What is the Bias Point (VDS, IDS) in a MOSFET Circuit?

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Discussion Overview

The discussion revolves around determining the bias point (VDS, IDS) in a MOSFET circuit. Participants are exploring the calculations necessary to find these values, including the relationships between various voltages and currents in the circuit.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant begins by calculating the gate voltage VG using a voltage divider formula, but expresses confusion about the next steps to find IDS.
  • Another participant points out an error in the initial calculation of VG and suggests that it should be 2VDD instead of VDD.
  • Subsequent posts highlight further corrections regarding the calculation of VGS and the impact of the -VDD term, indicating that the participant is struggling with these concepts.
  • One participant eventually arrives at a correct formulation for VG and VGS, but questions whether the small value of λ (0.02 V-1) can be ignored in their calculations.
  • Another participant advises retaining the value of λ in the final answer, suggesting that it should not be disregarded even if it is small.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether λ can be ignored in the calculations, indicating an unresolved aspect of the discussion. There are also multiple corrections and refinements to the initial calculations, reflecting ongoing debate about the correct approach.

Contextual Notes

The discussion includes various assumptions about circuit parameters and the relationships between voltages and currents, which may not be fully articulated or agreed upon by all participants.

Jalo
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Homework Statement



Given the following circuit find the bias point (VDS,IDS).

30b2pad.png


Homework Equations





The Attempt at a Solution



The first thing I did was find the value of VG, the voltage from the gate of the mosfet to the -VDD source.

VG = VDD * R2/(R1+R2)

To find the current IDS I needed the voltage from the gate to the source, VGS, which is:

VGS = VG - RsID

Since we're operating in the saturation region I can replace ID by:
VGS = VG - Rsk(VGS-VT)^2 (1+λVDS)

where VDS is the voltage between the drain and the source.
I also know that:
VDS = 2VDD - (Rd+Rs)ID

This is as far as I can get. I'm a little confused regarding what I should do next.

If anyone could help I'd appreciate.

Thanks.
Daniel.
 
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Jalo said:

Homework Statement



Given the following circuit find the bias point (VDS,IDS).

30b2pad.png


Homework Equations





The Attempt at a Solution



The first thing I did was find the value of VG, the voltage from the gate of the mosfet to the -VDD source.

VG = VDD * R2/(R1+R2)



Not a good start. Need to fix that first.
 
rude man said:
Not a good start. Need to fix that first.

Sorry, I meant to write 2VDD instead of only VDD.
 
Jalo said:
Sorry, I meant to write 2VDD instead of only VDD.

Better, but still not right.
 
Jalo said:


To find the current IDS I needed the voltage from the gate to the source, VGS, which is:

VGS = VG - RsID



That equation is also wrong.

You're having trouble with that "-V_DD".
 
rude man said:
That equation is also wrong.

You're having trouble with that "-V_DD".

Thanks! I finally got it right.


VG = VDD (R2-R1)/(R2+R1)
VGS = VG - RsID + VDD

ID = k(VGS-VT)2

Replacing VGS in the expression (1) I managed to find the correct solution. I still have one doubt tho.
The value of λ is very small, only 0.02 V-1. When the value is that low can I simply ignore it and solve the problem as if it was zero?
 
Jalo said:
Thanks! I finally got it right.


VG = VDD (R2-R1)/(R2+R1)
VGS = VG - RsID + VDD

ID = k(VGS-VT)2

Replacing VGS in the expression (1) I managed to find the correct solution. I still have one doubt tho.
The value of λ is very small, only 0.02 V-1. When the value is that low can I simply ignore it and solve the problem as if it was zero?

OK now.
Since you were given a number for lambda I would retain those values in your answer.
 

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