Electronics > RF, Microwave, Ham Radio

Amplifier Research 50A220 RF amp teardown

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ocw:
I recently picked something similar--a 20 watt Ailtech/Cutler-Hammer/Eaton 1020 amplifier.  Its expanded rating is for 500 kHz to 225 MHz operation.  Attached are a couple of pictures of it.  The shown 13.5 watts output was produced by -2 dBm input.  I'll wait to open up the RF PA until it needs some repairs.

KJDS:
Here's a teardown I created a long time ago of an AR amplifier

D Straney:
Interesting to see both of those.

I'm impressed with the size of the power supplies in the AR amp.  Unsurprisingly I guess, the first 3 stages also look identical to mine down to the individual components (may be different values or transistors, but the same arrangement) and each of the paralleled output stages looks identical to my 4th stage.  I'm curious how the power combining works on the output with all the transformers and 50-ohm resistors.  I guess if the transformers have a high-enough impedance then the outputs could be just stacked in series?  Whether a high-enough impedance for that is realistic, I have no idea.

T3sl4co1l:
Oh ha, just ran across this for unrelated reasons.

FYI, the schematic looks correct, with a couple of mis-seen capacitors, and I guess whatever the bias network(s) may be doing.  Not a bad job!

Between stages 1 and 2: seems to be a Zobel network sort of thing, looks like reducing gain at LF.  Amps usually have less gain at HF, in a slowly varying manner, so this helps flatten overall frequency response.

All stages have generous shunt feedback (or neutralization, a bit of both), which should give fairly low gain, but there's a lot of stages to make up for that.  And overall gain is what, 40 or 60dB or something I'm guessing?  So they're going to need a bit, while keeping the bandwidth up.

First error at output stage 3, shows a shunt cap; I think the coax shield is bypassed to GND there, and signal coupled to core.  The coax is at stage 4 input bias, so this makes sense.  And then, the two power amps can be biased independently, or maybe there's some resistance between them (56R?) so they vary together a bit, but can still be set over enough range to deal with offset between them.  It'll be class AB something or other; how much A, depends.  Probably not too shallow (class B ish) to avoid distortion, and, if it's on a huge metal plate heatsink kinda thing, it could even be fairly aggressive (up to maybe 50%, i.e. similar Pdiss ~ 2 x Pout; those packages are good for what, 100W or so dissipation..?).

And what are the caps, guessing they're pretty massive, a couple uF ceramic (dipped chip on leads) kinda thing?  They use them quite generously, and it seems they're not so bulky that HF performance is impacted.  Which seems reasonable enough, 220MHz is still a pretty long wave compared to what's in here.

So the 4th stage output coupling, the "?" network.  The input to it is cross connected coax, so they act in parallel.  If the coax is Zo = 50R, then that's 25R between output terminals (C-C or D-D whether BJT or FET), or 12.5R per transistor.  Which is easily 25W each out of say 25V supply, so I'm guessing supply is in the 28-35V range, something like that; and that'd be very typical for an RF amp like this.

Also the supply bias for that stage, I think the big inductor is differential mode, though it is hard to see from these angles.  And that makes sense, no need for CM filtering to a push-pull amp.

The cross-connected coaxes are then wired in series, so that the white coax would be 100 ohms (if they are 50).  The shields just tie together (by coupling caps; actually, there should be no DC voltage between these so they could maybe be shorted, but I guess it doesn't hurt this way, and it's only three more caps).  Note that the CM impedance at this node is poorly defined, because it's the midpoint between two RFCs in series: the pink coaxes from amp to here, and the white coax from here to GND (output).  (Probably the orange cores are lossy enough that this ends up well enough damped not to matter; high-Q resonances in locations like this can lead to DM-CM coupling and thus affect frequency response, so this would be helpful if true.)  But the white coax seems to go straight to the output, so I wonder if it might be specialty 25 ohm coax, the pink stuff.  In that case, supply voltage may be lower, as the transistors would see 6.25 ohms load each.

