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Old 24th May 2015, 03:37 AM   #1
rjm is offline rjm  Japan
Richard Murdey
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Default MOSFET amp instability

This circuit (nominally based on the AT-HA5000) is kinda-sorta working, but is highly unstable. Can anyone tell me why, and how best to bring it under control?
Attached Images
File Type: png HA5000.png (32.4 KB, 130 views)
File Type: png AT-HA5000.png (98.0 KB, 135 views)
Attached Files
File Type: asc HA5000 mod.asc (5.2 KB, 12 views)
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Old 24th May 2015, 08:57 AM   #2
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You have no phase margin on you 0dB frequency. You need to roll off gain a bit earlier. There are several ways of doing this. Will show you an example later.
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File Type: png Screenshot 2015-05-24 18.24.57.png (91.9 KB, 118 views)

Last edited by Sergey888; 24th May 2015 at 09:12 AM.
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Old 24th May 2015, 09:12 AM   #3
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There are a few variant. They are not optimized in any way, just a quick one to show the idea.
There are more ways of doing this. Some of them are less practical than those I've shown.
Attached Files
File Type: asc HA5000 mod_3.asc (6.4 KB, 18 views)
File Type: asc HA5000 mod.asc (5.9 KB, 7 views)
File Type: asc HA5000 mod_2.asc (5.9 KB, 10 views)
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Old 24th May 2015, 06:38 PM   #4
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I can't see Sergey's files, but I can see his phase/frequency graph. It confirms my suspicions.

You either need to slow down the Vas stage with a Miller capacitor (or one on each pair of the differential Vas transistors), or else use faster output/output driver devices. A less desirable alternative would be to configure the circuit for higher gain, thus reducing the loop gain.

The most desirable alternative would be to employ faster output and output driver devices.

That's a nice circuit, by the way.
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Old 24th May 2015, 06:51 PM   #5
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Quote:
Originally Posted by rjm View Post
This circuit (nominally based on the AT-HA5000) is kinda-sorta working, but is highly unstable.
Can anyone tell me why, and how best to bring it under control?
Try a series RC network between the two input stage collectors, using a 1k in series with 3.3nF.
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Old 25th May 2015, 12:40 AM   #6
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Thanks for all the suggestions. Let me digest this and get back later.
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Old 25th May 2015, 12:53 AM   #7
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Nobody had a look at HA5000 mod.asc, but it is actually not the original one
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Old 25th May 2015, 01:43 AM   #8
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I was searching for an explanation, and my own explanation came up.

JLH 1969 first amplification stage, need explanation

Quote:
Haphazard placement of poles will sometimes cause them to "stack up" and produce attenuation slopes of 12 dB/octave, which if inside the loop gain will likely cause oscillation or poor stability. This is why conventional designs often employ a "Miller capacitor" or Cdom or dominant capacitor. This method slows the circuit down so all slopes of more than 6 dB/octave occur well below unity gain.

When you mix and match active devices in an existing circuit, sometimes this strategy must be reworked. There are several approaches but in my opinion the best approach is to make the output stage as fast as possible and roll off the Vas stage so there's no gain in the trouble zone. 300 kHz is sufficient for audio
According to Serge's simulation, your circuit is right on the ragged edge of instability. This is why it's unstable, but not an outright oscillator.

Once again, I credit Walt Jung for helping me understand Nyquist stability criteria from a practical perspective.

Last edited by Fast Eddie D; 25th May 2015 at 01:47 AM.
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Old 25th May 2015, 01:49 AM   #9
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C3, 4 at 22p or larger make the circuit stable. The larger they are, the less open loop gain at high frequency.

C2 doesn't do anything unless C3,4 are not present. In that case, anything over 220p seems to make the circuit stable, though the phase plots indicate it only makes a tiny change for the better.

Which is better to implement, C3,4 or C2 or both?

One unrelated question, for the Vbe multiplier R4-R19-Q23 is most of the current supposed to go through the transistor or the resistors? What's the ideal ratio?
Attached Images
File Type: png HA5000.png (65.0 KB, 83 views)
Attached Files
File Type: asc HA5000 mod.asc (5.7 KB, 2 views)
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Last edited by rjm; 25th May 2015 at 01:52 AM.
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Old 25th May 2015, 03:43 AM   #10
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This version seems to be stable and have enough bandwidth with any reasonable combination of gain / feedback resistor combination R20,21.

C3,4 seem to be doing all the heavy lifting though. R16,17, C2 don't seem to have any effect at all.

Circuit notes:

In the original R3,R8 is supposed to be a trim pot to zero the output offset.

R14 is 1k at present which sends 7-8 mA though Q5,Q6. This is probably excessive, but I suppose with 200 mA through the mosfets it doesn't matter.

Q23 would be in thermal contact with the mosfet heatsink, hopefully that's enough to drop the mosfet bias on overtemp. Original circuit also had protection relays on the output.
Attached Images
File Type: png HA5000 stable.png (58.1 KB, 79 views)
Attached Files
File Type: asc HA5000 mod.asc (6.2 KB, 4 views)
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