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Discussion in 'CB and Export Equipment and Accessories' started by Mk121, Mar 18, 2019.
As said in the thread, use a switched power supply feed line (Like TX power). Don't feed the Gate a voltage all the time, you already will be supplying the Drain to Source feed a power. Don't mix the two. The Gates are sensitive, and you're adding in a part that although does consume power, it is for a design used to CONVERT one type of power, RF to a trickle of DC power that can keep the Gate turned on.
A good rule of thumb,
But that "mod" is a self powered one that is for AM only radios - use at your own risk.
The one you are implying is to apply a voltage to the Gate as a trigger...
It's important to know the difference in what you suggest.
One, the top graphic, shown for AM-ONLY and is made for Class C or above (like a typical Class D Cobra 29 is includsive - an AM-Only radio)
Then that requires a different approach and you need to keep the voltage low - much like a bias circuit they used to use from the older days for Bipolar, only in this case you need a voltage - not a lot of current.
About 2 Volts to start, with about 4 VOLTS level that can cause a LATCHING event keeping the part on and the MOSFET simply sinks current until it blows up or something in the power routing to the part - finally blows. This can get expensive - so keep the STARTING / TRIGGER - (insert favorite switch phrase) TURN ON voltage low as possible until your comfortable and the part is stable.
The thread linked above is to keep the trigger voltages of when not used. They need a place to drain off to. The premise of the effort I posted in it is to understand the mA readings they (the radio maker) wants to use, and the support hardware circuit used, is so the RF and any residual power DC AC or RF is kept off the Gate when it's not needed and keep any RF out of the BIAS circuit and not allowed to leak back in. The mA seems high for such a high-impedance part, but they found issues with RF at low levels. The legs or solder pads and traces act like antennas which can leak RF back into the Gate region - so they use power flowing thru a divider as a easy method to stabilize the Gate Source and Drain regions from receiving power and allow power to be removed when you want it turned off. (Low-side of divider ties Gate to Ground thru a low-impedance path. Note their use of divider resistor values (fixed impedance) - keeps the Gate from seeing a secondary effect of re-rectifying RF and generating a self-oscillation and LATCHING event . The power flow I discuss is similar to using a strong stream of water to remove water thru the process of "cavitation", siphon or a venturi principle that displacement can allow electrons a place to "Fill holes" left behind - sounds exotic but it's not - it's a simpler explanation for the definition of the word "stabilize" - as in - tie it to ground.
That low voltage is fine for the Gate, but you will need to keep an eye on points of entry RF can use to sneak into a circuit and "turn on" parts you don't want to lose.
This includes the IRF520, 13N10FQP as well as just about every other MOSFET part that even have a resonance (turn on - turn off or Rise and Fall times within 32pico-seconds) even close to the 27 MHz band - they can self oscillate due to the "Tank effect" the Gate, it's capacitance and the low-inductance effects the Drain To Source provide.
Grab a schematic to the Galaxy series that use MOSFET's - you'll see why they use the above - not just a simple diode -for Class AB type of SSB modes - it's for the purpose of not overvolting the gate with too much voltage damaging it.