MOSFET usage examples

[EHHS1979]

Both transistors and MOSFETS have advantages and weaknesses, and I have discovered multiple uses of MOSFETS that make it a handy device.

Few parameters to keep in mind.
1. Max drain current
2. VTH (Threshold voltage)
3. Max VGS (Voltage across source-Drain)
4. Max Power dissipation

Two types of common MOSFETs - enhanced mode and depletion mode

The voltage difference between source and gate acts like a water faucet

- Enhanced Mode: with VGS = 0, the Source-Drain channel is pinched off, no current flow.  Current flows as VGS voltage approaches VTH, current starts to flow between source and drain. 

- Depletion Mode: The source - drain channel is fully open with VGS = 0.  As VGS increases, the channel starts to pinch off until VGS = VTH.



Posted on: November 21, 2013, 01:04:50 01:04 - Automerged
Example of a depletion mode MOSFET is current limiters.  Especially for mixed voltage systems using current input analog channels, and digital optically isolated inputs.

Let's take an optical isolated digital input, a series resistor is used to limit current input from a digital signal source.  The series resistor will be one value for 5 VDC input, and another for 15 VDC input.  A depletion mode MOSFET in series with the optical isolator can eliminate the need to change component values and allow input voltage to be from 3.3 to 32 VDC without resistor value change.

Place the depletion mode MOSFET in series with the series resistor, then the optical isolator.  The gate of the MOSFET is connected to the junction with the resistor and optical isolator.  At 3.3 VDC, the voltage developed across the resistor may not be sufficient to cause the MOSFET to pinch off, but the resistor itself is the current limiter.  But when you appli 24 VDC to the input, the current through the series resistor will cause the MOSFET to pinch off, regulated by the voltage across the series resistor.  The max current through the resistor, and the optical isolator is roughly VTH/R.  if VTH is -1.5 VDC, and nominal optical isolator current is 10 mA (> 5 mA), then the resistor value of 150 ohms would be sufficient.  With 3.3 VDC, 150 ohm resistor, and forward voltage of the optical isolator to be apoprox 2.1, the current is calculated to be about 8 mA, which is greater than 5 mA.

[Gallymimu]

you might also want to mention some of the challenges of using MOSFETs in linear mode for instance how their power handling capability is greatly limited in this mode (Safe Operating Area discussion for instance)

[EHHS1979]

Since I use MOSFETS either as switches, or in small signal circuits, power handling is not a problem.

[Gallymimu]

Since I use MOSFETS either as switches, or in small signal circuits, power handling is not a problem.

I thought you were providing an education on MOSFETs thread.  If that is the case then you should be complete in the information and basic usage cases?!

[EHHS1979]

This is the place to share information, not play the battle of the egos...

MOSFETS can play in the linear region.  And there are power MOSFETS to handle high power.

[Vineyards]

EHHS1979, I read your initial post with interest.As a matter of fact, with all these big brains around we ought to have more helping hands at Sonsivri.

If you can post a few circuit schematics to visually support your topic, it would be very helpful.

A while ago, I tried to design an amplitude stable wien oscillator using an RMS to DC converter and a FET which is used as a VCO. The amplitude was stable within a few millivolts however thermal stability was simply not present. I toyed with NTC's and digipots but after a certain point the circuit grew unacceptably complicated. If you also have a few suggestions about providing thermal stability in the ohmic region of a FET, I would appreciate.

[pickit2]

This is not a pissing contest... FFS the post's in this topic have little value.
less in fact than the data sheets or a Google search.