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Author Topic: Best MMIC amplifier?  (Read 2289 times)
KB1GMX
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Posts: 1830




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« Reply #15 on: May 20, 2019, 10:28:52 AM »

MMICS are nice when I need gain above 200mhz, below that I
have to live with the excess wide band width and possible effects.
Using a mimic without selectivity before it will nearly always end
up with some RF from some unwanted source causing havoc.


Like many I've used the G4AON and SV1AFN circuits for the last 40+ years. 
They are old and the devices are well known with predictable performance.
It seems every 10 year or so they get rediscovered.

FYI the 2n7000 in a suitable circuit is very much similar to those and only
slightly more modern.

The last compendium of wideband HF amplifiers in my collection is
September 2002 73 mag.  I'm sure there are others since.

Allison
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G0HZU
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Posts: 163




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« Reply #16 on: June 07, 2019, 06:16:15 PM »

For low frequency stuff (up to the 2m band?) you can make a reasonably good wideband alternative to a 50R MMIC amplifier using a BFR91 BJT transistor and a few resistors and caps to define the feedback. It won't have the same bandwidth but it should work well as a basic low power 50R gain block up to maybe 150MHz and it would be 'strong' enough to use as a driver amp for a level 7 diode ring mixer for example.

One downside would be poor reverse isolation but this affects a lot of MMICs as well. It would work best up to about 70MHz with a decent frequency response similar to a 50R MMIC. It would probably only be good to about +7 to +10dBm in terms of the 1dB compression point but it depends on how hard it is biased.
« Last Edit: June 07, 2019, 06:33:14 PM by G0HZU » Logged
G0HZU
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Posts: 163




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« Reply #17 on: June 07, 2019, 06:32:41 PM »

Use a pair of BFR91 BJT transistors as a classic wideband DC coupled amplifier (50R in and out) and you can get flat gain to >200MHz and also 60-70dB reverse isolation across the HF bands. The noise figure won't be great but this might not matter as much as the reverse isolation. It won't be a very strong amplifier either but that doesn't always matter either.
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N3DT
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Posts: 1793




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« Reply #18 on: June 08, 2019, 06:12:22 PM »

Best for what? There are too many variables. There's also the PGA-103+ that's pretty hot for a few bucks, but easiest place to buy less than 20 pieces is ebay. I've made some nice amps with it, wide band and all, low NF, but they are pretty unstable unless you filter the dickens out of them to the freq you want and you need to work with a microscope. Those one piece gain blocks are pretty nice, but you pay a filter price for them. Through even 70cm discrete stuff is still easy to work with and there's plenty of designs out there. It's all fun.
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KZ4AK
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Posts: 13




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« Reply #19 on: July 23, 2019, 03:16:29 PM »

I agree with Chuck, WD4HXG with one minor exception.  Although it can be tedious with tiny parts, I often use dead-bug construction with no issues.  Most often when I do this, I use a sort of stripline on the board ground plane.  To do this, I shear 1/16" board to form the stripline (~0.1" wide), cut to appropriate lengths, then super glue it to the board ground plane.
Larger chip caps and resistors can be used by mounting on an angle - One end soldered to the main board, the other elevated to the stripline.  The continuous, uninterrupted ground plane works great.

Typically, I use Minicircuit devices.  The caveat: Last time I ordered from them, they had minimum quantity requirements.  Not too bad since it is always nice to have a few extras in the junque box.

Have never killed one except in the case of careless overdrive...

Woody - KZ4AK
kz4ak.com
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KM1H
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Posts: 5569




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« Reply #20 on: July 23, 2019, 05:09:45 PM »

My experience with MMICs is to not use them in a RX circuit that will be subject to lots of strong signals as they simply fall apart. The specs are written in a lab with one signal input. One popular VHF transverter kit used them and the ones I picked up for almost pennies at hamests all had blown MMICs in the 28 MHz IF amps. Now with real devices tacked in dead bug they have been fine for a few decades. Nothing like a few beads on leads if they get a bit squirrely.

Ive also used regular FETs in place of blown GaAs FETs from TX leakage or looking at them wrong when ultimate NF isnt needed. Put those closer to the antenna with a good sequencer.

Heck, I built a W2AZL 2M converter in the 60's and in the early 2000's took it to some VHF/UHF conferences and had the NF measured. That WE 417A front end helped it to a 1.6dB NF that beat quite a lot of SS. I still have it on a shelf Grin

Cant beat the 2N5109 or U-310 when the going gets tough.

Carl
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