Using a p channel mosfet to switch a RF switch

Question

I'm very new to electronics, i have the following RF switch (CG2179M2) which i need to control using the output signal of my MCU, which will provide a 3.3V HIGH for transmit mode and LOW for receiption mode. The final product will be running on a battery, thus i need the most efficient way to control the RF Switch. I've been suggested that i shall use a p channel mosfet instead of a PNP transistor because the transistor will consume some power in the On state. My final product will stay most of the time in the reception mode, therefore, i need to keep one pin of the RF switch (the pin used by the reception channel) always on HIGH state. I thought to do this by connecting the drain to the pin controlling the reception of the RF switch, Source to the battery and Gate to the control pin of the MCU. So what i expect to happen is, the MCU enters the reception mode by setting LOW the control pin -> the p channel mosfet getting a LOW on Gate will set the Drain to HIGH which in turn will set the reception channel pin of the RF switch to HIGH.

These are some of the questions i have:

Is this the correct way to do this? Or shall i need a PNP transistor instead of the p channel mosfet?

I found the following mosfet on mouser, will it be fine for the job? NTR2101P

Shall i use a pull down resistor on the mosfet to keep it always open unless i set it HIGH to cut it off?

EDIT: I thought it's much easier to just use an inverter IC. I found the following one on Mouser (74LVC1G04), do i need to add passive components (resistors or capacitors) between the 74LVC1G04, RF Switch and the MCU?

Thanks in advance, Bilhan.

Answer 1

1. There is no "better" option per se. It depends on your application and design choices such as maximum switching frequency, power requirements, size, cost etc. For low frequencies and small signals, both perform decently well and BJTs tend to be a little cheaper than FETs. For high current situations, a BJT would not be suitable and you might have to use FETs or even IGBTs. But since you say you are beginner and I am beginner too and I found FETs to be cheap enough, reliable enough and easy to understand and use.

2. A P-channel MOSFET's state depends on the potential difference between the Gate and the Source. When V_GS (depends on the rest of your circuit) is less than threshold voltage V_Th (Depends on the MOSFET), the switch will be ON and otherwise, it will be OFF. So, in your case the gate voltage is either 3.3V or 0V, but you have to provide the battery voltage at the minimum to see if a MOSFET is suitable for your use case. Other than V_Th, you still have to check maximum current, switching frequency etc. while deciding which MOSFET to get.

3. It is not strictly necessary as far as I can see but it wouldn't be a bad idea in my opinion. Again, your circuit and design choices may affect this.

Answer 2

Assuming you have two open pins, why can't you just drive the switch directly from your microcontroller? It's compatible with 3.3V and uses less than 10uA.

The data sheet says the control signal pins will work with a signal voltage of between +1.8V to +5.3V and consume a max of 10uA. The datasheet also said you need to drive VC1 and VC2 pins opposite each other to route the RF signal on RFC between RF1 or RF2 (so you'll need two driving pins or a signal inverter to generate the complementary signal).