Simulating MOSFET Transistors

MOSFETs as Electronic Switches

We want to use transistors to power on/off devices remotely and/or automatically.  One useful tool at our disposal is the Metal Oxide Semi-Conductor Field Effect Transistor (MOSFET).  There are two primary types of MOSFETs that you should be aware of:

  1. P-Channel  (+)
  2. N-Channel (-)

Use a P-Channel MOSFET if you would like to switch a circuit before the load and with no power at its gate resulting in an ON switch.  Applying voltage to a P-Channel gate will disconnect the load.  Normally On

Use a N-Channel MOSFET to switch a circuit at its return or “common” wire. With no power to at its gate results in an OFF switch.  Applying voltage to a N-Channel gate will allow current to flow through the transistor.  Normally OFF

Hint: A simple way to simulate a load is by adding a resistor.

Now, how do we know how much voltage to apply to the gate?  Well.. our goal is to reach the saturation point of the transistor, where the maximum amount of current will flow through the Drain – Source junction:

NMOS Voltage - Current Profile

NMOS Voltage – Current Profile

Critical equation:  Vgs = Vds – Vt

Translation:  set Gate Voltage equal to Source to Drain Voltage minus Threshold Voltage (or higher)

Refer to component specification sheet for threshold voltage and other information.  EXAMPLE

Can’t find the LT Spice components used in this tutorial?  Please refer to my previous video for instructions on how to download and install them.

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How to Download Spice Components

How to Download LT Spice Components

As you begin simulating circuits, you may find the LT Spice library to be rather limited.  You want to find an exact component online and load it into Spice.  This is one way to do it:

Grab Some Example Components

Download two files:

1.  ___.sub

2. ___.asy

 Save these files to the desktop, as it is forbidden to save directly to the LT Spice library.

Move ___.sub to c://program files/LTC/LTSpiceIV/lib/sub

Move ___.asy to c://program files/LTC/LTSpiceIV/lib/sym

Restart LT Spice.  Your new components should now be available!

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