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# The Basic Knowledge of the Operational Amplifier (3)

1. The input signal is directly added to the same phase input end, and the reverse input terminal is grounded by resistance. Why is U_ =U + =Ui ≠ 0? Isn't that virtual ground?

Besides, forming the "virtual short" has certain conditions. Does the virtual ground also need  certain conditions?

(1) in a same-phase amplifying circuit, the output through feedback makes the U (+) track U (-) automatically, so that U (+) -U (-) will be close to 0. It appears that both ends were short-circuited, so it is called "virtual short".

(2) due to the high input resistance of the virtual short phenomena and operational amplifiers, the current of the two input terminals of the operation amplifier is very small, close to 0, which is called "virtual open". (virtual open is derivative by virtual short, so don't think the two conflict)

(3) the virtual ground is in the inverse phase operational amplifier circuit. (+) end is grounding, and (-) terminal is connected to the input and feedback network. Because of the existence of virtual short , U(-) and U(+) [potential equals 0] are very close, the (-) end is called false grounding -- "virtual ground".

(4) about the condition: "virtual short" is an important feature of the in-phase amplifier circuit in closed loop working state, and "virtual ground" is an important feature of the reverse phase amplification circuit in closed loop working state. Pay attention to understanding the conditions for "virtual short" (such as "nearly equal").

2. What is the amplification principle of the operational amplifier?

The core of the operational amplifier is a differential amplifier.
It is the two triodes back to back connected to share the current of a constant current source. A triode is the forward input of the operational amplifier, and the other is the reverse input. The positive input triode is amplified and sent to a power amplifier circuit to amplify the output. In this way, if the voltage of the forward input end rises, the output will naturally become larger. If the reverse phase input end voltage increases, because a constant current source is shared by the reverse phase triode and the forward triode, the current of the reverse triode is large, and the forward one will be small, so the output will be reduced. Therefore it's called reverse input.
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