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Old 30th December 2011, 06:33 PM   #1
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Default weird DC offset at HF driving passive filter

Hi everybody!

I am having some trouble trying to drive with a 40dB gain stage a passive LP filter feeding another gainstage.

At about 100KHz I can clearly notice that, by increasing the input voltage, although the output of the gainstage has 0 DC offset and is not nearly close to the clipping area, the output at the filter shows a "DC" component.
The signal is all pushed below ground and the more I increase the voltage and the more the signal gests shifted down thus saturating the second stage.

Contrary, if I drive the same filter with my signal generator, as expected, there is no shift at the output of the filter nor at the output of the second stage.

The output of the first stage is capable of driving 10-12mA into the load.

I am wondering if anybody has a clue of what this shift could be caused by and wants to share a thought with me.

Thanks to all for your attention.
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Old 30th December 2011, 07:33 PM   #2
dhaen is offline dhaen  Europe
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Quote:
Originally Posted by Stefanoo View Post
Hi everybody!

..

The output of the first stage is capable of driving 10-12mA into the load.

..

Thanks to all for your attention.
Is the driver stage capable of the same current in each polarity?
Is this an overload/over-current effect?
Is the power supply impedance (local decoupling at the driver stage) poor?
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Old 30th December 2011, 07:40 PM   #3
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interesting points:

1) driving capability is symmetric
2) I am not sure what you meant here
3) power supply has decoupliung by the gain stage and scoping the PSU shows a residual noise under 100uV


I forgot to meantion that if I replace the capacitor with the equivalent impedance (only the resistive value at least) at the specific frequency, the offset disappears.
I think this should sort out any driving or eventual asymmetry problem.

Thank you very much dhaen for your input, if you have any further thoughts please feel free to post them.
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Old 30th December 2011, 07:52 PM   #4
djoffe is offline djoffe  United States
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if you can post a schematic, that might help...
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Old 30th December 2011, 08:12 PM   #5
dhaen is offline dhaen  Europe
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Hi Stefano,

By point 2, I meant that perhaps the driving circuit becomes asymmetric at the point of overload. In particular, even an opamp with symmetrical output topology will probably have an asymmetrical input topology, thus if there is any "funny business" (point 3) on the supply rails, it could create an asymmetrical output.

Replacing the reactive load with a resistive load for testing can be misleading. Look at the many audio amps that measure well into resistive loads!

....I'm only brainstorming
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Old 30th December 2011, 08:14 PM   #6
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here is the schematic first stage with the filter.

If I replace C2 with a proper resitive vale, the DC shifting disappears.
I don't understand why would that happen.
Attached Images
File Type: png first stage.png (14.8 KB, 68 views)
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Old 30th December 2011, 08:17 PM   #7
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Quote:
Originally Posted by dhaen View Post
Hi Stefano,

By point 2, I meant that perhaps the driving circuit becomes asymmetric at the point of overload. In particular, even an opamp with symmetrical output topology will probably have an asymmetrical input topology, thus if there is any "funny business" (point 3) on the supply rails, it could create an asymmetrical output.

Replacing the reactive load with a resistive load for testing can be misleading. Look at the many audio amps that measure well into resistive loads!

....I'm only brainstorming
I see what you are saying.
Nevertheless the POwer supply is really stable and properly decoupled by the gain-stage.
If what you are saying could be the problem:

1) how can I detect the nature of it?
2) wht could be a possible fix?


Thanks.
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Old 30th December 2011, 10:33 PM   #8
djoffe is offline djoffe  United States
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it's probably oscillating, owing to the capacitive load.

if you could add values for r, c, and transistors, that might also help.
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Old 30th December 2011, 11:44 PM   #9
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It's not oscillating.
I did although notice that this behavior starts happening when it gets close to saturation.
My observation is: why wouldn't notice the same thing on resistive load?
How can I prevent this problem assuming is due to asymmetric operation close to sat point? Would matching solve or improve this issue?

In case instead it is sub-oscillation that I am not able to detect, what would be your recommended fix?
I did however try to add a small cap (150pF) across the series feedback resistor and it didn't change at all.
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Old 31st December 2011, 12:26 AM   #10
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As far as values are concerned, the filter has very low capacitive values. C2 is around 15-22nF while C1 33-45nF. The resistor in series around 50kohm which gives a great isolation thus preventing oscillations and resistor in parallel around 10k.
The stage is biased to accommodate max headroom and sweet spot for the fets.
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