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Author Topic: Ever push/pull block amplifier -- i have a few CA2818C  (Read 2286 times)
KC7NOA
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Posts: 73




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« on: February 24, 2014, 01:51:08 PM »

Im wondering if any one has knowledge ----

Id like to know if it was ever done ... setting CA2818 type wideband block amps for abit more power in a push / pull configuration ...

Or parallel i guess with step up/down wideband RF transformers ....

Yep .. its for my SDR ... 100mW max out ... 1.5mhz up through 700mhz ...

Mike KC7NOA
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WB6BYU
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Posts: 13331




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« Reply #1 on: February 26, 2014, 10:51:48 AM »

Yes, you could do that.

First, you have to make sure you don't overdrive the CA2818.  They are only rated
for a maximum of +14dBm input.  Even driving two in push-pull, you can still exceed
that with your 100mW.  For initial testing you probably want to limit it to 10mW output.

While the part is rated for 1W output power, that is at 1dB compression.  The PEP output
at -32dB IMD is only 600 to 800mW, so you'll be lucky to get 1.5W out of two such
modules on SSB while keeping the distortion at a polite level.

You should be able to drive them in push-pull with suitable transformers on the input
and output.  (Since you have lots of drive available, even a resistive divider would work
on the input.)  The worst input SWR is in the 200 - 300 MHz range, and the output power
drops by about 3dB by the time you get to 450 MHz, but with some care in winding the
transformers you probably can get it to work reasonably well from HF through 2m if
you don't try to over-drive it or drive it to high IMD.
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KC7NOA
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Posts: 73




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« Reply #2 on: February 26, 2014, 02:34:12 PM »

Ok.. i just found a PDF today with an internal diagram .. talk about basic .. to the point that im not sure its accurate ... and its not a CA2818C .. just CA2818 ..

http://www.datasheetarchive.com/dlmain/Datasheets-115/DSAP00280.pdf

I was thinking of trying to replicate the input/output transformers of a FT-817 ... its HF to VHF ...

Anyways ..
               yea my drive is 100mW max .. but iv read that 60mW is the lowest IMD power out setting .... that's almost perfect ....for driving 2 ...

Thanks for the reply ... i think ill try just setting them parallel ...

The thinking is that if they are able to drive 100ohm system ... 2 @ 100ohm in (parallel) is 50ohm .. (but close should be fine)
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WB6BYU
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Posts: 13331




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« Reply #3 on: February 26, 2014, 05:27:06 PM »

Quote from: KC7NOA

yea my drive is 100mW max .. but iv read that 60mW is the lowest IMD power out setting .... that's almost perfect ....for driving 2 ...



60mW divided to two devices is about +15dBm each, which is just over the rated
maximum limit.  With a resistive divider (50 ohm resistor in series with each input,
giving two 100 ohms combined in parallel) that will reduce the drive to +12dBm,
which is just within the rated maximum limit, and should be pretty close to driving
each module into compression.

With a typical gain of 19dB and a target output of, say, 600mW (+27.5 dBm) from
each device to keep good IMD, you still want less than 10 dBm (10mW) drive per
device, so 20mW max (or 40mW with 3dB loss in a resistive divider.)
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KC7NOA
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« Reply #4 on: February 27, 2014, 08:54:59 AM »

Im sorry .. your correct ....

I did leave off a bit of information .... i have a bandpass filter for the hf and 6M bands that has a loss of 4db ... it would of been lower but i used chip inductors instead of hand winding torroids ... which seems like it will work out for me after all (i haven't installed it because i didn't care for the loss -- and it can only handle 800mW max)
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