Operational amplifier circuit design knowledge, have you noticed?

　　Operational amplifiers are a very common type of IC. The following are common problems and appropriate solutions in operational amplifier circuit design. I hope it will be inspiring for everyone and help us improve together!

　　How to compensate bias current

　　For the inverting operational amplifier we commonly use, the typical circuit is as follows:

　　In this case, R3 is a balancing resistor, and its value is generally easy to calculate. In this way, the current compensation of the op amp can be well guaranteed, making the positive and negative bias currents equal. Even when the value is larger, it will produce greater noise and drift. However, it should be greater than the internal resistance of the input signal source.

　　Engineers who are good at thinking will think about what the principle is when it is a common-phase amplifier. Now let's review the design circuit of the common-phase amplifier:

　　In the in-phase proportional amplifier, when the calculated Rp is negative, the bias resistor needs to be moved to the positive end and connected in series with R1 at the input end. Here is an extra sentence: the in-phase proportional amplifier has the characteristics of high input impedance and low output impedance, and is widely used in the preamplifier circuit.

　　Zero adjustment circuit

　　Today, op amps have developed rapidly, with various additional functions. For example, some op amps have external zero adjustment ports. In this case, the op amp zero adjustment can be completed by selecting appropriate resistors according to the data sheet. For example, the LF356 op amp has a typical circuit as follows:

　　Some other low-cost op amps may not have these automatic adjustment functions. As designers, we can achieve fixed zero adjustment through simple addition circuits, subtraction circuits, etc. (although sometimes this approach is ineffective). When it is necessary to perform temperature compensation, a triode circuit is usually added to the compensation circuit to use the temperature characteristics of the PN junction to complete the temperature compensation of the op amp. For example, in the typical circuit of LF355, the triode circuit is embedded between V+ and the 25K feedback resistor.
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　　How to choose phase compensation

　　When we read the data sheet of an integrated operational amplifier, we will find that the inside of the integrated operational amplifier is actually a multi-stage amplifier. Therefore, it is inevitable to introduce poles into the system, which requires phase compensation of the circuit. Lead compensation, lag compensation and lag-lead compensation are usually used. The so-called lead compensation is the compensation for phase shift reduction. In layman's terms, it is to make the circuit appear zero point, and the output signal at this frequency is 45° ahead of the input signal in phase. A zero point is artificially designed by calculating the frequency point where the pole will appear, so that the system becomes stable.

　　Hysteresis compensation can usually be understood as compensation that increases phase shift, which can reduce the main pole frequency and narrow the amplifier band, so that the op amp circuit can have only one pole within a limited bandwidth, making the op amp circuit easier to adjust.

　　The third is lead-lag compensation, which is to use appropriate methods to process the op amp unit.

　　How to deal with capacitive load

　　In the usual electronic circuit design, the circuit may become oscillating due to carelessness or inattention to the characteristics of the load. At this time, we should pay attention to the characteristics of the load. Usually, when the load is capacitive, by estimating its capacitance value to be less than 2000pF, a small resistor is connected in series between the load and the output of the op amp to eliminate the oscillation. The value of resistor R2 is between 10-300Ω.

　　When the load is large, we use the following solution to eliminate it:

　　The compensation capacitor C2 and the feedback resistor R3 form an advance compensation network, forming a new zero point to offset the new pole formed by the capacitive load Cl and the output resistor Ro of the op amp, thereby achieving the purpose of eliminating oscillation. At this time, the size of the compensation capacitor C2 is C2=Cl(Ro+Rk)/R3, and Rk takes an empirical value of 10-300Ω.