Re: Amplifier chain design - best practices
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Allison, Modelling confirmed to me that fewer active devices (and hence higher gain per stage), the better is the IMD. From an IMD perspective, higher gain with high standing current results in better IMD. That's why it makes sense to use a VHF power device as driver/pre-driver block. I did a low distortion (less than - 40dbc IMD) linear chain a few years ago. At 12v, It had to be an RD16HHF1 device biases for close to class A. At 1A of current, it gave 4 watts of power. The details are on the emrfd group's file section.? - f
On Fri, 21 Sep 2018, 21:21 ajparent1/KB1GMX, < kb1gmx@...> wrote: On Fri, Sep 21, 2018 at 09:19 AM, Jeff Omundson wrote:
1)Keep amplifier intos and outofs away from each other.
This also applies to all RF systems.??
2)Keep amplifier gain stages reasonable.? Add more stages instead of upping gain.
This is a serious and common error, forcing or wishing for gain that is not predictably there.
Conservative design rules insure that.
3)Pick the right device for the desired frequencies.? Obviously 2n3904's don't work for this application.?
First the 2n3904 can if one does not apply wishful thinking and employs it within the devices capability.
***Does picking a part with an fT about 40 times the highest desired frequency a safe bet??
Oddly the answer is yes if the stages has to deliver at least part of the gain desired.? For a?
30mhz amp with a 20db stage gain you will need an Ft of something approaching 3000mhz!
The real question is does it makes sense when two 3904s at 10DB each can also do that?
at 30mhz.??
***Can you pick an FT that is too high (like a npn transistor with and fT of 10 gHz for a 3-30mHz radio)???
The answer is yes and maybe.? IF you using resistive feedback amps (nearly the entire transmitter and both IFs)
the required FT is a product of the gain and the frequency so at some point you really have to ask is that much
gain per stage reasonable and stable.? IT might be prudent to back down the gain and use a more reasonable
transistor.? Also for higher power stages the cost for a supergain high power device might break the bank.
A lot of QRP designs more so older one try to be minimal and get the most out of a very few parts.
If you making one this is ok as you can dial it in and hand pick parts.? For quantity builds this is terrible.
You want every one to be the same as the last.? So care in not trying to get 101% of capability is advised..
This shows more often in transmitter circuits as out of a group most doe the expected watts, a few do
more and a bunch never do that.? Multiband transmitters due to bandwidth, power and layout require
greater attention.
4)Keep RF wires/traces away from each other and from the dc path.
Again this works for RX and TX circuits.? We do not want o induce unwanted signals into the receiver.
From Crystal Sets to Sideband, a very good starting place.? My Favorite are Solid State Design For Radio Amateurs,
and Experimental Methods in RF Design (Both ARRL).? Neither are light bits and do require several reading through
passes to pick up that what was mentioned here is often elaborated and related to things there.
***Makes me wonder how the ft817 does hf, vhf and uhf...?
Lots of tuned filters, careful layout the use of the chassis as cooling, shielding, and mechanical support.
Its worth looking at Elecraft K1 and K2 as the manuals and detail in them is very good as examples.
I'd add KNQ7A? and the KD1JV Sopbucket 20 as a simple monoband, and a few other as well.? Generally
the designers are creative people so watch for solutions to problems not outwardly discussed as its
often not featured but important.? Cooling and layout are often not obvious without analysis.? Same can
be said for component choices.
Allison
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