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Old 1st June 2005, 06:46 PM   #1
Duo is offline Duo  Canada
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Default Amplifier oscillates at 100MHz

Okay, I'm sure this is not a usual amplifier design problem, but I have an amplifier I built which oscillates at about 100MHz.

The amplifier is a typical three stage with differential amp, current mirror, current sources/sinks, class A VAS, diode bias, darlington output stage, etc.

The only thing I think is causing this is that it's built on a breadboard. At the moment, it's programmed for a gain factor of 23 and it does amplify audio properly.

The oscillation appears on the scope as a relatively clean sine wave and no matter what I do, it doesn't go away. By increasing the stabilization cap, I can make the amplitude smaller, but it is still there.

I've tried all kinds of bypassing techniques on the power supply and various stages. Any other time I've done this, I was able to get rid of oscillation problems. But also, any other time I've done this, I've never had it oscillate at such a high frequency.

Has anyone ever seen this happen before??
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Old 1st June 2005, 07:32 PM   #2
PMA is offline PMA  Europe
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Really 100MHz? (i.e. 10 nanoseconds period).

Well - it may happen. The highest power amp oscillation frequency I had faced was 30MHz.

You might try Zobel at the output terminals (some 33nF in series with 5 Ohms or so).
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Old 1st June 2005, 07:56 PM   #3
edl is offline edl  Hungary
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Few hints, hope one of them will help:

-let's solder 100uF elkos and 100nF folie cap on the rails
-lets increase Miller capacitor (cap in the collector and base of the VAS transistor) to 100pF
-solder 10ohm resistors in series with the output ransistors base-s
-solder 100ohm resistors in series with the driver transistors base-s
-solder a 330ohm resistor and a 330pF cap (in series) between the LTP's collectors
-solder 100nF folie caps paralel with the voltage reference (zener diode or silicon diodes) of the current sources
-solder 100nF folie caps paralel with the bias circut (diodes)
-insert a Zobel to the output, a 10ohm-2W resistor in series with a 100nF folie cap
-bulid the amp in a nice PCB
-etc...
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Old 1st June 2005, 08:11 PM   #4
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Default Re: Amplifier oscillates at 100MHz

Quote:
Originally posted by Duo
Okay, I'm sure this is not a usual amplifier design problem, but I have an amplifier I built which oscillates at about 100MHz.

The amplifier is a typical three stage with differential amp, current mirror, current sources/sinks, class A VAS, diode bias, darlington output stage, etc.

The only thing I think is causing this is that it's built on a breadboard. At the moment, it's programmed for a gain factor of 23 and it does amplify audio properly.

The oscillation appears on the scope as a relatively clean sine wave and no matter what I do, it doesn't go away. By increasing the stabilization cap, I can make the amplitude smaller, but it is still there.

I've tried all kinds of bypassing techniques on the power supply and various stages. Any other time I've done this, I was able to get rid of oscillation problems. But also, any other time I've done this, I've never had it oscillate at such a high frequency.

Has anyone ever seen this happen before??

Do you see it also when you clip the scope probe to the same point as the probe gnd lead?

Jan Didden
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Old 1st June 2005, 09:22 PM   #5
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do you use cascodes?
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Old 1st June 2005, 10:03 PM   #6
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sorry, let's assume I didn't ask, my fault.
this high freq. has nothing to do with cascodes or lack of them.
regards
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Old 1st June 2005, 10:13 PM   #7
Gofer is offline Gofer  Netherlands
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Default oscilation

Once I have silenced an high frequency oscilation by grounding or coupling with a capacitor (100nf) the heatsinks with starground.

maybe it helps.
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Old 2nd June 2005, 03:15 AM   #8
Duo is offline Duo  Canada
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Something leads me to believe that this problem is caused by capacitance in the breadboard.

I have already done most of the fixes found here and none have fixed the problem. They have reduced the amplitude and changed the frequency of oscillation, but it's still there.

I should try some experimentation with layout on there and see if I can improve it by changing the layout.

(I have tried slight adaptations of this circuit with printed circuit boards and it works fine that way.

The only important difference being the length of the wire and the inter-electrode capacitance seen in the breadboard itself.
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Old 2nd June 2005, 04:28 AM   #9
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Duo,

Maybe you need a 10MHz CRO?


seriously,

Breadboard lashups are intrinsically prone to such horrors. Really what you are doing is taking a perfectly good design on a neat, compact PCB and seperating every component by little inductors and some stray Cs and saying "Now work perfectly".

Did I tell you the one about Madrigal and such lashups....

You should bypass this step.
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Old 2nd June 2005, 01:29 PM   #10
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Lol, no I didn't hear about Madrigal, do tell if you wish.

And yes, I know perfectly of the fact that this breadboard idead is hellish when one expects a high gain, high speed amplifier to work properly.

My PCB layouts of amplifiers have always been very stable in operation under many conditions.

I just like the breadboard because it's a fast way to see things in action. The trouble is all of the added problems.

I suppose that still my fastest way to reliably experiment is using point to point soldering of parts. That always works well. In fact, that would lead to less stray L and C than even a PCB, no wonder the P-P jobs never seem to have problems when I try them.
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