MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) amplifiers are widely used in electronics due to their high input impedance, low power consumption, and excellent amplification capabilities. Designing a MOSFET amplifier involves selecting biasing components, calculating gain, and ensuring stability. In this guide, we’ll walk through the steps to draw and calculate a common-source MOSFET amplifier circuit.

1. MOSFET Amplifier Basics

A MOSFET amplifier typically operates in one of three configurations:

• Common-Source (CS) – Provides high voltage gain (most common for amplification).
• Common-Drain (CD) (Source Follower) – Provides unity voltage gain but high current gain.
• Common-Gate (CG) – Provides moderate voltage gain and low input impedance.

Here, we’ll focus on the Common-Source Amplifier, which is widely used for signal amplification.

2. Drawing a Common-Source MOSFET Amplifier Circuit

Components Needed:

• N-channel MOSFET (e.g., IRF540N, 2N7000)
• DC power supply (V_DD)
• Resistors (R_G1, R_G2, R_S, R_D)
• Capacitors (C₁, C₂, C_S)
• Load resistor (R_L)

Circuit Diagram:

         VDD
          |
          RD
          |
GND---||--D
      C1  |
          |  
      RG1 |  
Vin---o---G  
      |  
      RG2 |  
          S  
          |  
          RS  
          |  
         CS  
          |  
         GND  

Explanation:

• R_G1 & R_G2: Voltage divider to set gate bias (V_GS).
• R_S: Source resistor for stability (bypassed by C_S for AC signals).
• R_D: Drain resistor sets DC operating point and gain.
• C₁, C₂: Coupling capacitors to block DC and pass AC signals.
• C_S: Bypass capacitor to maximize gain (shorts R_S at signal frequencies).

3. Calculating DC Bias Conditions

Step 1: Determine V_GS (Gate-Source Voltage)

Using the voltage divider rule: VG=VDD×RG2RG1+RG2VG​=VDD​×RG1​+RG2​RG2​​

Assume MOSFET is in saturation (V_DS > V_GS – V_th).

Step 2: Find I_D (Drain Current)

For an enhancement MOSFET:ID=K(VGS−Vth)2ID​=K(VGS​−Vth​)2

Where:

• K = Transconductance parameter (from datasheet).
• V_th = Threshold voltage.

If R_S is present:VGS=VG−IDRSVGS​=VG​−ID​RS​

Step 3: Calculate V_DS (Drain-Source Voltage)

VDS=VDD−ID(RD+RS)VDS​=VDD​−ID​(RD​+RS​)

Ensure V_DS > V_GS – V_th for saturation.

4. Calculating AC Parameters (Small-Signal Gain)

Step 1: Find Transconductance (g_m)

gm=2KIDgm​=2KID​​

orgm=2IDVGS−Vthgm​=VGS​−Vth​2ID​​

Step 2: Voltage Gain (A_v)

For a common-source amplifier with bypassed R_S:Av=−gm(RD∣∣ro∣∣RL)Av​=−gm​(RD​∣∣ro​∣∣RL​)

Where:

• r_o = MOSFET output resistance (from datasheet, often ignored in basic designs).
• R_L = Load resistance.

If R_S is not bypassed:Av=−gmRD1+gmRSAv​=1+gm​RS​−gm​RD​​

Step 3: Input and Output Impedance

• Input impedance (Z_in) ≈ R_G1 || R_G2 (very high due to MOSFET gate).
• Output impedance (Z_out) ≈ R_D || r_o.

5. Design Example

Given:

• V_DD = 12V
• MOSFET: K = 0.1 A/V², V_th = 2V
• Desired I_D = 5mA
• Desired A_v = -10

Calculations:

1. Find V_GS:5mA=0.1(VGS−2)2  ⟹  VGS=2+0.05≈2.23V5mA=0.1(VGS​−2)2⟹VGS​=2+0.05​≈2.23V
2. Set R_S = 100Ω (for stability):VG=VGS+IDRS=2.23+(5mA×100)=2.73VVG​=VGS​+ID​RS​=2.23+(5mA×100)=2.73V
3. Choose R_G1 & R_G2:
   Let R_G1 = 1MΩ, R_G2 = 270kΩ (using voltage divider).
4. Calculate R_D for gain:gm=2×0.1×(2.23−2)=0.046Sgm​=2×0.1×(2.23−2)=0.046SAv=−gmRD  ⟹  10=0.046RD  ⟹  RD≈220ΩAv​=−gm​RD​⟹10=0.046RD​⟹RD​≈220Ω
5. Verify V_DS:VDS=12−5mA(220+100)=10.4V(validforsaturation)VDS​=12−5mA(220+100)=10.4V(validforsaturation)

6. Simulation & Testing

• Use SPICE (e.g., LTspice) to verify bias points and gain.
• Measure input/output signals with an oscilloscope.
• Adjust R_D, R_S, or bypass capacitors to optimize performance.

Conclusion:

Designing a MOSFET amplifier involves:

1. Setting proper DC bias conditions.
2. Calculating small-signal parameters (g_m, r_o).
3. Determining voltage gain and impedance.

By following these steps, you can successfully design and analyze a MOSFET amplifier circuit for various applications.