Op amp theory Q's

[audioguru2]

Hi TG,
There is your problem. You were using an OPA637 opamp that has full output at the top of the AM radio band! In order for it to go so high, its design causes it to oscillate with a gain of less than 5. Its high frequency response is so good (?) that in a non-inverting situation the pcb's wiring capacitance causes positive feedback making it unstable at gains much higher than 5.

The NE5532 and the TL072 have the same full output bandwidth of 100kHz.
Since the 5532 uses bipolar input transistors, it has a high input current but has low noise with a very low input impedance source like a phono cartridge (remember them?). It has a fairly high idle current and can drive a 600 ohm load well.
The TL072 has FET imputs with extremely low input current, so has low noise only with a high impedance source. It has a low idle current and doesn't drive 600 ohms. 

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[indulis]

I'm confused... Right off the bat I'll say audio is not my thing, but I am very familiar with things in the analog world. In a "first order" DC analysis of the circuit, R3, R4, R11 and R12 do nothing, as one input to all the op-amps is hard tied to ground, and the other is at a "virtual ground" of sorts. Aside from the input offset voltage, there is no voltage potential across any of those resistors, and the additional current the op-amp would have to source or sink due to these resistors is a few orders of magnitude smaller than the normal current flowing. So, my question is this, what exactly do they do from an audio perspective??? What is the math behind the phenomena, as it doesn't figure into the DC gain equation?? As a general observation, in the analog instrumentation world, none of those op-amp inputs would be tied directly to ground. In that particular configuration, they would be connected to ground thru a resistor that is equal in value to the parallel equivalent of all the resistors connected to the op-amp minus input node.

 

[audioguru2]

Hi Indulis,
Those resistors from the inverting input to ground are used to make unstable opamps more stable, by adding "noise gain". I use stable opamps so have never used them.

Opamps with ridiculously high operating frequencies are made with inadequate compensation capacitors, so they oscillate if their closed loop gain is less than 5, and/or have a capacitive load. The addition of noise gain allows the opamp to operate with low closed loop gain but adds noise. Usually a capacitor is added in series with the noise gain resistor so that the input offset voltage isn't amplified.
Here is an applications note from National Semi that explains it pretty well:
http://www.national.com/an/LB/LB-42.pdf

 

[tadgesualdo]

With regard to R3, R4, R11, and R12.

The value you want depends on the other values you are using in the loop.

 

[audioguru2]

some op amps are on the verge of oscillation and will exibit some problems

That's why I use ordinary audio opamps, then I don't worry about instability and don't need to use RF construction methods. Without adding noise gain, then my circuits have the low distortion provided by full negative feedback and are quiet.

I think using an opamp for audio that has full output up to 1.7MHz is ridiculous. 

 

[tadgesualdo]

Audioguru,

Why don't you tell us how you really feel, eh?

 

[ante1]

Hi TG,

Here is a book on the subject unless you’ve already got it : http://www.electronics-lab.com/forum/index.php?topic=5379.56

 

[audioguru2]

Hi TG,
The OPA2134 is an excellent audio opamp but isn't much better than a TL072.
Its power bandwidth is 230kHz, not 8MHz. The power bandwidth of a TL072 is 100kHz.
An opamp's output is slew-rate-limited above its max power bandwidth but still has gain up to its gain-bandwidth-product (GBP), which for the OPA2134 is 8MHz to 10MHz. The GBP of a TL072 is 3MHz. At the GBP the internal gain of an opamp is unity.

Like all normally compensated opamps that work at a gain of unity without oscillating, its internal frequency response is reduced above only 10Hz.
It has a fantastic gain of 2 million below 10Hz but at 1kHz its gain is only (!) 10,000. Therefore with negative feedback its distortion emerges from its noise above 1kHz.

The distortion of all opamps increases with a higher output level, with a higher amount of closed-loop gain and with a higher amount of loading.

Look at what this expert has to say:
http://www.dself.dsl.pipex.com/ampins/ampins.htm
I have Ante's book but haven't opened it yet.
I frequent DIYaudio.com but some audiophiles there don't know what they are talking about, describing audio like it is visual. Most don't know the difference between a resistor and a capacitor. 

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[audioguru2]

Hi again, TG.
I forgot. Your addition of noise-gain increases the level of noise and distortion so it becomes audible:

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[tadgesualdo]

How do I open Ante's book, what program?

TG

 

[wvengineer]

It's a rar file which is a compressed folder so I would normally use WinRar to extract the file/files within which is probably in pdf format.

 

[ante1]

Hi TG,

You can find Winrar here: http://www.rarlab.com/download.htm

 

[tadgesualdo]

I found one thanks, I have been reading, this is what I needed, but to be quite honest you do need a little bit of background to grasp all of it.  It would be nice to for things to be written in a bit more common language, I miss some of the detail stuff.

TG

 

[ante1]

OK TG, how about this one then?



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[tadgesualdo]

Hi all,

Ok I have another question regarding op amp theory.  What is the best way to make a op amp buffer?  Most times you see two 10K resistors for this with a non-inverting input, but would not just a short between the output and the negative input work the same?  What is the difference?

Now with inverting inputs this seems to get more complex, because doesn't the input inpedance of the opamp have an effect on the feedback circuit, and needs to be taken into account?

Thanks,
TG