Author Topic: Why connecting a capacitance across Gate and Source of a MOSFET makes it Stable?  (Read 1075 times)

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Offline Arsh ahmadTopic starter

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I am designing a constant current source using OPMAP and a P channel MOSFET. At low resistive loads the MOSFET goes unstable but when I connect a 100nF capacitor between Gate and source the current becomes stable. But I am not able to figure out why is it so.
 

Offline ZeTeX

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Because you're slowing the response of the loop. The capacitor has to charge to appropriate voltage, thus slowing down the speed of your feedback loop. This might reduce oscillation with specific op amps and load.
The common way to stabilize the loop and reduce oscillation is by a few compensation techniques.
Have a look at the following thread, and read the comments
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/
 
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Offline Roehrenonkel

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Hi Arsh ahmad,
 
maybe try a gate-resistor (100R...1k) instead of the cap.

Good luck

Edit: To clarify: Not replacing the cap, but in series to the gate (as near as possible), ommit the cap.
« Last Edit: July 12, 2022, 09:16:56 pm by Roehrenonkel »
 

Offline SiliconWizard

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This of course is a compromise between stability and speed. You don't want the current source to oscillate, but you may have requirements in terms of speed, in particular if you either need to set your current source from a varying voltage, or if the load itself is not stable, or a combination of the two.
 


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