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Mosfet Source Follower Calculator

Reviewed by Calculator Editorial Team

This MOSFET source follower calculator helps you analyze and design source follower circuits by calculating key parameters such as voltage gain, output resistance, and voltage transfer characteristics. The calculator uses standard MOSFET equations and provides visual output for better understanding.

Introduction

A MOSFET source follower is a basic amplifier configuration that provides voltage buffering between stages. It's commonly used in analog circuits to maintain a constant voltage difference between the input and output while providing some current gain.

The source follower configuration is particularly useful in applications requiring high input impedance and low output impedance. It's widely used in operational amplifiers, voltage regulators, and signal conditioning circuits.

How MOSFET Source Followers Work

The basic operation of a MOSFET source follower can be understood through the following principles:

  1. The MOSFET operates in the saturation region where the drain current is controlled by the gate voltage.
  2. The source voltage follows the input voltage minus the threshold voltage, creating a voltage buffer effect.
  3. The output resistance is determined by the MOSFET's transconductance and the load resistance.

Key Equations:

Voltage gain (Av) = gm * RL

Output resistance (Rout) = 1 / (gm * (1 + gm * RD))

Where gm is the transconductance, RL is the load resistance, and RD is the drain resistance.

Key Parameters

The MOSFET source follower calculator computes several important parameters:

  • Voltage Gain: The ratio of output voltage to input voltage
  • Output Resistance: The resistance seen looking into the output
  • Voltage Transfer Characteristics: The relationship between input and output voltages
  • Current Gain: The ratio of output current to input current

These parameters help engineers understand the circuit's behavior and make appropriate design choices.

Using the Calculator

To use the MOSFET source follower calculator:

  1. Enter the MOSFET parameters: transconductance (gm), threshold voltage (Vth), and drain current (ID)
  2. Specify the load resistance (RL) and supply voltage (VDD)
  3. Click "Calculate" to compute the circuit parameters
  4. Review the results and adjust parameters as needed

Note: The calculator assumes the MOSFET is operating in saturation. For subthreshold operation, different equations would apply.

Example Calculation

Consider a MOSFET with gm = 0.01 mhos, Vth = 1V, and ID = 1mA. With RL = 1kΩ and VDD = 5V:

  1. Voltage gain (Av) = 0.01 * 1000 = 10
  2. Output resistance (Rout) ≈ 1 / (0.01 * (1 + 0.01 * 1000)) ≈ 10Ω
  3. The output voltage will follow the input voltage with a gain of 10 and low output impedance of 10Ω

This example demonstrates the voltage buffering and current gain characteristics of the source follower configuration.

FAQ

What is the difference between a source follower and a common drain amplifier?
The terms are often used interchangeably, but source followers typically refer to configurations where the source is connected to the output, while common drain amplifiers may include additional components for specific applications.
When would I use a MOSFET source follower instead of a bipolar transistor?
MOSFET source followers are preferred for their high input impedance, low output impedance, and compatibility with digital circuits. Bipolar transistors may be used when higher current gain or different voltage characteristics are required.
How does temperature affect MOSFET source follower performance?
Temperature variations can affect the MOSFET's transconductance and threshold voltage. The calculator assumes room temperature operation, and significant temperature effects may require additional analysis.
Can the calculator handle depletion-mode MOSFETs?
The calculator is designed for enhancement-mode MOSFETs. Depletion-mode operation would require different equations and parameters.