Expressing Operating point of a NMOSFET

[Teszla]

NMOSFET circuit analysis

In the following circuit all capacitances could be seen as coupling capacity, i.e. they block direct voltages (the supply voltage +V_DD) but lets through (short circuits) time varying signals (input signal V_in(t)).

**broken link removed**

Determine R₃ expressed in the operating point (V_GSQ, I_DQ) of the transistor and the supply voltage V_DD.

I know that there will be no current through R₁. The voltage at the source could be expressed as V_R3 = R₃I_DQ, which means that R₃=V_R3/i_DQ. But how can we express V_R3 in terms of the known variables?

 

[bmandl]

cant' see circuit...

 

[keith1200rs]

cant' see circuit...

Seems fine to me.

Keith

 

[Teszla]

Updated image link.

 

[bmandl]

And which are the data, that you have?

 

[Teszla]

And which are the data, that you have?

R₃ should be expressed in terms of V_GSQ, I_DQ and V_DD.

 

[Audioguru]

Properties for the image say, "Not Available" for most properties. I do not know why the image is posted on another website instead of being posted here. I copied it and here it is:

R1 applies the entire power supply voltage +VDD to the gate of the Mosfet so it is turned on as hard as it can go, like a piece of wire between R2 and R3.
Instead, R1 should be connected to ground then the current in R3 biases the gate-source.

 

[Teszla]

R1 applies the entire power supply voltage +VDD to the gate of the Mosfet so it is turned on as hard as it can go, like a piece of wire between R2 and R3.
Instead, R1 should be connected to ground then the current in R3 biases the gate-source.

I understand the MOSFET is turned on, but how do you mean "R1 should be connected to ground"? There are components between R1 and ground.

Also what do you mean with that "the current in R3 biases the gate-source". The voltage above R3 must be lower than VDD, for there to flow current iD, right?

 

[bmandl]

Like this:

 

[Teszla]

Is this image of yours actually explaining something? Why did you switch places of components? What is R4, where did it come from? Please explain the image. Is it helping answering the problem I'm asking?

 

[Audioguru]

Tesla changed all the parts designation numbers. Also he draws a P-channel Mosfet instead of an N-channel Mosfet.

R1 in the original circuit is supposed to connect to ground because Mosfets are enhancement type. Then the gate is at 0V and current in the Mosfet drain to source in R3 in the original circuit biases the Mosfet to its operating point.
R4 in Tezla's schematic is the load resistor because every amplifier except yours has a load.

 

[keith1200rs]

The circuit shown by bmandl is still NMOS not PMOS. "Biasing" an NMOS with the gate to GND can work in some circumstances but most of the time won't turn on the transistor or produce any results (for enhancement NMOS). Unfortunately we don't know the nature of the input signal.

Keith

 

[FvM]

If we are talking about linear amplifiers, ground connected R1 is only good for depletion mode FETs, e.g. JFETs.

Depending on the threshold voltage, an enhancement MOSFET can either work with a Vdd connected gate resistor, or more flexible, with a voltage divider.

 

[Audioguru]

Duhh! Of course.
An enhancement mode Mosfet must have a bias voltage (not VDD) on its gate for it to be a linear amplifier.

Every Mosfet in Google Images shows the arrow direction reversed and in the middle, not at one end.

 

[Teszla]

How will this circuit be different?

 

[bmandl]

This is much better. It is most common biasing circuit with 4 resistors.

 

[Audioguru]

How will this circuit be different?

View attachment 98163

Thank-you for showing a NORMAL image of an N-channel Mosfet.
R1 and R2 feed a certain voltage (not the entire supply voltage) to the gate to turn on the Mosfet a little, then the current in R4 develops a voltage at the source to turn the Mosfet off a little making it balanced and linear.

 

[FvM]

Thank-you for showing a NORMAL image of an N-channel Mosfet.

I don't agree with "NORMAL".To be exact, it's a depletion mode NMOSFET symbol (continuous line). The original circuit shows an enhancement mode NMOSFET (broken line).

 

[Teszla]

Thank you for expanding the discussion around these tasks. However these circuits are from a task book, and the question of the task is to find an expression for R3 (in the first circuit) and for R4 in the second. I.e. R3 and R4 should be expressed in terms of the other known variables. Do you understand the purpose of the tasks? If so, can you please me how to solve the tasks in question?

 

[Audioguru]

It is difficult to calculate the values for R1, R2 and R3 because a Mosfet has a range of gate-source voltage for a certain current and a range of current for a certain gate-source voltage.
The range of voltage gain of a Mosfet complicates the calculations.