Suppose we have a voltage that we refer to as a input. Suppose we want to multiple this input by a number to produce another voltage that we will refer to as the output. Or, suppose that we have several input voltages, and we wish to add them together, and for the output voltage to represent their sum.

Operations such as these, as well as many far more advanced calculations can be performed with circuits that have what we call an operational amplifier, typically referred to as an "op amp". And in this passage, we will discuss circuits that have an "ideal op amp".

An op amp has two input terminals and one output terminal. It also has two terminals for providing power to the device.

Current can never flow into or out of the input terminals, but it can flow into and out of the output terminal. The current that flows into or out of the output is supplied by the two terminals providing power to the op amp. In most of the cases, the two terminals providing power to the op amp are typically not shown. But it is important to keep in mind that the op amp only has the ability to produce output voltages that are in between the voltages of the two power terminals. One of the two inputs has a plus sigh next to it, and the other input has a minus sign next to it. The op amp takes the voltage value of the "plus" input, and subtracts from it the voltage value of the "minus" input. The op amp then takes this difference between the two input voltages, and multiplies it by a very large number. The op amp then tries to make the output voltage equal to the value of this result. This means that if the "plus" input is even slightly lower that the "minus" input, the op amp will try to make the output voltage equal to the largest negative number it is capable of producing. And if the "plus" input is slightly higher than the "minus" input, the op amp will try to make the output voltage equal to the largest positive number it is capable of producing.(Comparator)

Now suppose that we do something that we call providing negative feedback.

These two upper resistors cause a portion of the output voltage to be added to the value of the op amp's "minus" input terminal. Suppose that the voltage of the op amp's "plus" input terminal is lower that the value of the op amp's "minus" input terminal. This will cause to op amp to want to decrease the voltage value of the output terminal, which will then also end up decreasing the voltage of the op amp's "minus" input terminal. The output voltage will stop decreasing when the op amp's "minus" input terminal is almost exactly equal to the value of the op amp's "plus" input terminal.

Let's consider a new scenario. Suppose that the voltage of the op amp's plus input terminal is higher than the value of the op amp's minus input terminal. This will cause the op amp to increase the voltage value of the output terminal, which will then also end up increasing the voltage of the op amp's minus input terminal. The output voltage will stop increasing when the op amp's minus input terminal is almost exactly equal to the value of the op amp's plus input terminal. The presence of negative feedback always forces the op amp's two input terminals to always be at almost the same voltage value. Once the voltages of the op amp's two input terminals are almost of the same value, the diff between them is a very small number. The output voltage is then this very small number multiplied by a very large number.

(To be continued)


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