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.

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;

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;

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;

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

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.

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

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;

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.

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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.

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.

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.

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

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

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

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.

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.

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

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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;

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

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

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

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