Electronics > RF, Microwave, Ham Radio

Trying to understand MMIC PA usage

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I've been trying to learn RF electronics lately for a project. I'm at a point where my current focus is on power amps, and in an attempt to not get bogged down with designing a discrete unit, I've been looking at PA ICs. Actually, now I'm siding more towards complete (and cheap) eval boards. That said, there are a few specs and parameters that I'm having trouble wrapping my head around.

First off, I've been seeing output power specs that don't seem to make sense for the specified load impedance and supply voltage. For example, the CMX90A004 (https://surf.cmlmicro.com/wp-content/uploads/2021/03/CMX90A004_ds.pdf) states "Output power 32.5 dBm @ 3.6 V" with an output matched to 50 Ohms. If 32.5dBm is ~1.8W, that requires a Vpp of 26.6V. So what am I missing here? Even if we say the 32.5dBm figure is with the amp operating in saturation, it would still take ~9.2V DC to produce 1.8W of power through a 50 Ohm load. So how do they figure the "32.5 dBm @ 3.6 V"??

What are some other common snags I should be made aware of?


The AC output voltage of an RF circuit will often exceed its DC supply voltage. Current flowing into the output does not have to come directly from the DC supply. Nor does current flowing back, during the other half of the cycle, have to disappear straight to ground. Charge can be stored in capacitors, and energy can be stored in inductors.

Consider what happens when you drive DC current from a 3.6V source into an inductor connected to ground, then disconnect the ground end and connect it to a capacitor instead. The stored energy in the inductor can force current into the capacitor and charge it to far above the input DC voltage. This is the principle used by DC-DC boost converters. They take a low voltage, high current input, and convert it to a higher voltage, lower current output. The power in and out is the same, less any inefficiency.

The role of the output matching network in an RF amplifier is actually quite similar. The voltage at the drain of a common-source FET amplifier is constrained by the supply voltage, but the transistor can sink a very high current. So the transistor itself has a very low source impedance, e.g. a few ohms. The output matching network then steps this up to higher voltage, at lower current, to match a 50 ohm load.

I think what's confusing is that the data sheet presents it as a broadband part with 50-ohm output impedance (presumably pure resistive), no matching network needed.   If the output impedance is really 50 ohms, it is indeed impossible to turn 3.6V into almost 2 watts at a 50-ohm antenna.

The matching network will be internal to the part. It's an MMIC, not a discrete transistor.

--- Quote ---CMX90A004is highly integrated for ease of use, minimising external component count and reducing board area. RF input and output matching is incorporated on-chip, as well as active bias circuitry and an input DC-blocking capacitor.
--- End quote ---

True, but I'm still surprised the math works out.  The Q can't be more than 10, so getting 32 dBm out of 3.6 volts is interesting.  I'd expect them to require an external boost regulator.


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