by neutral1 Fri Oct 08, 2021 2:32 pm

Hi Harry and Ivan,

Thank you both for your reply’s, they are very useful.

Ivan you mentioned for the ferrite binoculars I could glue 6 to 8 small toroids together instead, did you mean ferrite beads and if so any particular type/size?

What I want to do initially is to use a PA like this to give a WSPR signal, 200mV or less a bit more power. Would a WSPR signal with frequency shift keying be ok with this PA and if required run at 4 watts or what ever the maximum power that can be produced with out overheating or signal distortion, running continuously?

What type/size heat sink would be suitable for running 4 watts?

Just to confirm, T1 and T2 are really just one transformer is that correct?
What is the number of turns required?

I’m not to sure if I understand the winding diagram although it is probably straight forward.

What is the PA input and output impedance?

Sorry for all the questions.

Thank you again.

Andrew.

2E1HSU.

by **admin** Thu Oct 07, 2021 11:30 pm

Hi Andrew,
Ivan covered the construction methods.

As regards the tansistor type, almost any modern transistor intended for HF (CB) will work, such as the 2SC2078, sSC1969, 2SC1945, but you may get away with silicon audio/switching transistors, such as the TIP41 for the lower bands. You should be able to get the 2078 easily enough - they are as common as dirt.

If you do not mind changing the bias method, then you can even use a power MOSFET, such as IRF-510. All you need do is inject a constant voltage into the centre-tap of T1. IRF-510 will give you well over 10 Watts, higher gain and can operate to over 50MHz.

The IRF-510 needs about 1.0V to 3.0V (variable with a pot) to set the standing current to a couple of 100 mA.

Have I answered your question?

Very best regards - Harry - SM0VPO

by **Ivan** Thu Oct 07, 2021 6:50 am

Hi Andrew,
the article shows the tested version. You may try any bipolar NPN RF transistor with the same or bigger power capability you have at hand - it will probably work. The idle current needed may be different in that case.

Manhattan and ugly bug styles are good at HF, but less mechanically stiff. Keep the leads short.

Ferrite binoculars for VHF have become rare now. I bought my last lot in China for a fairly high price. You can glue 6 or 8 small toroids together instead: make two columns of 3 or 4 pcs and glue these columns together side by side.

I personally would prefer a thicker wire if you do not have #22 at hand, not a thinner one.

VBR Ivan OK1SIP

by neutral2 Wed Oct 06, 2021 5:29 pm

Hi Harold,

I was reading the article above and wondered if you know of a more readily available and cheaper transistor than the 2N5590 specified, that could be used in it’s place?
Could this circuit be built on a copper clad board using Manhattan style construction, ugly bug style or whatever it is called?

Do you also know of a more readily available 2 hole ferrite core than the one specified that would be suitable?

22 SWG wire is specified for the balun, would it be ok to use 26, 27 or 28 SWG instead?

Thank you.

Andrew.

2E1HSU.

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