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Op-amp basic circuits questions

I've been reading about some basic op-amp circuits. The book has the circuit and then it gives the analysis. The analysis doesn't make much sense to me. For example, it says that when the op-amp is working in it's linear area (sorry if the translation from the book's lanugage is sloppy) the two inputs have the same voltage.

But how do we know that the op-amp is working in its linear area? I'm trying the circuits on SPICE and it seems that if I use a circuit with feedback

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the op-amp will work in its linear area, but if I remove the feedback then it won't. Why?

Should I worry too much about those details or should I just learn the circuits and move on?

Reply to
qwerty
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Yes. This is another way to say that an ideal opamp has a very high differential gain.

Its output is not saturated against either of the supply rails, but is producing a live, controlled (by the feedback network) output signal.

If you remove the feedback, it is almost impossible to keep the input signal small enough and well enough matched to keep the two inputs almost exactly (but not quite) exactly the same. This is what negative feedback does. If the two inputs are not almost exactly the same, the output voltage changes in the direction that makes the two input voltages more alike. When they are almost perfectly matched, there is only enough difference between them (when multiplied by the high differential gain of the amplifier) to produce the output voltage that got them to that state of match.

Reply to
John Popelish

The bit about the inputs being at the same potential is not a property of op-amps. It's a property of op-amp circuits using negative feedback (pretty much all of the ones which work well). Without feedback there is no way for the op-amp to alter the potentials on its inputs. With feedback the output is driven one way or the other (we design the circuit so that it's the right way) till there is zero voltage differential at the inputs.

Tim

Reply to
Tim Auton

On Fri, 25 Aug 2006 18:03:49 +0000 (UTC), in message , qwerty scribed:

John summed it up quite well. The large open loop gain of the device makes even the tiniest difference in potential at the input appear as an enormous swing at the output. That's why op amp design (as differentiated from comparators) uses degenerative feedback - the gain is thus made controllable, and easily determined through mathematical calculation.

That depends entirely on how well you wish to understand the fundamentals of the craft.

Reply to
Alan B

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