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Old 20th August 2012, 08:16 PM   #21
Calvin is offline Calvin  Germany
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Hi,

Thanks for trying to explain, but I still don't get it.
Most power amplifiers are designed as power operational amplifiers, taking useage of voltage feedback. The definition of an opamp only describes the requirements regarding the number of inputs and their impedance, the number of outputs and their impedance and gain and bandwidth of the "black box". Power level is not defined or restricted to line level.
Its the nature of such a op amp to try to steer its output, so that the inverting input is a mirror of the noninverting input and hence the difference between noninv and inv becomes 0. Both inputs are high impedance points. The inverting input may be regarded as virtual gnd, but it is not low impedance. In a current feedback opamp it would be low impedance though.

jauu
Calvin
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Old 20th August 2012, 11:29 PM   #22
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I think what we are dealing with is a semantic issue.

As already pointed out the name operational amplifier was originally used to perform operations in analogue computers, the classic one uses five discrete transistors.

The two stage differential amplifier/common emitter configuration of the standard discrete op amp, found its way into audio because it is a simple way to convert single ended inputs to double ended two rail outputs and by a.c. coupling the feedback to ground it has 100% d.c. feedback so it has d.c. stability, other direct coupled topologies need a servo to achieve this.

Power amps and later integrated op amps added a complimentary emitter follower to the output, enabling larger output currents and less quiescent current.

So essentially the appellation op amp came about as a historical accident as they were extensively used in military electronics and scientific/ industrial instrumentation to do mathematical operations in analogue computers, that were used in control systems and by scientists. to solve differential equations.

It is true that so called current feedback op amps are not strictly op amps by the formal definition, but neither are audio op amps because they are intended to amplify a.c. signals and not dc ones.
rcw
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Old 21st August 2012, 02:03 AM   #23
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK
Quote:
Originally Posted by Calvin View Post
Hi,

Thanks for trying to explain, but I still don't get it.
Most power amplifiers are designed as power operational amplifiers, taking useage of voltage feedback. The definition of an opamp only describes the requirements regarding the number of inputs and their impedance, the number of outputs and their impedance and gain and bandwidth of the "black box". Power level is not defined or restricted to line level.
Its the nature of such a op amp to try to steer its output, so that the inverting input is a mirror of the noninverting input and hence the difference between noninv and inv becomes 0. Both inputs are high impedance points. The inverting input may be regarded as virtual gnd, but it is not low impedance. In a current feedback opamp it would be low impedance though.

jauu
Calvin

Hi,

The difference between the two inputs is zero therefore the inverting
input is exactly the same voltage as the the output of a unity gain
gain amplifier.

The impedance of the inverting input is the the two feedback
resistors in parallel, in a power amplifier typically 1K and 28K
so just under 1K is very typical.

Op-amps simply don't support the voltage swing of power
amplifiers, and even if they did they cannot provide the
current or power to drive speaker loads.

Op-amps comes from operational amplifiers and analogue
computers based on mathematical operations where the
only issue is accuracy and this doesn't need high power.

Power amplifiers are not remotely op-amps.

Click the image to open in full size.

Above is a high pass Sallen/Key filter with a unity gain op-amp.

I've described how to use it with a power amplifier with gain.
(You move R1's connection to the output to the inverting input,
you should be able to work out why its effectively the same.)

And FWIW assuming an ideal op-amp for the above diagram
the inverting input is zero source impedance as its connected
directly to the output, so your waffling bigtime on details.

rgds, sreten.
__________________
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When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 21st August 2012 at 02:13 AM.
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Old 24th August 2012, 03:56 PM   #24
Davey is offline Davey  United States
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Location: Bremerton, WA.
Sreten,

You've managed to confuse this thread pretty well. However, I believe the OP's original query has been answered.

Anyways, is an LM3886 in typical configuration an "op-amp?"

Cheers,

Dave.
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