| 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?? |
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| PMA |
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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| edl |
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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| janneman |
| 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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| darkfenriz |
| do you use cascodes? |
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| darkfenriz |
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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| Gofer |
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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| Duo |
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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| amplifierguru |
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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| Duo |
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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| phase_accurate |
Some designs are picky when it comes to the position of the feedback takeoff and speaker connection. And yes the grounding scheme is important as well.
Neither part of the NFB path (signal and ground) should ideally carry output current.
Regards
Charles |
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| Duo |
| Hm, this is interesting. You say the feedback path shouldn't carry any current?? Well, the current flows from the transistors at the output and the feedback must be taken off somewhere around that point. No matter where it's taken off, current must flow through a part of the feedback loop, be it a very small portion. |
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| phase_accurate |
| quote: | | be it a very small portion. |
This one is O.K. but problems can arise if feedback current and output current share the same path !
Regards
Charles |
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| Mr Evil |
| quote: | Originally posted by Duo
Hm, this is interesting. You say the feedback path shouldn't carry any current?? Well, the current flows from the transistors at the output and the feedback must be taken off somewhere around that point. No matter where it's taken off, current must flow through a part of the feedback loop, be it a very small portion. |
Obviously current must flow through the feedback loop as a whole, but I think what phase_accurate was trying to say is that feedback should be taken at the output of the amp, not with any intervening track. If feedback is taken from the wrong point, then the amp will by trying to control the wrong voltage. See D. Self's measurements of increased THD when not taking feedback from exactly the centre of a push-pull output stage. |
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| Duo |
What I'm saying is that feedback current and output current must share the same path because they are connected to the same output.
Certainly, I understand that placement of these points is critical; however, I can't see any way that the feedback current and output current don't share the same path at a point. |
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| sam9 |
What Self was saying is that the trace leading to the the feedback resistor should not originate directly at one the Re resistor leads (Rc if CFB topology). It should originate a some point after the P and N (top and bottom) signals from the output devices have clearly merged.
The trap is that we get used to the schematic (and SPICE) representation of circuits where conditions at all points in a node are indentical. Most of the time that's essentially valid in the real world. Sometimes in the real physical world the "node" is an oddly shaped piece of copper with non-zero resistance and the above generalization is no longer a sufficiently accurate descriptor.
A very analogus caveat is the one never to locate your star point right at the PS filter caps where the ground currents are merging. |
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| destroyer X |
And despite good frequency response in 1 Megahertz can produce better wave shape around 20 Kilohertz.... transistors used that can work in frequencies around 150 Mhz are really more problems than solution...as amplification is oscilation, audio is oscilation...and a transistor that can go up....sometimes goes!
Last year i made some experience with 220 Mhz Radio frequency transistores.... but could not have good 20 Kilohertz wave shape...because had to control using a lot of capacitors.... always going to oscilate and overheat around 440 Megahertz.
Hehe....modern age, modern components will produce better sound, and sometimes more hard problems to fix.
regards,
Carlos |
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| Miles Prower |
100MHz? That's pretty impressive. :D I built an audio amp that did a real good imitation of an 80m (3.54MHz) QRP rig (into an 8 ohm speaker, no less! Hardly an RF device). I had to take the whole circuit apart, and rebuild. (Big tip-off: this amp cliped at a very low volume. Which it would with all that RF superimposed on the audio. You could tune it in loud and clear on an SW receiver. Scoping the output showed nice sinewaves at 3.54MHz.)
So, I treat any audio project as if I were building for RF. That means baffle shields between sections, damper resistors at bases or gates, either tantalum electrolytics, or ceramic monolithics across aluminum electrolytics. And, finally, keeping leads as short and direct as possible.
Since then, I haven't had any RF oscillation problems. |
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| Duo |
Another interesting point to state is that I'm not using transistors that are designed to work so fast..
I believe these have a transition frequency around 20MHz.
Something leads me to believe that there's a fishy problem somewhere in the circuit. |
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| peranders |
| quote: | Originally posted by janneman
Do you see it also when you clip the scope probe to the same point as the probe gnd lead? |
I think Mr. Didden is trying to say that you may test this to rule out that you are picking up FM radio. What happens if you turn of the power? |
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| anli |
I think, Jan Didden is the most close to the reason of oscillations: probaly half of all different scopes I have worked with oscillate with shorted probe and probe gnd when max (and near) Y scope sensitivity is selected. Such oscillation takes place wile measurements too.
I don't insist on I have worked with the best scopes, but it is a reality of my life :-). |
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| Duo |
| Yes, I have observed that the oscillation went away when the amplifier power was disconnected. (I suppose there is still the possibility that it is picking something up. |
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| jleaman |
| Maybe build up a small pcb board and see if the problem goes away.. |
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| Duo |
| I'm pretty sure the scope isn't the problem. It's a high end scope and seems to be immune to a lot of the problems that most scopes I've worked with have. |
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| jleaman |
| quote: | Originally posted by Duo
I'm pretty sure the scope isn't the problem. It's a high end scope and seems to be immune to a lot of the problems that most scopes I've worked with have. |
What kinda scope is it. ? what is the frequancy ? Could it be your probe's ? |
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| Duo |
The scope is a Tektronix 2247A with original probes that came with it.
I think I'll try it later with some of my other scopes; though I'm far more likely to believe it's the breadboard and not a scope problem. |
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| amplifierguru |
You could contact the 'chief' amplifier engineer at Madrigal - he had a similar problem when I landed the revelation on him that LAYOUT MATTERED, after I advised a reroute of a Vas output PCB track away from the input diff'l pair on his rehash (of someone else's competent design) project amp. Immediately the THD dropped 2 orders of magnitude - to which he replied "Does layout matter?" . I asked "Who did the PCB layout?". "The draftsman!" "What criteria?" "Fit!?" He then asked how to do it. I explained that you need to identify all the sensitive low level areas/tracks and all the high current and voltage radiating tracks/blocks and keep them apart. "Can you identify them for me?". "Sure", I replied. I marked out the various sections of his 12" sq board and he hacksawed it up, connecting sections with 3-4" jumpers, fired it up while he moved the pieces around in 3D space!
Then it went up in flames and the engineering manager came racing out with a fire extinguisher and nuked it!
Seriously! You had to be there!
The *******s picked my brains for 4 months then didn't pay me! |
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| Duo |
Yes, I do understand that layout is of utmost importance in amplifiers. As I said, my PC boards work very very well, never any stability issues.
I tried this amplifier on the breadboard again at home today (I did the other one at the highschool) and this time it works perfectly as expected.
The only thing I did differently this time was spread the various stages of the amplifier farther apart. I also used a start ground instead of the rail in the breadboard. I found that 68pF was enough for the stabilization capacitor and the amplifier works well.
I still haven't determined exactly what the deal was with the 100MHz oscillation with the one at the school, but I intend to figure out the problem before I re-do the layout anyway. It would be interesting to know. |
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| amplifierguru |
| So your 100MHz is gone? |
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| Eva |
100Mhz is too high a frequency for a global feedback loop oscillation. It must be local oscillation across one or two transistors and breadboard capacitance is the most probable cause
As you have found, this is a layout issue |
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| Duo |
| Yes, that is what I figured it was. |
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