Reading Notes on the Good Book "Operational Amplifier Parameter Analysis and LTspice Application Simulation" 02 DC Bias

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Reading Notes on the Good Book "Operational Amplifier Parameter Analysis and LTspice Application Simulation" 02 DC Bias [Copy link]

I made this mistake about DC bias when I just started working, and I remember it very clearly. When we were studying in school, the textbooks ignored these details. The textbooks were all some principle drawings and DC op amp amplification circuits, so it was these details that caused a lot of confusion in the actual design. One of the most common problems in the application of AC-coupled operational amplifiers or instrumentation amplifier circuits is that there is no DC loop for the bias current. The author talked about the causes of bias current and offset current in the book and made a detailed derivation. The comparison between the actual components of OP07 and ADA4077 tells us the great progress in the performance improvement of the first generation of op amps to the sixth generation. A failed case is also used to analyze how to find and solve this problem.

I think the above picture can be used to illustrate the principle in a simple way . It can be explained in a more understandable way (not very rigorous, just to explain this one thing). Picture + "Speaking in a way that people can understand" ------ As shown in the figure, a capacitor C is connected in series to the non-inverting (+) input of an operational amplifier (red box). This AC coupling method is a simple method of input DC voltage isolation, which is particularly common in high-gain applications. When the gain is high, even a small DC voltage at the input of the amplifier will affect the dynamic range of the op amp and may even cause output saturation. However, the capacitive coupling into the high-impedance input of the op amp can be understood as the concept of "virtual disconnection" in the op amp. The current flowing into the N+ of the op amp is extremely small, and the capacitor needs to be charged and discharged. This will cause the circuit to work normally for a short period of time after powering on, and then lose its amplification effect. The reason for this is that the capacitor cannot be released after being fully charged-equivalent to an open circuit, and the subsequent AC signal naturally cannot be coupled to the op amp, so there is no amplification. Therefore, the DC bias problem of the non-inverting AC amplifier circuit is caused in this way, so a bias resistor (blue box) must be introduced to provide a loop for the AC signal. The practice of providing a DC path for the current in the N+ positive input terminal will bring some problems. In order to minimize the offset voltage caused by the input bias current, it is necessary to consider the matching problem of the two input terminals of the op amp. This book explains how to solve them. Pay attention! Inverting amplifiers generally do not have this problem! That's it for today. I will continue to read the book next time.

soso

Great, looking forward to the sequel.