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Old 11th December 2014, 11:08 PM   #1
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Default Anotehr lesson learned in the fight against oscillating amplifiers.

I have designed quite a few mosfet amplifiers.
In that time I have come across oscillation on the output a few times.
So increasing the VAS capacitor or adding a small capacitor in the feedback loop usually got rid of it.
Going from other peoples designs I learned to use gate resistors to slow down the input to the output mosfets.

I recently designed a bipolar amplifier.
Lo and behold it oscillated when I built one up.
The usual VAS capacitor and feedback capacitor trick didn't work.
The amount of oscillation seemed to depend where the bias pot was set.
So I went back to my mosfet design for inspiration and added 10R resistors in series with the bases of the output transistors. The oscillation went away.

Last edited by nigelwright7557; 11th December 2014 at 11:12 PM.
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Old 11th December 2014, 11:23 PM   #2
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If you decide to read Bob Cordell's power amplifier book one day, you'll find that he discusses those resistors at length. Both in BJT amplifiers and in MOSFET amplifiers. Nelson Pass recommends them too for the esoteric and rare power-JFET amplifiers he builds.
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Old 11th December 2014, 11:47 PM   #3
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Found a good article here on the problem.
Elliott Sound Products - Audio Power Amplifier Design Guidelines
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Old 12th December 2014, 02:37 AM   #4
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Here is my bodge to get it working.
When a transistor blew earlier I cut both pnp's out but only one was blown.
Until a new transistor arrives I bodged the old to work.
Click the image to open in full size.

Last edited by nigelwright7557; 12th December 2014 at 02:39 AM.
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Old 12th December 2014, 04:27 AM   #5
jcx is offline jcx  United States
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not an avid reader of Ott, Johnson, anyone on loop area, HF layout?
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Old 12th December 2014, 10:06 AM   #6
AndrewT is offline AndrewT  Scotland
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10r for a base stopper on the output is high.
1r0 is possibly at the useful low limit.
Somewhere in between might be better.
try 2r2 and 4r7 to see if either work sufficiently.

Worth also putting base stoppers on the drivers as well. Typically about 10times the value of the output stoppers. i.e. 10r to 47r.

On the few amplifiers that need neither, or only need one, of these there remains the option to use 0r0 links when the PCB has been designed for stoppers.
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Old 12th December 2014, 08:38 PM   #7
CBS240 is offline CBS240  United States
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Just use old homotaxial transistors with low Ft.
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Old 12th December 2014, 08:45 PM   #8
wg_ski is offline wg_ski  United States
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Then it will oscillate for other reasons.
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Old 12th December 2014, 09:15 PM   #9
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In the end 27R got rid of all the oscillation at any setting of the bias pot.
The output transistors have a gain of 75 so 27R isn't limiting the current too much.
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