RF amp

John Larkin wrote in news: snipped-for-privacy@4ax.com:

Sheesh...

15Mhz+rf+amp&_sacat=0

Class C depends on the resonant LC circuits that follow to reduce the disto rtion. If you want a cleaner signal, use a push-pull configuration. The old Motorola Power RF handbook had a lot of good information. It should be ava ilable for download on Bitsavers or Archive.org.

We have already bought three of the cheap Chinese RF amps like that from Amazon. All were junk.

They generally have balun-type transformers driving a pair of unbiased bipolar transistors, probably fake Motorola parts, which has zero gain at low level and gobs of nonlinearity and distortion, if they work at all. The single-MMIC ones output a fraction of the claimed power.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

That's interesting. "Class A" implies that they at least bias the transistors some.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

The data sheets say that either can be damaged when driving an opan load. I'd rather not have that hazard for a production test setup.

Maybe we could buy the 5W unit and always use an attenuator. It's $1020 so we'd need to mount it and power it and heat sink and all that.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Kit Sold Out...PCB available only Build this 2-30 MHz 5 Watt Output Class-C Amplifier for $29.

I have no idea if this will fit your need, also it has some circuitry you don't need, but just don't put the parts in.

Mikek

That seems to be a common circuit. Q2 and Q3 are bipolars running unbiased, which I would expect to have lots of distortion and nonlinearity. They do suggest that the user can add their own lowpass filter, which would help over a narrow frequency range.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

You may need to prepare for an attenuator between the amplifier and the DUT because of the varying load impedance. Nearly all semiconductor RF amplifiers I know about dislike bad SWR at load.

A way out could also be a 6CL6 with suitable tanks, as it gets red in good time before blowing up.

Have you thought of a switchable load for termination in the time of Hi-Z load tsting?

--

-TV

Could be worth checking out this part:

Its demo board (with part mounted) is $99.95, good for +28 dBm into

50 ohms. Since you need to drive 500 ohms you could use a transformer to get some more voltage.

For a while they were selling HELA-10 eval boards for $50. These were great because they ran on a single +12V supply and could put out 1W at 1 GHz. They arrived fully populated in a milled aluminum block with tapped 2-56 holes, ready to bolt into onto whatever.

Eventually Mini-Circuits realized what they were selling and jacked up the price, and their amplifier demo boards are no longer sold as finished assemblies. I bought 3 of those HELA amps and I wish I'd bought 10 more.

As far as running into an open circuit is concerned, that should be safe enough with a broadband amp as long as the open circuit in question isn't at the end of a transmission line. Most of their MMIC parts are unconditionally stable, so it's just a matter of not sending a lot of reflected voltage back at them.

-- john, KE5FX

Well cut some eye holes in it or you might burn yourself. :^)

If the copper clad is double sided, you could drill some holes and them with solder or something. (two holes is nice 'cause you can see when the solder wets the pad, down one hole. I've done that with plated through holes.)

Hey if an opamp is not enough then some opamp driving some fet class A, would be my next try. (Mind you I have no idea which opamp or FET, and 14 MHz, well you've gotta start thinking about wavelenghts and impedances. TBH I wish I had more RF wisdom.)

George H.

oops ^then fill... I'm equally bad at speaking. GH

I just got an offer for this kit:

Comes with two transistors, just in case you blow one. $50, now 50% off with code NXP40IFU until end of august. But maybe 100W is a bit of overkill...

Arie

Here's another idea: make a big square wave and bandpass filter it.

which simulates a very nice sine with a 2-pole bandpass.

The T577 is a little totem-pole switch board that I did as an experiment. I may have found a use for it.

We were thinking that we could use a mosfet gate driver chip to get the square wave, but none of them seem to like to switch at 14 MHz. The cute little T577 will switch 60 volts or so rail-to-rail in about a nanosecond.

It's a mouse-bite component that can be used on bigger boards.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I should maybe make some power-pad SO adapter boards, with lots of thermal vias and a big pour on the bottom. Next proto board!

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Drill a hole and dispense cold spray on the pad. ;)

(I've done that sort of thing occasionally.)

Cheers

Phil Hobbs

>

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Here's one way we could do it.

The T577 is my little totem-pole ganfet switch board.

If we lay this out to fit into one of our standard enclosures, we could call it a product. The output filter could be a bandpass for the sine generator function, or a lowpass, or nothing, for a pulse output.

Input could be AC or DC coupled, and there are lots of possibilities for the power and amplitude programming bits.

I might add a crystal oscillator just for fun. Digikey will program an XO to basically any frequency one wants.

This one only needs 3 mA of supply current:

I would be linear regulating from up to 56 volts!

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Those programmable oscillators are square wave output. 14MHz is a stock frequency at Digikey, and can be bought for as little as 16 cents each, per 1000.

You can use the crystals to generate tthe signal, as well as to clean it up with a crystal filter.

Cheap chinese function generator and fed into a step up transformer

Can not be much simpler than that

Use your own lab generator if you do not need one fixed for the test station

You don't even need to do the transformer, just buy a signal transformer on Digikey

Cheers

Klaus

Some have resonance frequencies below 14MHz, but you don't really care about the phase