Virtual short and virtual break are two important concepts of ideal integrated op amps in analog circuits.
Virtual short means that under ideal conditions, the potentials of the two inputs are equal, as if the two inputs are shorted together, but in fact there is no short circuit, called "virtual short". The necessary condition for the virtual short is that the op amp introduces deep negative feedback.
The virtual cutoff refers to the ideal current flowing into the input of the integrated op amp to zero. This is due to the fact that the input resistance of an ideal op amp is infinite, as if the op amp was open between the two inputs. But in fact, there is no way to open it, called "virtual break."
Application examples:
Figure 1: The grounding of the op amp is 0V, the reverse and the same end are short, so it is also 0V. The input resistance of the inverting input is very high, the virtual disconnection, almost no current injection and outflow, then R1 and R2 Equivalent to being in series, the current flowing through each component in a series circuit is the same, that is, the current flowing through R1 and the current flowing through R2 are the same. The current flowing through R1 is I1 = (Vi - V-) / R1 ... a current flowing through R2 I2 = (V - - Vout) / R2 ... b V - = V + = 0 ... c I1 = I2 ...... d Solve the above equation for junior high school algebra Vout = (-R2/R1)*Vi This is the input and output relationship of the legendary inverting amplifier.
Figure 2: Vi and V-virtual short, then Vi = V- ... a Because of the virtual break, there is no current input and output at the inverting input, the current through R1 and R2 is equal, and the current is I, which is obtained by Ohm's law: I = Vout/(R1+R2) ... b Vi is equal to the partial pressure on R2, ie: Vi = I*R2 ......c is abc-type Vout=Vi*(R1+R2)/R2 This is the legendary The formula for the same direction amplifier.
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