1. MOSFET DC circuit analysis Common-Source Circuit
Coupling capacitor acts as an open circuit to dc but allows the signal voltage to be coupled to the gate
The source terminal is at ground potential and is comon to both the input and output.

2. MOSFET DC circuit analysis Common-Source Circuit
Applying the voltage divider rule;
Assume that the MOSFET is biased in the saturation region i.e; and
Also;
The dc equivalent circuit;

3. MOSFET DC circuit analysis Common-Source Circuit
If the calculated vDS is greater than vDS(sat), the MOSFET is biased in the saturation region as we have initially assumed, and our analysis is correct.
The dc equivalent circuit;

4. MOSFET DC circuit analysis Common-Source Circuit
If the calculated vDS is smaller than vDS(sat), the MOSFET is biased in the triode region and we have to recalculate the current using the equation for the triode region namely;
The dc equivalent circuit;

5. MOSFET DC circuit analysis Common-Source Circuit
The power dissipated in the MOSFET is;
The dc equivalent circuit;

6. MOSFET DC circuit analysis Example 1 Common-Source Circuit
Calculate the drain current, ID and drain-to-source voltage, VDS and the power dissipated in the transistor PT.

7. MOSFET
DC circuit analysis
Common-Source Circuit
Example 1 – Solution

8. MOSFET DC circuit analysis Example 1 – Solution (cont’d)
Common-Source Circuit
Example 1 – Solution (cont’d)
Assuming that the transistor is in saturation mode;

9. MOSFET DC circuit analysis Example 1 – Solution (cont’d)
Common-Source Circuit
Example 1 – Solution (cont’d)
Since VDS > VDS(sat), the transistor is indeed in saturation mode as we have initially assumed.

10. MOSFET DC circuit analysis Example 1 – Solution (cont’d)
Common-Source Circuit
Example 1 – Solution (cont’d)
The power dissipated in the transistor;

11. MOSFET DC circuit analysis Example 2 Common-Source Circuit
Design the circuit such that;

12. MOSFET DC circuit analysis Example 2 – Solution Common-Source Circuit
Hence;

13. MOSFET DC circuit analysis Example 2 – Solution (cont’d)
Common-Source Circuit
Example 2 – Solution (cont’d)
Substituting values;

14. MOSFET DC circuit analysis Example 2 – Solution (cont’d)
Common-Source Circuit
Example 2 – Solution (cont’d)
But;

15. 11/02/2008

16. MOSFET DC circuit analysis Example 2 – Solution (cont’d)
Common-Source Circuit
Example 2 – Solution (cont’d)
Solving the equations gives us;

17. MOSFET DC circuit analysis Example 2 – Solution (cont’d)
Common-Source Circuit
Example 2 – Solution (cont’d)

18. MOSFET DC circuit analysis Example 3 Common-Source Circuit
For the circuit in figure; VTN = 1 V and Kn = 0.5 mA/V2.
Determine VGS, ID and VDS.

19. MOSFET
DC circuit analysis
Common-Source Circuit
Example 3 – Solution

20. MOSFET DC circuit analysis Example 3 – Solution (cont’d)
Common-Source Circuit
Example 3 – Solution (cont’d)
Substituting for
Hence;

21. MOSFET DC circuit analysis Example 3 – Solution (cont’d)
Common-Source Circuit
Example 3 – Solution (cont’d)

22. MOSFET DC circuit analysis Example 3 – Solution (cont’d)
Common-Source Circuit
Example 3 – Solution (cont’d)

23. MOSFET DC circuit analysis Example 3 – Solution (cont’d)
Common-Source Circuit
Example 3 – Solution (cont’d)

24. MOSFET DC circuit analysis Load Line and Mode of Operation
The load line equation is;
or;
From the above equation, we obtain the following two points: when ID = 0, VDS = 5 V and when VDS = 0, ID = 5/20 mA

25. MOSFET DC circuit analysis Load Line and Mode of Operation
The two points are used to plot a load line on the iD – vDS characteristic curves as shown.

26. MOSFET DC circuit analysis Load Line and Mode of Operation
The Q-point is given by the drain dc current and drain-to-source voltage and is always on the load line.

27. MOSFET DC circuit analysis Load Line and Mode of Operation
If vGS is less than VTN, the drain current is zero and the transistor is in the cutoff mode.

28. MOSFET DC circuit analysis Load Line and Mode of Operation
If vGS is just greater than VTN, the transistor turns on.

29. MOSFET DC circuit analysis Load Line and Mode of Operation
As vGS increases, the Q-point moves up the load line.

30. MOSFET DC circuit analysis Load Line and Mode of Operation
The transition point is the boundary between saturation and non-saturation (triode) region.

31. MOSFET DC circuit analysis Constant-Current Biasing
The figure shows a MOSFET amplifier where RS is replaced with a constant current source, IQ.
The gate-to-source voltage, VGS adjusts itself to correspond to the current IQ.
IQ causes the bias to be independent on the transistor parameters and hence stabilizes the operating (Q) point.

32. MOSFET DC circuit analysis Constant-Current Biasing
The figure shows the corresponding dc equivalent circuit.
The gate terminal is at the ground potential because we assume zero gate current.
IQ sets the value of VGS, VD, VDS and hence the Q-point of the MOSFET.
The current source IQ can be constructed using MOSFETs but this topic will not be covered in this note.

33. MOSFET DC circuit analysis Example 4 Constant-Current Biasing
The MOSFET parameters in the figure are;
Determine VGS and VDS.

34. 15/02/08

35. MOSFET DC circuit analysis Example 4 – Solution
Constant-Current Biasing
Example 4 – Solution
The dc equivalent circuit is as follows;
Assuming the MOSFET is operating in the saturation mode;
Substituting values;

36. MOSFET DC circuit analysis Example 4 – Solution (cont’d)
Constant-Current Biasing
Example 4 – Solution (cont’d)
The source potential;
Assuming zero gate current;

37. MOSFET DC circuit analysis Example 4 – Solution (cont’d)
Constant-Current Biasing
Example 4 – Solution (cont’d)
The drain-to-source voltage;

38. MOSFET DC circuit analysis EXERCISES Problems: 3.23; 3.25; 3.27 and
3.28

39. QUIZ