We need 28 volts p-p at 14 MHz to test our IQ modulator box. I've bought two Chinese RF amps from Amazon and both are garbage. So we're going to build our own class C amp, as a dremel'd prototype.
I get to teach a young engineer my Dremeling secrets.
Why not some opamp*? My rigol sig gen does ~20 Vp-p, a gain of
2... oh you probably need more current drive.George H.
*says the man with mostly opamps in his tool box.
We could almost do it with an opamp. We'd need a gain of 3 from our source. THS3091 might almost work, at its abs max Vcc.
New buf634a perhaps?
torsdag den 18. juli 2019 kl. 22.25.14 UTC+2 skrev John Larkin:
if the output doesn't have to grounded use two in a bridge?
THS3491 would be the right part for that job, it sounds like.
-- john, KE5FX
That just about works. We could push the Vccmax a volt or so if needed. 28 p-p is right on the edge.
If we used a 50 ohm resistor to drive our short coax, and then the 500 ohm DUT load, it doesn't work again.
That would drive the load nicely, but we'd still need to get the gain from somewhere. Maybe a passive resonant circuit.
The load is single-ended. A transformer could convert a bridge drive to se.
The mosfet class C isn't bad. One of my guys is Spicing it now.
fredag den 19. juli 2019 kl. 00.18.03 UTC+2 skrev John Larkin:
could also give you some voltage gain
The link doesn't work here right now but if it's for a bench test you could also use a shortwave transceiver for ham radio. They all do 14MHz because that is one of the ham bands.
Make sure the first aid kit is nearby when the engineer tries.
If it's for testing and not sale, then sometimes parts will work a few volts over the max... you'd have to test. (I blew up an lt1013 at ~52 V... it ran fine at 50V for ~1 hr. I only 'looked' at my schematic after I blew it up.)
Well, it's in a loop driving a darlington so no power.
George H.
We use to slice thin walled stainless with dremel grinding discs. 'eye protection' required!
George H. And you should never be in the machine shop alone... though I've broken that rule many times in my stupid youth. (How is it most boys/ men live to reproduce?)
The class C thing has a lot of distortion in Spice.
This might be OK.
I think I can make the 1:1 transformer on a pot core that we have, maybe 2 turns twisted pair wire-wrap wire, bifilar.
The THS6022 amp has a power pad on the bottom, which would be tricky to cool on a proto board. Maybe we can glue a heat sink to the top. The dual THS would dissipate a half watt maybe, not too bad. Or we could use two separate amps, just to spread the heat.
On a sunny day (Thu, 18 Jul 2019 12:39:33 -0700) it happened John Larkin wrote in :
For that sort of thing maybe a simple NPN PNP totempole amp with feedback would do, then 50 ohm resistors to the outputs.
I have used that for video several times up to 10 MHz. Most transistors go much higher, No inductors, capacitors, and wideband. low distortion.
What is the load impedance ? If 50 ohms, that is just 2 W, any amateur radio transmitter could provide it.
If much less than 50 ohms, just use a bigger transmitter and a step down transformer to transfer 50 ohms down to your low required impedance.
If you insist of doing it all yourself with a few volt op-amps, use some op-amp followed by a very beefy complementary emitter follower and a step-up transformer to get 28 Vpp.
I've flipped opamps with power pads over.. dead bug, and then strapped a piece of copper tape front to back, soldered to copper clad and pad. (Probably too ugly for a gold plated proto. :^)
George H.
I could put a bag on my head and do that.
I'd just buy a boxed RF power amp if I could find a decent one for a low number of kilobucks. My frequency, 14-15 MHz, excludes a lot of possibilities. I can find some nice stuff for $8K.
The DUT is switchable to be 50r or HiZ. We could test it in either mode.
The problem is to get a fast, high-current opamp that has the voltage swing. A few of the THS parts almost work.
Any suggestion for an amateur transmitter that would make maybe 2 or so watts sinewave into 50r at 14.5 MHz?
fredag den 19. juli 2019 kl. 16.09.54 UTC+2 skrev John Larkin:
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