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role of optocoupler for biasing drain of amplifiers

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john23:
Hello,In the diagram sown below I have a MOSFET called NDT3055L, the mosfet sends the current into the Qorvo TGA2590-CP drain.
When we test the circuit the source of the mosfet is basically floating although when I will connect the amplifier to source then the QORVO amplifier will introduce a load to the source of the mosfet.
Somehow the optocoupler helps with the transition of the state when we don't have QORVO amplifier load on the source of the MOSFET.
What is the role of the optocoupler in this situation?
I'll be glad to have some analog design logic behind putting this component in this position.
Thanks.

https://www.ti.com/lit/ds/symlink/ucc5304.pdf?ts=1713292114392

TGA2590-CP - Qorvo
www.qorvo.com www.qorvo.com

xvr:
There is no optocoupler in this schema. UCC5304 is an isolated driver, but not optocoupler.
As for using it here at all - it used to decouple ground of schema from Source of MOSFET. MOSFET controlled by voltage, applied between Source and Gate. Original signal to control MOSFET based to ground of all schema, but MOSFET Source has floating potential relatively ground, so control signal can not be directly passed to MOSFET Gate.
UCC5304 rebased this control signal to MOSFET Source - it used as ground potential on output (isolated) side of UCC5304.

BTW - there is no supply attached to isolated side of UCC5304 in schema. I think something was forgotten :)

john23:
Hello XVR, an updated version is attached of the schematics.

Why do we connect floating ground to a place where we would get the amplifier connected.
The TGA 2590 amplifier will be the load the mosfet source(when we connect them together)
What is the logic of connecting floating ground there?
Thanks.

"it used to decouple ground of schema from Source of MOSFET"

xvr:
> an updated version is attached of the schematics.

Floating power still unconnected. You should connect right Vdd of UCxxx to +24V net.

> What is the logic of connecting floating ground there?

Output of UCCxxx should be referenced again Source of MOSFET - this is the way how NOSFET works (it controlled by Gate-Source voltage).
And doesn't matter what else is connected to MOSFET Source.

moffy:
I think you need a diode from +24V to Vdd, because the source shifts up to +24V when the MOSFET is on, but +24V exceeds the maximum voltage of the UCC5304, and is also too high for MOSFET gates. You probably need a zener  in series with the diode to drop the voltage down to say 15V. I also think you need some moderate value resistor from your floating ground to ground otherwise there will be no current path to charge the driver IC supply unless the opamp can provide that path.

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