What are the most cost effective options for buying or building a linear amp for general lab work that will do DC up into the early tens of Mhz?
I would prefer to use a single unit, rather than several to cover the entire frequency range. It would be powered by a commercial signal generator. Maximum output required would be 20W into a non-reactive load.
Richard, you know of many types of wideband DC amplifiers going beyond 30MHz, ranging from those in scopes to wideband opamps, but none can meet your 20W requirement. Furthermore, most any say 2 to 30MHz amplifier you find that can deliver 20 watts or more will do so only under restricted conditions, e.g. into a 50-ohm resistive load.
I have made a 50W 1-30MHz amplifier, using military surplus RF stages. It heavily employs large hideously-expensive RF transistors, operating class-A with special thermal-bias-set feedback. It uses balanced transmission-line transformers in and out, wound on large RF toroids to get down to 1MHz.
It's designed for 50 ohm loads, but it's fairly tolerant of non-resistive loads, and has been used continuously for many years to make swept magnetic fields to cool atoms in light- stopping experiments. The amplifier won't deliver "50 watts" into an inductive load or a capacitive short at 30MHz, but it will deliver several amps of current, creating a wideband 1-30MHz tunable magnetic field.
I imagine my RF amplifier could be mated somehow to a DC-3MHz power amplifier (I have two candidate designs that have gone through maturing iterations over the years, as we've made about 30 of them), but even thinking about such a thing is highly unappealing to me. It would be one ugly, fragile messy beast.
Andy replies: 1) Go to the Communications Concepts website. They have these modules, and you can buy the parts there, but you have to assemble them yourself.
2) Check copies of The Radio Amateur's Handbook. Windband amps in the 20 watt range driving a 50 ohm load have been around a long time. Expect to pay $50-$100 in parts, but, like Comminications Concepts, you have to build it yourself.
3) Check our ebay for ham band linear amplifiers.
4) Check your local amateur radio club..... You will probably need a ham license before anyone will sell/trade you one.
That is why I stocked up heavily on LH0063 buffers before they became extinct, so I don't have to go through that messy beast scenario. Nice heavy-duty TO-3 style 8-pin package that can be bolted onto a heat sink. The only downside is that their supply abs max limit is 40V. Riding the supplies of one on the outputs of two others is something I have never dared to do with these since they oscillate easily.
The 1982 databook officially calls them 'damn fast buffers'. Which they are, rolling off around 100MHz. I guess they couldn't write 'damn' into a data sheet these days anymore.
Once I had to drive a transducer of a few ohms really hard. So I paralleled a bunch of them and they actually blew up the transducer.
When I was in college we built RF amps up to 30MHz with large FETs but back then these were expensive. The total power output was pretty much limited by the trade off between the FET budget and the beer budget. Beer had a higher priority ;-)
Actually, with tubes it was worse because of the needed infrastructure. Microwaves weren't old enough back then to find sufficient quantities in the scrap yards, to get the HV caps out of them. I remember having to plunk down about $20 for one 6uF 5kV capacitor. Ouch. That really hits hard if you are in college.
In contrast, the scrap yards in that university town had tons of old mainframes flying around. You could scavenge 10,000uF+ caps to your hearts desire out of these, plus big transformers and so on. Typically you paid by weight and not by what you scrapped out. I still have a
100,000uF cap from a RAM backup module. Paid next to nothing for it. And some diode arrays with huge lag bolts on them.
What kind of phase distortion can you tolerate? If you don't mind waiting a few microseconds or so for your gazintas to get to your gazoutas, you could use a distributed amplifier. They can be made amazingly wideband, but they're real power hogs.