# intresting way to bias an RF amplifier

#### yefj

Hello i have a configuration as shown bellow which i dont understand its logic:
This circuit is supposed to pass 3.3V to the amplifier.
I dont have any knowledge about the drain in the mosfet.
What is the drain voltage in this case, how can i see if the transistor is saturated or not?

#### dick_freebird

The "1V" may be a rough OP value that represents a more complex circuit's imposed control voltage such as a closed loop active bias scheme.

#### yefj

Hello dick_freebird, the big problem is when we connect the MOSFET to an amplifier in such way.
What is the value of the drain of the mosfet?

I can assume we have Vsg>Vt so its open but i cant know the drain value to see if its saturated or not?
Thanks.

#### frankrose

Hi,

I think it is a simple switch, not the bias circuit itself.
FET is a PMOS, its Vsg is -2.3V, you should suppose it is enough to close the switch.

If DC current doesn't flow through it into the input of the amplifier it is in deep linear OP region.
With small DC current the Vsg is small enough to keep it in linear OP region, if the DC current is relatively high is the case it will be in saturation region, not so possible, but lot of details are missing (Vth,beta,Idc,discrete/integrated FET type?...)

#### dick_freebird

Hello dick_freebird, the big problem is when we connect the MOSFET to an amplifier in such way.
What is the value of the drain of the mosfet?

I can assume we have Vsg>Vt so its open but i cant know the drain value to see if its saturated or not?
Thanks.
The drain voltage would depend on the load, the
load is the input bias current of the amplifier.

Seems peculiar, to drive the PMOS with a not-ground
voltage for turnon if this is supposed to be a "STFU"
switch. But we'd want a datasheet link for the amplifier
if we were to make sense of a switch or a current source
theory - what does its front end do with, or want from,
a switch or a pullup current source?

If this were attached to the amplifier front end,
you'd expect a small MOSFET to be an aggravating
noise source and a large one, a distortion issue
with its significant nonlinear drain capacitance).