Engineering MOSFET Design: Solving Q4 with R1, R2, k, V_T and I_D

  • Thread starter Thread starter wirefree
  • Start date Start date
  • Tags Tags
    Design Mosfet
Click For Summary
SUMMARY

This discussion focuses on solving a MOSFET design problem, specifically the calculation of the drain resistor (R_D) using parameters R1, R2, transconductance (g_m), threshold voltage (V_T), and drain current (I_D). The user, wirefree, outlines a procedure involving the calculation of source voltage (V_S) and the application of the saturation condition to determine R_D. Despite following a logical approach, wirefree's calculated R_D does not align with the expected range of 16-19 kOhm, prompting a request for feedback on potential errors in the methodology.

PREREQUISITES
  • Understanding of MOSFET operation and parameters such as V_T and I_D
  • Familiarity with voltage divider circuits
  • Knowledge of KVL (Kirchhoff's Voltage Law) in circuit analysis
  • Experience with saturation conditions in MOSFETs
NEXT STEPS
  • Review the calculation of source voltage (V_S) in MOSFET circuits
  • Study the implications of the saturation condition on MOSFET performance
  • Learn about the impact of drain resistor (R_D) on circuit behavior
  • Explore common sources of error in MOSFET design calculations
USEFUL FOR

Electrical engineers, circuit designers, and students studying MOSFET applications who are looking to deepen their understanding of MOSFET design and troubleshooting techniques.

wirefree
Messages
110
Reaction score
21
Homework Statement
Determine the value of R_D for which the amplitude of drain voltage swing will be maximum
Relevant Equations
Saturation condition: v_DS >= v_GS - V_T


Drain current:
i_D = k*(v_GS - V_T)^2 * (1+y*v_DS)
where y: channel length modulation parameter
V_T: Threshold Voltage
V_XY: Voltage between X & Y
[D: Drain S: Source G: Gate]
I greatly appreciate the opportunity afforded by this forum to submit questions.

I am presently tackling a 10-part question, fourth of which this post is concerning. Values provided at the start are: R1, R2, k, V_T and I_D.

In the previous 3 parts, Source Resistor, Transconductance (g_m) and slope of i_D-v_DS curve (r_o) have been calculated.

My attempt at a solution is pretty straight-forward but the answer does not match any of the provided options. The said approach involves:
- ascertaining V_G via Voltage Divider
- invoking the Saturation condition to obtain R_D
- Further still, setting V_D to halfway between VDD and Ground to allow maximum swing and finding R_D again

My working is for your purview in the attachment.

I would be indebted for advice on possible sources of error.

Best regards and Namaste,
wirefree
 

Attachments

  • D8A87674-CD76-470A-8DCE-88748153C16F.jpeg
    D8A87674-CD76-470A-8DCE-88748153C16F.jpeg
    35.4 KB · Views: 239
Physics news on Phys.org
As I keenly await Respected Members to respond, I am taking this opportunity to better explain my procedure.

Given quantities & circuit:
1566404760609.png

1566404776150.png

* The last quantity on the right is V_S (it's calculated)

The procedure to determine R_D is as follows:
1) Calculate V_S for given I_D and R_S (calculated earlier using the Drain current equation)

2) Determine minimum value of V_DS from Saturation condition and add to it V_S to obtain V_D
1566404956499.png
3) Center V_D at mid-point between 0V & 5V, i.e. 2.5V, and apply KVL to obtain R_D
1566404978163.png
This procedure ensures that V_DS meets the Saturation condition, and that maximum swing is available for V_DS, as much as I understand the situation.

However, the value of R_D does not match any of the given answers, which are in the 16-19 kOhm region.

Kindly correct me where I am wrong.
wirefree
 
Do you see an inconsistency between these two statements?
wirefree said:
2) Determine minimum value of V_DS from Saturation condition and add to it V_S to obtain V_D
wirefree said:
3) Center V_D at mid-point between 0V & 5V, i.e. 2.5V,
 
  • Like
Likes wirefree
Tom.G said:
Do you see an inconsistency between these two statements?

I do now.

Thank you, Sir.
 
  • Like
Likes Tom.G