The op amp
model is always considered a bit strange. The first is the "virtual short", because of the "virtual short", when the op amp is connected into the same amplifier, the two input potentials are the same, Which is like a common-mode signal, in fact, there are still tiny differential mode signals on the two inputs, but the general instrument can not be measured. However, as a result, " virtual short" artificially increases the common-mode signal at the two inputs, thus it challenges the performance of the op amp. Why do op amps use like this ?
(1). The common-mode signal of non -inverting amplifier in requirements of CMRR a is much larger than the that of inverting amplifier .
(2.) My Opinion on Common Mode Signal Suppression Capability of Both Inverting and Inverting Amplifiers The advantages and disadvantages (db value) of the common mode signal rejection ratio of an op amp mainly depend on the degree of symmetry of the internal (only internal) differential amplifier within the op amp And gain. It is obvious that no op-amp provides its common-mode rejection ratio at the same time as the structural conditions of the external circuit are added. For single-ended input, no matter is in phase or reverse phase, its equivalent common-mode values are enter the value of a half. However, due to the input amplification with the phase is usually larger than the inverting amplifier, the anti-interference ability is less accordingly.
As mentioned above, when making the inverting input, the inverting terminal voltage is almost zero, so the differential pair of tube collector voltage changes only one tube. When making the non -inverting input, the inverting voltage and phase voltage is equal, so the common-mode voltage and input voltage is equivalent! That is to say, the differential pair collector tube voltage has the same amount of change in the same direction except for the two tube portions that change in different directions at the same time. This is the common mode output voltage. It is in-phase with the voltage of one of the tubes. So it easily leads the tube to tend to saturation (or cut off), but fortunately the common mode voltage amplification is only one part of the differential mode magnification.