Simple analysis of a circuit for collecting the average value of AC voltage

Aguilera

Simple analysis of a circuit for collecting the average value of AC voltage [Copy link]

This circuit feels like I have seen a similar circuit in the analog circuit class. After reading other people's simple explanations, I understand a little bit. Let me talk about my specific understanding. For the sake of simplicity, IN- is directly regarded as GND, and the output of the transformer should be the same-name terminal with the same turns ratio; The front-stage op amp rectifies and takes out the negative value. This circuit is a precision rectifier circuit. I have read many articles explaining precision rectifier circuits, but they did not explain why it is a precision rectification circuit and why the conduction voltage of the diode can be ignored. This is because the amplification factor of the op amp is very large, about 500,000 times, which means that when the input voltage is 0.7V/(5.0*1.0e6), that is, 1.4μV, the diode will be turned on. The circuit waveform at this time is equivalent to the following figure: Simple analysis of a circuit for collecting the average value of AC voltage There is no R' in the figure above. In order to balance the input resistance of the op amp, it is scientific to add R'. The op amp at the latter stage forms a low-pass and summing circuit, and converts the circuit to positive. This type of circuit is still fresh in my memory. I remember that when I was interviewed by Sande, I didn't answer these questions. Later, I made up for it and figured out some circuit calculations of active filters. It is very simple to use the virtual short and virtual break of the op amp to list the transfer function of the circuit. Finally, look at the overall circuit. At this time, when the input voltage Vi<0, the output of the first-stage op amp is equivalent to a short circuit, and the voltage on the right end of R9 is Vi. The output voltage of the second stage is 1 times amplified, that is, Vi. When Vi>0, it is a 2x amplification circuit, but the second stage is a summing circuit, which results in the output voltage still being Vi, thus completing full-wave rectification. That is, precise full-wave rectification.