Applied Electronics - Outcome 1 - MOSFETs

MOSFETs at a glance:

N-channel MOSFET

• MOSFETs are similar to transistors, but they are voltage operated devices. 
• This means that (ideally) it draws no current. (This means that you can use ohm's law in your calculations to find the Vin unlike a transistor where you must use the voltage divider equation because the transistor will draw current from the supply/voltage divider)
• They can be N-channel, or P-channel
• It has a very high input resistance, and no (low) current, therefore has a low power consumption
• It has a slower switching speed than a transistor
• It is very sensitive to static!  Beware when building with them!

•  MOSFETs operate above a threshold voltage VT.  When the voltage between the gate and the source (VGS) is above the threshold voltage, the MOSFET will saturate and ID will be its maximum and will not increase past this point.
• Until full saturation the ID will increase proportionally.  It is then possible to calculate the transconductance which is the ratio between ID and VGS as demonstrated below:

•  MOSFET circuit calculations involve mostly ohm's law.  This diagram shows the voltages dropped and the currents in the circuits:

 

So, a similar question to the homework - Use the diagram above to help you understand the labels and relationships between the voltages and currents.

This MOSFET has a Threshold voltage of 5v. When saturated the ID is 3mA and the source resistor is 2k (The supply voltage is 12v).  The current flowing through the voltage divider is 5uA.

Find the values of R1 and R2 (in the voltage divider) to ensure saturation.

So if ID = 3mA then IS will equal 3mA.  If the source resistance is known then the VS can be found.

VS = ISRS

        = 3x10-3 x 2x103 

        = 6v 

If we know VS and we know VGS (remember that if the MOSFET is saturated then VGS = VT) so we can find VG which is the voltage dropped over R2.

VG = VGS + VS

       = 3 + 6

       = 9v

If we know VG and we know that the current flowing through the voltage divider, then we can work out the two resistances.  Remember that we can use Ohm's law because no current goes into a MOSFET so it is all flowing through the voltage divider.

 VG = V2 = IR2

           9  = 5 x10-6 x R2  

         R2 = 9/5 x10-6

               = 1.8M Ω

V1 = VCC - V2              R1 = 3/5 x10-6

      = 12 - 9                 = 600kΩ

      = 3v 

Find the VDS if the Drain resistor is 500Ω.

 VD = ISRS                                      VDS = VCC - VD - VS

          = 3x10-3 x 500                 = 12 - 1.5 - 6

          = 1.5v                            = 4.5v