School has probably taught most of us that an op amp is a black box with a set of ideal and non-ideal characteristics. We often easily derive the output equations using ideal assumptions, which includes infinite open loop gain, zero input offset, zero input bias currents, and zero output impedance, etc. But in order to fully appreciate its capabilities and limitations, it helps to know why they came about in the first place. And it is primarily because its use with negative feedback.
It always helps me to think that op-amp is basically a device that takes a differential input and amplifies it with its large open loop gain. One can't go wrong with the inputs that are always labeled with positive and negative. A small differential input would drive the output large positive or negative depending on the polarity of the differential input.
Negative feedback is what makes the op amp very useful in applications such as amplifiers and signal conditioning circuits. The following explanation will refer to the input terminals of the op amp as positive and negative input.
When used with negative feedback, the op amp tries to provide the necessary output needed to force the negative input to follow the positive input. This forms the basis of the ideal assumption that the inputs are equal, i.e. has zero potential between them. [1] To further illustrate of what actually goes on in an op amp feedback circuit, consider when an input is first applied. When the positive terminal sees a voltage higher than that of the negative terminal, because the open loop gain is large, the output will head toward the positive rail and will stay there if the loop is open, i.e., if there is no feedback. The feedback signal to the negative input will cause the output to head the opposite way as it becomes higher than the potential at the positive input, opposing the initial output reaction.
This feedback action will eventually make the output settle to a value such that the voltage at (-) input terminal is equal to the (+) input voltage. Actually, a small error appears between the input terminals, because the output is equal to the differential input voltage, V(+) minus V(-), multiplied by the open loop gain (A) of the op-amp. The differential error voltage is usually a very small value since the open loop gain is typically very large in the thousands. This is where the term virtual short comes from. In the non-inverting circuit above, the output is a larger scaled version of the positive input, where as already mentioned the input voltage is equal to the voltage at the negative terminal node, with closed loop gain of 1 + (Rf/Rin) ignoring the small error due to the open loop gain. The overall closed loop gain of the circuit now becomes largely determined by the feedback resistors as a consequence, and is insensitive to potential large variations of the open loop gain!
These devices however are also useful with positive feedback. Positive feedback generally finds its use in oscillators and comparators. The use of feedback makes the op-amps very popular in a wide variety applications such as amplification, analog computation circuits, oscillators, active filters, etc.
References:
Analog Circuits World Class Design, Robert A. Pease, Editor, Copyright 2008
It always helps me to think that op-amp is basically a device that takes a differential input and amplifies it with its large open loop gain. One can't go wrong with the inputs that are always labeled with positive and negative. A small differential input would drive the output large positive or negative depending on the polarity of the differential input.
Negative feedback is what makes the op amp very useful in applications such as amplifiers and signal conditioning circuits. The following explanation will refer to the input terminals of the op amp as positive and negative input.
When used with negative feedback, the op amp tries to provide the necessary output needed to force the negative input to follow the positive input. This forms the basis of the ideal assumption that the inputs are equal, i.e. has zero potential between them. [1] To further illustrate of what actually goes on in an op amp feedback circuit, consider when an input is first applied. When the positive terminal sees a voltage higher than that of the negative terminal, because the open loop gain is large, the output will head toward the positive rail and will stay there if the loop is open, i.e., if there is no feedback. The feedback signal to the negative input will cause the output to head the opposite way as it becomes higher than the potential at the positive input, opposing the initial output reaction.
This feedback action will eventually make the output settle to a value such that the voltage at (-) input terminal is equal to the (+) input voltage. Actually, a small error appears between the input terminals, because the output is equal to the differential input voltage, V(+) minus V(-), multiplied by the open loop gain (A) of the op-amp. The differential error voltage is usually a very small value since the open loop gain is typically very large in the thousands. This is where the term virtual short comes from. In the non-inverting circuit above, the output is a larger scaled version of the positive input, where as already mentioned the input voltage is equal to the voltage at the negative terminal node, with closed loop gain of 1 + (Rf/Rin) ignoring the small error due to the open loop gain. The overall closed loop gain of the circuit now becomes largely determined by the feedback resistors as a consequence, and is insensitive to potential large variations of the open loop gain!
These devices however are also useful with positive feedback. Positive feedback generally finds its use in oscillators and comparators. The use of feedback makes the op-amps very popular in a wide variety applications such as amplification, analog computation circuits, oscillators, active filters, etc.
References:
Analog Circuits World Class Design, Robert A. Pease, Editor, Copyright 2008
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