Why is this LM158 oscillating?

Outside the FB loop and after the pulldown resistor.

Cheers

Phil Hobbs

Forces the output stage to sort of be class-A.

I'll look further tonight. Maybe it's an instability in the output stage. ...Jim Thompson

More like +2.2

Hide it anyway.

Usually to prevent crossover distortion.

How much capacitive loading do you have on the op-amp? Eg. a long cable, scope probe, or something like that. Try putting 50-100 ohms in series with the output right at the op-amp.

Jim Thompson wrote

Yes, agreed.

Spehro Pefhany wrote

It is about 1.5 metre of 50 ohm coax, going straight into a Tek 2465B scope. The coax OD is 5mm; not sure of the capacitance. I wasn't using a 10x scope probe because it was going into Input #3 which has only two sensitivity settings ;) and I only have 2 scope probes to hand :)

I have now fixed the 1MHz stuff, with a 4k7 to the -15V rail, so can't try other things.

The weird thing is that I have been using the classic LM358N since c.

The other circuits in the data sheet don't show the 6.2k... why not?

The signal source is a HP3314A, 50 ohm source impedance. 100Hz sinewave.

Den tirsdag den 15. oktober 2013 22.44.57 UTC+2 skrev Peter:

from the datasheet: .. For ac applications, where the load is capacitively coupled to the output of the amplifier, a resistor should be used,from the output of the amplifier to ground to increase the class A bias current and prevent crossover distortion. Where the load is directly coupled, as in dc applications, there is no crossover distortion.

Capacitive loads which are applied directly to the output of the amplifier reduce the loop stability margin. Values of 50pF can be accommodated using the worst-case non-inverting unity gain connection. Large closed loop gains or resistive isolation should be used if larger load capacitance must be driven by the amplifier. ..

the figure with the 6.2k pull down is ac coupled, your coax is probably 100pf

nothing really suprising

-Lasse

Lasse Langwadt Christensen wrote

In that case, how does the resistor to the -ve rail fix the problem?

Maybe driving it in "Class A" improves the stability?

This isn't crossover distortion. This is a lack of stability when the

I also tested it with a 1k from the output to GND. That changed the instability but really just shifted it along.

I will certainly test the circuit with varying capacitive loads...

It's not because it's in class A per se.

Op-amps oscillate with capacitive loads because the load shunts the output impedance of the op-amp, and introduces a pole. When the thing is running class B and it's neither pushing nor pulling then the output impedance will be higher than otherwise -- this will make the effect worse.

Putting the resistor to the -ve rail does the same thing as the current load in the schematic -- it tends to bias Q6 on all the time, so that the output impedance is Q6's emitter impedance.

"just shifting the instability along" with your 1k to ground makes sense

-- you're not changing the fact that the output stage goes into class B, you're just changing where it happens.

I'd seriously consider isolating the amp from the load with a resistor, per Spehro and Phil, or at least test the thing to make sure that you've got plenty of stability, don't have a circuit that's just hanging on to sanity by its fingernails, waiting for a temperature change or component value drift to go bonkers.

If you _really_ want to be safe, go the full output-isolated method that's traditional for driving a cap: resistor from op-amp to load, cap from op-amp output to -input, and feedback from the load side of the isolating resistor. For such a wimpy op amp you'd probably need more than 100 ohms for guaranteed stability, but I'm not going to hazard a guess at just how much (I don't do enough circuit design to be able to pull a number out of my ear and have it be right -- I'd actually have to WORK to get a good answer).

Keeps the output impedance low. With no pull-down it's probably oscillating in the dead-band region..

...Jim Thompson

"Peter"

The LM358 is not suited to general audio use and is rarely seen used that way.

It has advantages with single supply rail and battery operation, as it has low idle current.

For general audio, the TL062 is far superior with a similar low drain and ability to work from +/- 3.5 volt rails.

If low drain is not a issue, then go for the TL072 or NE5532.

... Phil

"Lasse Langwadt Christensen"

IF you use a single + supply and loads are not AC coupled then the output stage only sources current.

But if you use split rails, the output stage has to source and sink current so there will be HEAPS of crossover distortion.

.... Phil

=lm158-n&fileType=pdf

The 741 and the 748 had a truly vile output stage incorporating a lateral P NP transistor with a current gain of about 3. You used to be able to get th e 748 output to oscillate by asking it to sink current when close-ish to th e negative rail. Bob Widlar did better in the LM301 and LM307.

The LM158 data sheet shows a PNP in the output stage ...

I suspect that the 100pF capacitative load in the coaxial cable on the outp ut is the guilty party. Twisted pair did that to me once - I got more paran oid thereafter.

I thought you guys were in love with JRC 4558s?

Best regards, Spehro Pefhany

"Spehro Pefhany" "Phil Allison"

... Phil

Although stated as Unity gain stable, judging by 30% overshoot on small signal, it is going to be affected by load impedance and resistor to V- increases the Class B output bias which lowers the impedance. ( read poor stability margin)

ignal, it is going to be affected by load impedance and resistor to V- incr eases the Class B output bias which lowers the impedance. ( read poor stab ility margin)

. Try 20k //22K without the Class B load pull-down resistor.

Also 30pF per foot cable capacitance will eliminate whatever phase margin y ou had and cause oscillation with low gain. Adding a series resistor outsid e the loop fixes that. If you intend to drive it long distances , use imped ance matching divider network, otherwise use FET probe or 10:1 probe with coaxial barrel to 2 pins on board with no ground clip.

Tim Wescott wrote

I can have up to about 100R in series with the output (without re-wiring the feedback) so I will do that, and I will test it with a capacitor box to make sure it is stable for all values.

The temp range is a good point - that's why I used the 158 in the first place...

Here is the "standard" way to isolate OpAmps from a capacitive load...

...Jim Thompson

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Peter, that's what I was describing with words -- Jim's just given you a nice schematic.

The idea behind it is that the R2 isolates the op-amp output somewhat from the load, C2 provides some lead compensation without letting the DC value of the op-amp change the feedback, and R1 provides the necessary DC feedback. You unavoidably reduce the bandwidth of the circuit, and the higher that CLOAD gets the bigger C2 needs to be, which means the lower the bandwidth goes. That's something that can't be avoided*, but stable and slow is usually better than unstable.

• You push the bandwidth up by getting a faster op-amp with a lower output impedance. Faster means lower C2; lower output impedance means lower R2 and R1. When you're shopping, you usually know that the output impedance is lower because the amplifier is specified for higher output current, although occasionally a vendor will actually specify an open- loop output impedance number.