Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Post History

71%
+3 −0
Q&A MOSFET drain current ringing in saturation region

It's got nothing to do with the MOSFET's miller capacitance. Miller capacitance causes problems in common-source circuits but, your circuit is common-drain (or source follower) hence, with a steady...

posted 9mo ago by Andy aka‭  ·  edited 9mo ago by Andy aka‭

Answer
#3: Post edited by user avatar Andy aka‭ · 2024-03-10T12:16:45Z (9 months ago)
  • It's got nothing to do with the MOSFET's miller capacitance. Miller capacitance causes problems in common-source circuits but, your circuit is common-drain (or source follower) hence, with a steady DC voltage on the drain (your power supply), there can be no problematic feedback via the miller capacitor to the gate.
  • Any issues with the stability (from to a step change in demand) are due to the 50 k&ohm; gate resistor and the gate-source capacitance. You might say "hey, it's a source follower so gate-source capacitance doesn't come into play" and, that would be a fairly valid point should the MOSFET source follower have near unity voltage gain (like a BJT). But, it doesn't so, about 50% of the gate-source capacitance can be modelled as sitting between gate and 0 volts.
  • This produce a decent phase lag that approaches 90&deg; in the feedback loop and takes you close to instability. In fact, many of these types of circuit are so unstable that local feedback around the op-amp are needed to stabilize them.
  • Then you should ask yourself, do you really need a very fast response in load current from a demand change and, if not, then put an RC filter between demand input and non-inverting input of the op-amp.
  • It's got nothing to do with the MOSFET's miller capacitance. Miller capacitance causes problems in common-source circuits but, your circuit is common-drain (or source follower) hence, with a steady DC voltage on the drain (your power supply), there can be no problematic feedback via the miller capacitor to the gate.
  • Any issues with the stability (from to a step change in demand) are due to the 50 k&ohm; gate resistor and the gate-source capacitance. You might say "hey, it's a source follower so gate-source capacitance doesn't come into play" and, that would be a fairly valid point should the MOSFET source follower have near unity voltage gain (like a BJT). But, it doesn't so, about 50% of the gate-source capacitance can be modelled as sitting between gate and 0 volts.
  • This produces a sizable extra chunk of phase lag that approaches 90&deg; in the feedback loop and takes you close to instability. In fact, many of these types of circuit are so unstable that local feedback around the op-amp are needed to stabilize them.
  • Then you should ask yourself, do you really need a very fast response in load current from a demand change and, if not, then put an RC filter between demand input and non-inverting input of the op-amp.
  • Regarding the variation in overshoot with supply voltage, this might be because the drain-source capacitance increases as drain-source voltage decreases. That capacitance can be regarded as being in parallel with the load hence, stability changes as supply voltage changes.
#2: Post edited by user avatar Andy aka‭ · 2024-03-10T10:01:29Z (9 months ago)
  • It's got nothing to do with the MOSFET's miller capacitance. Miller capacitance causes problems in common-source circuits but, your circuit is common-drain (or source follower) hence, with a steady DC voltage on the drain (your power supply), there can be no feedback via the miller capacitor.
  • Any issues with the stability (from to a step change in demand) are due to the 50 k&ohm; gate resistor and the gate-source capacitance. You might say hey, it's a source follower so gate-source capacitance doesn't come into play and that would be a fairly valid point should the MOSFET have near unity voltage gain (like a BJT). But, it doesn't so, about 50% of the gate-source capacitance can be modelled as sitting between gate and 0 volts.
  • This produce a decent phase lag that approaches 90&deg; in the feedback loop and takes you close to instability. In fact, many of these types of circuit are so unstable that local feedback around the op-amp are needed to stabilize them.
  • Then you should ask yourself, do you need a very fast response in load current form a demand change and, if not, then put an RC filter between demand input and non-inverting input of the op-amp.
  • It's got nothing to do with the MOSFET's miller capacitance. Miller capacitance causes problems in common-source circuits but, your circuit is common-drain (or source follower) hence, with a steady DC voltage on the drain (your power supply), there can be no problematic feedback via the miller capacitor to the gate.
  • Any issues with the stability (from to a step change in demand) are due to the 50 k&ohm; gate resistor and the gate-source capacitance. You might say "hey, it's a source follower so gate-source capacitance doesn't come into play" and, that would be a fairly valid point should the MOSFET source follower have near unity voltage gain (like a BJT). But, it doesn't so, about 50% of the gate-source capacitance can be modelled as sitting between gate and 0 volts.
  • This produce a decent phase lag that approaches 90&deg; in the feedback loop and takes you close to instability. In fact, many of these types of circuit are so unstable that local feedback around the op-amp are needed to stabilize them.
  • Then you should ask yourself, do you really need a very fast response in load current from a demand change and, if not, then put an RC filter between demand input and non-inverting input of the op-amp.
#1: Initial revision by user avatar Andy aka‭ · 2024-03-10T09:58:58Z (9 months ago)
It's got nothing to do with the MOSFET's miller capacitance. Miller capacitance causes problems in common-source circuits but, your circuit is common-drain (or source follower) hence, with a steady DC voltage on the drain (your power supply), there can be no feedback via the miller capacitor.

Any issues with the stability (from to a step change in demand) are due to the 50 k&ohm; gate resistor and the gate-source capacitance. You might say hey, it's a source follower so gate-source capacitance doesn't come into play and that would be a fairly valid point should the MOSFET have near unity voltage gain (like a BJT). But, it doesn't so, about 50% of the gate-source capacitance can be modelled as sitting between gate and 0 volts.

This produce a decent phase lag that approaches 90&deg; in the feedback loop and takes you close to instability. In fact, many of these types of circuit are so unstable that local feedback around the op-amp are needed to stabilize them.

Then you should ask yourself, do you need a very fast response in load current form a demand change and, if not, then put an RC filter between demand input and non-inverting input of the op-amp.