MOSFet Biasing??!!

[Alpha579]

Hey all,
Im having a nightmare of a time biasing this IRFP450 Mosfet/ Mosfets in general. I just dont know where to start??? :oops:
Help plse!

[Alpha579]

~#bump#~

[Alpha579]

Come on guys....

[RDV]

http://www.geofex.com/Article_Folders/mosboost/mosboost.htm

It's here.

RDV

[Jered]

Come on guys....

 Well, for anybody to even venture a guess we need more info. What circuit are you using it in, voltage readings at the drain, gate, and source, etc.
 The more info the better.
 Jered

[Alpha579]

Sorry, no offense ment...
I am trying to bias a 2n7000 now, and the readings im getting at the moment are:
G:2.8
D:8.97
S:0.006???!!!!
Im new to Mosfets, so i know almost nothing abput them  :oops:

[Jered]

Still don't know what circuit your trying to use this in. As RDV posted above go to the geofex mosfet booster write up. RG has forgoten more about mosfets than I'll ever know. Good luck.
  Jered

[gez]

Take a look at the EA trem over at Runoff.  It uses a MOSFET input buffer.

Trimpot bias is the easiest way to go with MOSFETS.  Block off a small amount of supply voltage for a source resistor to set the quiescent drain current, select a drain resistor to bias the drain at around half the supply, then adjust the trim from ground till you get the voltage readings you calculated.

Simple!

[R.G.]

I am trying to bias a 2n7000 now, and the readings im getting at the moment are:
G:2.8
D:8.97
S:0.006???!!!!

That looks quite reasonable. MOSFETs have a threshold Vgs (gate to source voltage) below which nothing happens. After you pass that, the device conducts at its transconductance rate. For the big IRF devices, the transconductance tends to be about 1A/V and Vt is about 3 volts, so for an IRF device you usually get something like
VGS             Id
0                 0
1                 0
2                 0
3                 just a touch of current
4                 1A
5                 2A

For the smaller devices, the Vt and transconductance vary more. Vt for the 2N7000 is 0.8 to 3V (as the mentioned article at GEO goes through), so the 2.8V you are showing for gate to source is reasonable, if at the high range of normal. Your particular device is just now starting to conduct, and apparently is doing just enough to raise the source by 6mv on whatever resistor is between it and ground. The drain is dropped down from the power supply by the same current times whatever resistance it flows through.

I think your device is just starting to turn on. However, as the GEO article points out, the range between barely on and fully saturated in a small signal circuit can be under 50mv, so increasing the gate voltage just a hair can make it really start up.

Read the article at GEO - there's a lot there.  I get tired of typing so I write the things down at GEO instead of retyping.