Regarding the application of operational amplifiers in circuits. The following describes the use of two types of operational amplifiers in circuits.

#### Inverting proportional operational amplifier

According to the characteristics of virtual break, the current flowing through points A and B is 0,

and it can be seen from the figure that the voltage at point B is 0. According to the characteristics of virtual short, the voltage at point A is equal to the voltage at point B, which is equal to 0.

According to Kirchhoff’s current law, the current through R1 is equal to the sum of the current flowing to point A and the current in Rf. Because the current at point A is 0, the current through R1 is equal to the current through Rf.

According to Kirchhoff’s voltage law, the voltage of Uo is equal to Ui minus the voltage on R1, minus the voltage on Rf.

###### Expressed by the formula:

It can be seen from the formula that the output voltage of Uo is equal to the inverse value of the product of the input voltage and RF/R1. By changing the ratio of Rf to R1,

we can get the required amplification factor, but the direction of the output voltage is opposite to the direction of the input voltage.

The resistance of R2 is equal to the value of R1 in parallel with RF, which is called the static balancing resistance.

Inverting proportional operation circuit

#### Non-inverting proportional operational amplifier

According to the characteristics of virtual break, the current flowing through point A and point B is 0.

It can be seen from the figure that the voltage at point B is Ui. According to the virtual short characteristic,

the voltage at point A is equal to the voltage at point B and is equal to Ui. According to Kirchhoff’s current law,

the current through R1 is equal to the sum of the current flowing to point A and the current in Rf. Because the current at point A is 0,

the current through R1 is equal to the current through Rf.

According to Kirchhoff’s voltage law, the voltage at Uo is equal to the voltage at point A minus the voltage at Rf.

###### Expressed by the formula:

It can be seen from the formula that the output voltage of Uo is the input voltage plus the product of (RF/R1) and the input voltage. By changing the ratio of Rf to R1, we can get the required magnification. The resistance of R2 is equal to the value of R1 in parallel with RF:

R2=(R1*RF)/(R1+RF), this value is called static balance resistance. Ensure that when the amplifier circuit is static, the equivalent resistance to the ground of the non-inverting input terminal and the inverting input terminal of the op-amp is equal to reduce the impact of offset current on the circuit operation error.

The in-phase proportional operation circuit

It can be seen from the two circuit diagrams that the in-phase proportional operation circuit and the inverting proportional operation circuit are basically similar. When designing the circuit, you need to pay attention to the direction of the input end. There are many types of application circuits for operational amplifiers. Next time we will introduce the inverting summing circuit and the non-phase summing circuit.