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Learn the Basic Knowledge of Operational Amplifier (6)

1. Why do the amplifiers circut made of operational amplifiers generally use the inverting input?
(1) The major difference between inverting input method and the in-phase input method is:
Inverting input method, because in the in-phase termination a balanced resistance to the ground, and there is no current on the resistance (because of operational amplifier input resistance is great), so the in-phase end is approximately equal to potential, known as the "virtual ground", and on the inverting side with the in-phase potential is extremely close to it, so on the inverting side it is "virtual ground". The advantage of virtual land is that there is no common mode input signal, even if the common mode suppression ratio of this operational amplifier is not high, and there is no common mode output. And in-phase input connection, there is no "virtual ground", when using a single-ended input signal, will generate common-mode input signal, even with high common mode rejection ratio of operational amplifier, also will have a common mode of the output. Therefore, in general use, the inverting phase input method will be adopted.

(2) The positive phase is the oscillator, which can stabilize the amplifier and access negative feedback.

(3) In principle, it is possible to scale the circuit in the same phase.

However, when the actual application is amplified (i.e., differential signal), it is usually very small, and the noise suppression (usually expressed as a common mode signal) should be taken into account.
While the same phase proportional amplification circuit has poor suppression ability of the common mode signal, and the signal that needs to be amplified is drowned in the noise, which is not conducive to the post-processing. Therefore, the anti-phase proportional amplification circuit with better suppression capacity is generally preferred.

2. The magnification of the ideal integrated operational amplifier is how much input impedance is and what is the voltage between the same phase input and the inverse input terminal?
(1) the magnification is infinite, the input impedance is infinitesimal, and the voltage between the in-phase input and the reverting input is almost the same.
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