Analog circuit classic knowledge sharing op amp application skills

The operational amplifier is the core of the analog circuit. It is necessary to master the analog circuit and skillfully apply it to the design. It is necessary to master the use of the operational amplifier. At the same time, it is very important to grasp the parameters of the operational amplifier.

In general, the operation in the analysis remembers two characteristics, one is virtual short and one is virtual, these two concepts are the premise of the core of the various application circuits. Virtual short means that the voltage between the two inputs of the op amp is approximately short-circuited and the voltages are equal. A virtual break means that the current path between the two inputs of the op amp is open and no current flows between the inputs. With these two concepts, you can analyze all forms of op amp circuits. However, it is worth noting that the two concepts are established on the condition that the op amp needs to form reasonable negative feedback, otherwise it will not be established. The basics of op amps are not mentioned here. In an accidental design, I encountered a problem of insufficient power supply voltage. At that time, the power supply voltage of the op amp was positive and negative 15V, and I need to output 0-25V voltage. If I use the normal op amp negative feedback, at most Output 0-15V range, but I think that for -15V voltage, can not output 0-30V voltage?

If you have the idea, you will act immediately and continue to think. In fact, the key to the problem is not whether the op amp can output a lot of voltage range, but the supply voltage of -15V itself is also jittery and not stable enough, because the -15V power supply voltage is not Strict -15V, so the problem has arisen, I can not directly superimpose the 0-15V voltage of the op amp output to -15V, but must also introduce the -15V voltage into the negative feedback loop, so a problem is formed, the problem The focus is on how to construct a reasonable negative feedback loop. Finally, the circuit shown below is formed:

In the above figure, U5 is the main op amp. U6 and U7 form a negative feedback to form a loop, and finally U5 outputs to -VCC. R10 is the matching resistor and can be ignored during the whole analysis. U7 forms a 1:1 inverting amplifier. For U7, its output is:

In the formula, the parameter R36=R33=100K, R31=190K, and R32=10K are obtained: U7out=-Uout/20.

U6 constitutes a 1:1 inverting adder. The sum of the two voltages can be obtained from R34=R37 to the inverting input of U6:

Since it is the output of -Vcc, URL:Uout-(-Vcc)=20Uin. Thus a larger range of outputs can be obtained, in which case Uin must be >0, because the output is not likely to be negative, the output range of the entire circuit is changed from the previous -VCC to +VCC to 0 to 2 times VCC, and the whole process has Negative feedback stabilizes the output.

The above design finally meets the requirements. The key is to introduce -VCC into the feedback loop so that the output is for -VCC, not for GND, thus increasing the output range. In the above design, it should be noted that the value of the resistors that constitute the op amp proportional operation should be appropriately larger, otherwise it may cause a larger offset, resulting in inaccurate amplification ratio and asymmetry of parameters. In fact, the difficulty of op amp circuit design is the design of the feedback loop, how to introduce feedback, how to generate feedback voltage, are very worth considering in the design.