Why do many RF amplifiers use an external biasing resistor instead of a push pull halfbridge configuration? Is the external biasing main function to pull up the output since the amplifier only can sink current on its output?
example of an RF amplifier that requires external biasing: "
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Also can the same MMIC (monolithic microwave integrated circuit) amplifier be used either as a LNA or as an RF power amplifier? ie. the "Gali series": "
The technologies only produce fast N-type devices... silicon or InGaAs NPN bipolar transistors so, yes, they can only pull down. There have been a few MMIC-type amps with active pullups (NEC maybe? Maxim?) but they tend to be slow.
There's nothing inside most of these parts but a Darlington and a couple of resistors.
We had a thread a few weeks ago on the problems associated with creating a super-wideband inductor for use in the pullup.
Several reasons. It saves a pin and lets you use a four-pin package. It provides flexibility to use whatever rail voltage you have. And it removes one of the bigger power dissipation components from the package.
Andy replies: I've use the MMIC amps for a long time as both LNAs and Power amps. However, they are really great at either.
The noise figures, IM, and power diss aren't close to what you can get with a discrete design, when doing LNAs. However, if your requirements are not "best that can be done", they are extremely convenient.
For power amps, I think some of the manufacturers have devices in the 100 mw range, tho I haven't used one for that yet. The current requirements are pretty high, and the IM specs aren't great. But, if your specs aren't too stringent, it's very handy to plop one or two down on a 100 mil track on G-10 (FR4) to get 50 ohms, and move on...
They can even be directly paralleled to almost double the power output (but not the gain).
To use them as LO buffers which put out +10 dbm or so to a balanced mixer is a slam dunk...
In short, they answer to your question is YES, BUT.
The convenience and speed of design must be weighted against the specifications. Often , it is a good method.
Is that for 0.062" thick PCB? So you can run 100mil traces for all the RF signals if everything is 50ohms? :) Does this apply to a 2layer board with a ground plane on the bottom?
Do you use a common bias resistor/wideband inductor for this or one per amplifier?
Those are nice, but L is low and drops like a rock as the DC current increases. For the amp we're doing now, we needed 100 uH maybe at 200 mA, so we wound up with a string of three ferrite beads of increasing size, followed by two axial inductors, each with a shunt resistor. Inductors are sure non-ideal gadgets.
******** 100 mils on .062" FR-4 , ground one side, track on the other, forms a 50 microstrip (very very close). I have often just used an exacto knife to cut it out. I have also used adhesive copper tape on the ungrounded side.
This is known as "micro-strip" and is well defined by graphs of Wheeler's Equations. It's just as easy to make 35 ohm or
70 ohm, but I forget the widths right now.... Higher impedance means narrower track, and by 300 ohms the track is smaller than I want to mess with using an exacto knife.
Note that this subject is a complex one, but 50 ohm track is so common that RF engineers, who do their own assembly and building (getting rarer these days) remember certain dimensions as "rules of thumb"..... While the mathematical types will argue till hell freezes over about 2 or 3 ohms difference, as a practical matter it is immeasureable in performance....
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***** I believe that the mini-ckts lab designer handbook has some examples. I, personally, would uses separate biases and cap couple the outputs so one amp can't current hog, but , I may be being overly conservative....
The inputs MUST be cap coupled separately. Use chip caps.
Understand, in my opinion, this will work OK but I would use it only in non-critical applications, where the purpose is to get some quick increase in power output where there are no OVERRIDING other considerations. Quick and Dirty. But that's just my opinion...... Often certain parts of a design will not require any more than this, like line amplifier, LO buffers, etc. Experience usually is a good guide as to whether this method will cause unforseen problems.....
One advantage is that an exacto knife will open up the line so you can just solder coupling caps in. Also easy to tap the line with bias resistors and chokes........You will see immediately the advantages for quick assembly once you try it...
Good luck with your project. I would reccommend that you check out EDN's "Ideas for Design" each month to get a feel for some innovative ways to do different things. Also, LOTS of circuits are in The Radio Amateur's Handbook, and the UHF Experimenters Handbook...... If you have a copy of each of these in a stack in your bathroom, you should be in good shape...
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