intresting way to bias an RF amplifier

Thread Starter

yef smith

Joined Aug 2, 2020
268
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?
1623352722699.png
 

Audioguru again

Joined Oct 21, 2019
3,293
Every Mosfet has a detailed datasheet that tells you the range of Vgs that turns it on. Its part number is very important.
Your Mosfet symbol is missing the arrow that says if it is an N-channel or is a P-channel.
Your voltages do not say +3.3V or -3.3V. The same with the 1V.
 

Thread Starter

yef smith

Joined Aug 2, 2020
268
Hello Audioguru,this is a theorectical question only.
suppose my Vsg>Vt and the transistor is open but its drain is connected to the amplifier.
How do i know the value of the drain to see if its saturated or not?
Thanks.
 

Audioguru again

Joined Oct 21, 2019
3,293
A door opens and closes. A transistor turns on, amplifies or turns off.
A transistor part number has a range of specifications since some have a high gain, some have medium gain and some have a low gain.

A Mosfet like you show has a range of Vgs threshold which is 2V to 4V for an IRFxxx one producing a drain current of only 0.25mA.
Most circuits are designed with negative feedback so that the different gains and different threshold voltages all work fine in a circuit.

You show 1V at the gate of the Mosfet which keeps it turned off. For an N-channel mosfet it is barely turned on when its Vgs is 2V to 4V and must be about 10V for an IRFxxx one to be fully turned on.

Your amplifier is not biased when the Mosfet is turned off. When the Mosfet is turned on then the input of the amplifier becomes biased with the 3.3V but if the 3.3V is a low impedance then the signal is blocked and the amplifier gets no signal.
 
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