The output MOSFET could be slightly damaged to cause that behaviour. I remember having the exact same symptoms, a long time ago, on few occasions, when I worked for Jands electronics (PA amplifier of some 2KW per channel, bipolar and/or MOSFET's). I also recall that you played a lot with one pair, two pairs... of output MOSFETs, so you did a fair bit of touching-the-legs activity.
If the oscillations go down with 680ohm gate resistors, then replace all MOSFET's and try again. If the oscillations disappear, go back to the original value of gate resistors.
If the oscillations go down with 680ohm gate resistors, then replace all MOSFET's and try again. If the oscillations disappear, go back to the original value of gate resistors.
Hello all,
with original values of 221R gate resistor and 680pF capacitor amplifier goes into oscillation. It is nicely visible on square wave, where is heavy overshooting and ringing.
Increasing gate resistor to 680R reduces overshooting as well as ringing, but it is still there and causing oscillation. If I increase 680pF capacitor, I have nice square wave. Due to increase in gate resistor, I probably do not need to increase the value of capacitor too much. Im still looking for right value, maybe 1nF would be enough. Waiting for ordered components to test.
And yes, the mosfets are not best quality. They are heavily used IRFP244 get from some old device, as obtain NOS 244 is almost mission impossible. I can change it for 240 but first I wanted to hear the original design
So seems, this is the way to fix my oscillation. Thank you all for help, really appreciate.
with original values of 221R gate resistor and 680pF capacitor amplifier goes into oscillation. It is nicely visible on square wave, where is heavy overshooting and ringing.
Increasing gate resistor to 680R reduces overshooting as well as ringing, but it is still there and causing oscillation. If I increase 680pF capacitor, I have nice square wave. Due to increase in gate resistor, I probably do not need to increase the value of capacitor too much. Im still looking for right value, maybe 1nF would be enough. Waiting for ordered components to test.
And yes, the mosfets are not best quality. They are heavily used IRFP244 get from some old device, as obtain NOS 244 is almost mission impossible. I can change it for 240 but first I wanted to hear the original design
So seems, this is the way to fix my oscillation. Thank you all for help, really appreciate.
different reason, most probably ..... edit: certainly
servo circuit optimized for greater number of outputs , so with just one pair in situ , servo having excessive gain for tiny xconductance of just one pair ; when all outputs are in place, servo gain and OS gain are in equilibrium
easy to conclude after, when you did that test
servo circuit optimized for greater number of outputs , so with just one pair in situ , servo having excessive gain for tiny xconductance of just one pair ; when all outputs are in place, servo gain and OS gain are in equilibrium
easy to conclude after, when you did that test
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.