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Author Topic: First attempt at a solid-state RF amp  (Read 3077 times)
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KI4YAN
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« on: May 28, 2017, 10:01:16 PM »

First effort. Tear it apart-show me what I have done wrong. Broadband, 7 to 54Mhz operation, 25-30 watts output on SSB, 5-6W AM carrier.

I am positive that the NFB around the finals will have to be done again at the drivers, also some level of frequency compensation is gonna have to happen. But for now, here's the intended operation:



The NE592 differential video amplifier receives the RF input at no higher than 0dB level, and I took a stab at getting it into the differential inputs-the inputs need to be biased up to +6V though, I forgot to do that. It's frequency compensated, stable, and gain controlled via a single potentiometer.

The VR2 potentiometer sets gain of this opamp, it will need to be digitalized for MCU control. The microcontroller will output a 0-5V signal which I'll have to convert to a variable resistance, I'll tackle that later.

Q3/Q4 are actually MRF630, I just grabbed a generic NPN off the stack for the drawing.

TX1 is a bal-bal transformer, just to match the collectors to the bases. A pair of ferrite tubes glued together, 61 material.

Bias is supplied by Q1 and Q2, which are probably gonna be something like TIP3055's since I have a pile of them, and Q2 is bolted to the heatsink right between Q5 and Q6, the final amps. Q2's job is error amp/thermal compensation. Bias is set by VR1, R5 and R6 need to be calculated to drop appropriate voltages, they are not yet.

Output stage-I've wrapped the VHF finals in negative feedback to reduce the gain they'll have at HF/lowVHF, and try to control parasitic oscillations. Probably didn't even get close to correct with it yet. They are spec'ed for 132-180Mhz, and 6.3dB gain at that 175Mhz. I am assuming they will have too much gain at HF/VHF.

TX2 is a bal-un transformer, matching the 2.5 ohm collectors to a 50ohm output. If I am lucky, the transformer will have to deal with 25-30W output. I have half-a-dozen 1" OD, 1/2" thick, 61 material toroids to make a stack, or binocular core.

So, let's hear it, what have I left out, and what is just plain wrong?

I'm cribbing from Solid State Design for the Radio Amateur, ARRL 1978 handbook, and https://pa0fri.home.xs4all.nl/Lineairs/BLW60PA/pa-blw60eng.htm
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Pete, WA2CWA
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« Reply #1 on: May 28, 2017, 10:03:59 PM »

Is there a reason there's a black background on the schematic? I don't know about others, but I can't read it.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
KI4YAN
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« Reply #2 on: May 28, 2017, 10:44:08 PM »

Just the program I use to draw-I will attempt to flip it to white so more people can read it.

Edit: Here's the white-background version. The forum doesn't want to let me make it clickable to make it bigger, though.

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VE3ELQ
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« Reply #3 on: May 29, 2017, 07:03:01 AM »

Sorry OM still cant read it well enough, perhaps print it then scan into a .pdf and re-post.
Maybe contact the moderator and see if the original file extension can be added to the acceptable file extension list, same for Express PCB and Eagle PCB files.
73s  Nigel
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KI4YAN
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« Reply #4 on: May 29, 2017, 11:51:32 AM »

Here is he PNG file, maybe this will be readable.


* RFamp2.png (39.62 KB, 1660x836 - viewed 387 times.)
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