here is a simple constant current load circuit from eevblog.com, it's possible that U2 and Q1 might oscillate, a series resistor need to put between U2 and Q1 to stop it.

So what cause the oscillation here? and why?

 

Hello,

The input of the Mosfet will behave as a capacitor.
A resistor in series with the gate of the Mosfet will reduce the peak current into the gate.

Bertus

 

Yes this circuit can oscillate simply add capacitor between gate and source and additionally include op amp output resistance.
http://books.google.pl/books?id=d8E...q=Example 11.5: MOSFET current source&f=false

 

Sorry to dig this old post up, what if I replace the MOSFET with a bipolar transistor, does it still oscillate?

 

Some op amps are sensitive to output load capacitance and will oscillate unless there is a resistor added in series with the capacitance. A BJT is much less likely to cause oscillations in that circuit since it has a much lower input capacitance than a MOSFET.