Electronics > RF, Microwave, Ham Radio

Amplifier Research 50A220 RF amp teardown

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D Straney:
This is a 50W RF amplifier, which claims to work from 10 kHz - 220 Mhz.  It died at work driving some badly mismatched loads, from what I understand, but from the lack of smoke, obvious burned bits inside, etc. it seems it went pretty quietly.


The entire thing is pretty much one giant heatsink, with an RF board on top, and the power supply underneath.  I didn't dig down to the level of the power supply, as that would've been some pretty involved disassembly on a piece of equipment I don't own, and the power supply is almost guaranteed to be boring - just a transformer, bridge rectifier, and some capacitors.  Here's the interesting part though, the RF board!


A closer look, you say?



Luckily, since it's just a 2-layer board with easily-visible through-hole components, it's not that hard to figure out how everything's connected (especially as it's all 80% ground plane, as I'd expect from anything reasonably high-frequency).  RF amps are not something I have a lot of experience with though, so this interpretation is going to be shaky in a lot of places, but here goes...

There seem to be a few linear regulators at the top-right edge (1) for biasing, and maybe for the actual DC collector/drain power, but that seems less likely.  There's also what looks like a thermal cutoff switch (2) to kill the power if that giant heatsink gets too hot.  For the RF parts, a schematic should make it easier:

(For what it's worth, I have no idea if the transistors are bipolars or FETs, but I've drawn them as FETs just because)
RF input enters through coax on right (3); goes through a DC-blocking cap then back-to-back diodes for limiting, and gets AC-coupled to the 1st stage (4), with an AR153 transistor.  1st stage has some bias fed from a trimpot, and some feedback through a (large brown) resistor and a messily-wound inductor to the gate.  The drain is loaded down with a variable cap, and fed with bias from a series large inductor (marked 1.4mH) and a small inductor (the toroid) in series.  The large inductor is probably for low frequencies (it's supposed to operate over a really wide frequency range...1.4mH = +j88 ohms @ 10kHz) and the small inductor (the toroid) is probably for high frequencies where the SRF of the large inductor makes it no longer inductive.  The cap at the input to the large inductor provides some local decoupling.

1st stage output is AC-coupled to some kind of matching network.  2nd stage (5) has a different transistor (AR156) but otherwise similar; has the same feedback structure, variable drain cap, and dual-inductor load.

Input of the 3rd stage doesn't have any elaborate matching network, just a variable cap on the gate.  The DC bias, feedback, and load inductor is the same as the previous two stages.  Transistor is now AR157.

After that, my guesses get far more vague as my lack of knowledge really shows itself here.  The dual transistors and their connections suggest a class-B 4th stage ( 8 ), although it could be 2 out-of-phase class-As to cancel symmetric distortion or something; there's a couple trimpots which seem to set the DC bias for each transistor independently.  The right-most toroid wrapped with coax (7) I think is a balun to take the up-until-now single-ended signal and produce a differential signal to drive both transistors' gates out-of-phase.  Each transistor has its own local negative feedback through a tan resistor and a DC-blocking cap.  After the 4th stage, the signal path becomes symmetric and looks differential for a bit (notice the top and bottom toroids).  The drains get loaded with an interesting combination of differential (black cores) and common-mode (toroid at bottom-left, (9)) inductive loads.

I have no idea what the dual set of cross-connected transformers (marked as "?" on schematic) terminated in a triple capacitor is doing.

Finally, the top-left pair of toroids (10) probably are another balun, taking the signal from differential back to single-ended this time.  It took me a bit to get the weird side-by-side winding arrangement there, but I think it's just a more vertically-efficient way to stack two cores on top of each other: this gets you twice the core area for the same magnetic path length, which gives more inductance while keeping the saturation current the same.  The returned-to-single-ended signal then goes through some more AC coupling and variable-cap loading before leaving at the top-left to go to the output connector.

It's interesting that the transistors have part numbers (ARxxx) which match the manufacturer's name...wonder if they're custom parts, or at least custom-branded?  Couldn't find anything about these parts online.  The other numbers (9321, 9238, 9339) look like date codes in a [2-digit year, week number in the year] format, and if this is true, would indicate that this amp is from the early 90's, which wouldn't surprise me given the construction style.

coppercone2:
better suited in rf section, you will get more responses there

D Straney:
Ok thanks, hadn't seen any teardowns in the RF section so thought maybe that was frowned upon.  Seems I can't delete my own topic to re-post (and wouldn't want to do two identical posts), any chance of a move to the RF section from the mods, please?

coppercone2:
I find it interesting because I have a 100-500MHz 50W generator that looks similar but this one is shifted down in frequency.

I have yet to tear mine apart though. I will be looking at adding something like this at similar power levels to my amplifier rack one day. Need a better lab though, once you start getting into rack mount equipment it really needs a decent sized alone room.

KJDS:
I've not looked closely, but will guess that the output stage matching is a guanella balun

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