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Old 17th May 2013, 10:57 AM   #1
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Default Opamp power bypass

I realise this is a basic topic and has been covered many times, but I have not been able to find a definitive answer.

Many opamp datasheets calls for a small capacitor at the pin of the opamp in addition to the larger capacitors at the power supply, and apparently so does many experienced designer.

However, when I simulate such arrangement on something like PSUD2, the result will always be horrible. And I am aware of the discussion on detrimental effect of adding a film capacitor across large electrolytic capacitor on power supply decoupling, especially when the distance of the capacitors are far apart (inductance).

My question is, am I confusing these separate topics together?

The reason I am asking is, recently I built a simple single stage non inverting opamp preamp. I have the the power supply capacitor (2x47F) after the regulator mounted close to the opamp, at about 1 cm away. The I think it is oscillating (it hisses quite badly). My guess is it has to do with the power supply decoupling, otherwise I am quite stumped on how can something so simple go so wrong.

The opamp used is OPA2132, which has a reputation on being well behaved. But I ended up swapping it for LT1358 as it seems to be a tiny bit more stable.
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Old 17th May 2013, 11:19 AM   #2
AndrewT is offline AndrewT  Scotland
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psud2 is for power supply, at the source end.

You are referring to decoupling at the receiver end. Quite different.
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Old 17th May 2013, 11:31 AM   #3
Mooly is offline Mooly  United Kingdom
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Perhaps post a circuit of the preamp. You shouldn't hear any obvious "hiss" with any line stage op amp "gain block" (I'm assuming it is line level and not a phono stage)
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Old 17th May 2013, 12:10 PM   #4
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Quote:
Originally Posted by AndrewT View Post
psud2 is for power supply, at the source end.

You are referring to decoupling at the receiver end. Quite different.
Please enlighten me. Say if I have a 100F capacitor on the output of a power supply, and I put a 0.1F capacitor across it. I suppose it is a bad idea. But if I put this capacitor further down the line at the pin of the opamp, how is it different? Wouldn't it be worse off because of the longer distance?

But btw, I tied a 47nF film cap on the pin (not soldered), it motorboats like crazy.

Quote:
Originally Posted by Mooly View Post
Perhaps post a circuit of the preamp. You shouldn't hear any obvious "hiss" with any line stage op amp "gain block" (I'm assuming it is line level and not a phono stage)
It is a line level.

Say this basic non inverting arrangement:

Click the image to open in full size.

My components value are as follows:

C1 - nothing
R1 - 590kΩ (before it it has a 50kΩ potentiometer)
R2 - 1.4kΩ
R3 - 18kΩ
R4 - 10kΩ
R5 - nothing (I tried 22Ω with same result)

The resistor values are that way because those are what I happen to have on hand.
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Old 17th May 2013, 12:20 PM   #5
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From the datasheets, they are usually quite specific in saying that the 0.1uF bypass capacitors should be as close to the power supply pins of the opamp as possible.
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Old 17th May 2013, 01:13 PM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by Navyblue
But if I put this capacitor further down the line at the pin of the opamp, how is it different? Wouldn't it be worse off because of the longer distance?
It is different because of the distance, which adds resistance and inductance. This lowers the frequency and Q of any resonance.

People often speak of 'bypass' when they add an extra cap. Remember, the big cap does not need a bypass - but the circuit might need more decoupling (especially at HF). Think 'decouple', not 'bypass'. The aim is to keep HF circuit loops as small as possible, not to 'improve' the big cap.
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Old 17th May 2013, 03:46 PM   #7
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Originally Posted by DF96 View Post
It is different because of the distance, which adds resistance and inductance. This lowers the frequency and Q of any resonance.

People often speak of 'bypass' when they add an extra cap. Remember, the big cap does not need a bypass - but the circuit might need more decoupling (especially at HF). Think 'decouple', not 'bypass'. The aim is to keep HF circuit loops as small as possible, not to 'improve' the big cap.
Which reminds me of this.

Click the image to open in full size.

I suppose this is a bad idea? I tried simulating this and it's really bad. But people seem to swear that this is an improvement. I have a chip amp project down the pipeline but that is another topic. I have read the arguments I am leaning towards my simulation result, and Nelson Pass' PSU tend to have no such thing, and he seems to know what he is doing.

So what you are saying is, if I mount the 100nF cap on the PSU board, it will ring badly. But if I mount it on the amp board, it won't ring? While resistance lowers Q, won't inductance increases Q?

Last edited by Navyblue; 17th May 2013 at 03:49 PM.
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Old 17th May 2013, 04:34 PM   #8
Mooly is offline Mooly  United Kingdom
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The circuit in post 4 is fine. R2L can be shorted, it does nothing. For decoupling the opamp a small electroylitic (say 4.7 or 10uf 50v) soldered on the board directly across pins 4 and 8 should work well.

There should be no audible hiss from that circuit.

"But btw, I tied a 47nF film cap on the pin (not soldered), it motorboats like crazy

Not sure what you mean by that but touching either pin 4 or pin 8 should have zero audible effect.
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Old 17th May 2013, 04:46 PM   #9
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Originally Posted by Mooly View Post
Not sure what you mean by that but touching either pin 4 or pin 8 should have zero audible effect.
I have a a 47nF cap across pin 4 and ground, and another across pin 8 and ground. It certainly worsens it. And if it matters, there were 2x47F PSU cap 1 cm away from it.
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Old 17th May 2013, 04:58 PM   #10
Mooly is offline Mooly  United Kingdom
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It sounds like you have something amiss somewhere if its behaving like this. If there is any doubt over stability due to rail decoupling then a single cap across 4 and 8 stops it dead.

The only things I can think of are,

1) If adding caps from supply to ground causes something odd to happen then look carefully at the layout and wiring scheme. In theory, if there is noise on a rail, then adding a cap from that rail to ground injects that noise into the ground. If ground were truly a point of "zero" impedance and the same all over that wouldn't matter, but its not.

2) If the opamp is oscillating, then again this probably comes down to layout and wiring/grounding. You could try adding a 22pf to 100pf cap across R3L to see if that stops the "hiss". An OPA2134 should be rock steady in the circuit you have drawn.

3) If you still get nowhere then try disconnecting the supplies and try it on two 9 volt batteries.
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