Actually... I think the output must be class A.  Common mode supply impedance is very high, so as the supply current varies with load, it'll just flop around and fart all over your signal.  So probably these are running near limits, 100W or more quiescent, and you just eat all that tasty, tasty class A power dissipation in exchange for the low distortion.

As for what the orange cores are, that's a good question; they're quite square, and I've never seen a ferrite in that shade of orange.  They may well be stripwound steel.  Micrometals makes (or has made) cores in that shade before, I have a few.  And that would go with the probably laminated-steel E-core chokes from the input stages.  They have to be extra large, because not many turns can be afforded on them (coax length limited to fractional wavelengths, hopefully?).

And if the coaxes aren't different impedances, then the maximum length of the pink windings I think has to be much less than 1/4 wave, maybe 1/8 wave, at Fmax.  So, maybe all of 10cm.  And there's 7 turns on there, and I'm pretty sure they're getting more than 1.4cm length per turn, at least that's what it looks like.  So the coaxes must be different types.

There is one final out, which is the trim cap at the output; maybe that allows compensating for the mismatch of coax impedances or something.  But that seems kind of far fetched and probably it's just to tweak response a bit, along with all the other caps in the chain -- which will mainly be to do with the upper corner.  (Which, of the trim caps used, the first one (1st stage output) is maybe the most interesting: it's on an intentionally longer trace length, and the feedback resistor is connected there.  Probably this provides some peaking effect.)

And the output, the white coax coming off the pair of orange cores, it's not a cap network, but the shield and core are at VCC; that'll be a cap from shield to GND, and core to output (assuming this top-left fly lead goes to output, or maybe some more compensation networks, power meter or something).

----

Not mine, but I have pics and manual floating around, may be of interest:
https://www.seventransistorlabs.com/Images/AR60LA/
just give the index a browse.  Highlights:
https://www.seventransistorlabs.com/Images/AR60LA/AR60LA17.jpg Front panel
https://www.seventransistorlabs.com/Images/AR60LA/AR60LA15.jpg Rear open
https://www.seventransistorlabs.com/Images/AR60LA/AR60LA40.jpg Plate line detail, top
https://www.seventransistorlabs.com/Images/AR60LA/AR60LA60.jpg Plate line detail, side
https://www.seventransistorlabs.com/Images/AR60LA/AR60LA70.jpg Grid line (bottom)
https://www.seventransistorlabs.com/Images/AR60LA/D1000669_RF_board.jpg Schematic, finals

It's a distributed amplifier, using planar tetrodes (vacuum state!), single ended class A.  The grids are chained together as a lumped-equivalent transmission line, and likewise the plates; this allows the capacitances to cancel out (in the same way that the capacitance from one segment to the next of a transmission line is balanced by the inductance between them, and thus a wave propagates one way or the other along the structure at some velocity, and with some impedance).  Bandwidth goes up arithmetically with number of stages, limited by circuit losses (grid drive attenuates as it goes along, as the grid itself has some AC impedance: literally, work is being done upon the electron beam, as it's being modulated by the varying grid voltage!).  Tubes happen to be fairly good at this, so the 8-stage design is pretty effective.  MOSFETs aren't bad at this (though common power transistor types aren't good enough to go beyond some 10s MHz), while BJTs are pretty poor.  The highest bandwidth designs that exist today, use distributed MOS amps on InP, GaN or such, monolithic designs with bandwidth into the low 100s of GHz.

Oh sorry, those are air-cooled radial-beam types (see also 4CX250, etc.).  High performance, but not planar. https://frank.pocnet.net/sheets/079/8/8121.pdf
Not sure what voltage they were running at, the manual scans aren't complete it seems...
Planar types can go much higher, see https://frank.pocnet.net/sheets/140/2/2C39WA.pdf for example.

Tim

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