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1、Hello, and welcome to part six of the TI Precision Labs on op amp stability.This lecture will describe the Riso with dual feedback stability compensation method.大家好，欢迎来到 TI Precision Labs（德州仪器高精度实验室）关于运放 稳定性的第六部分。这个课程会讲述 Riso with dual feedback stability compensation method（双反馈 Riso 稳定性补偿方法） 。In the
2、 previous videos, we discussed the first capacitive load compensation technique using an isolation resistor. While the Riso circuit is both simple to implement and design it has a big disadvantage in precision circuits.The voltage drop from Riso is dependent on the output current or output load, and
3、 may be significant compared to the desired signal.As shown here, a 10mV signal has over 3mV (30%) of error due to a 250 output load.在前面的视频中我们讨论了第一种使用 isolation resistor（隔离电阻）来 补偿容性负载的方法。虽然 Riso 电路设计和使用都较简单，但它在精密电 路里有一大缺点。Riso 上的压降与输出电流或输出负载有关，并且 Riso 造 成的压降可能影响输出信号的准确度。如右图所示，对于一个 10mV 的输出 信号，由于 250
4、 的输出负载而产生超过 3mV 的压降，也就是 30%的误差。A solution to the voltage drop problem from the Riso circuit is to implement the “Riso + Dual Feedback” circuit shown here.为了解決 Riso 产生压降的问题， 我们可以使用这里所展示的 “Riso+双反馈” 的电路。The operation of the Riso + dual feedback circuit can be analyzed using the equivalent dc and ac re
5、presentations of the circuit. At dc, the feedback capacitor Cf acts as an open circuit and Rf closes the feedback loop around Riso.Since Riso is now in the op amp feedback loop, the op amp output will increase to overcome the Riso voltage drop such that the load voltage, Vload, is equal to Vin. At a
6、c frequencies, Cf acts as a short.When this happens, Rf can be thought of as an open-circuit because the impedance of Cf, XCF, will be much smaller than the impedance of Rf. Therefore, at ac, this circuit looks effectively the same as the standard Riso circuit.Riso+双反馈的工作原理可以用 DC 和 AC 等效电路来分析。 在 DC
7、时，反馈电容 Cf 是开路的，并且 Rf 闭合了包含 Riso 的反馈环路。因 为现在 Riso 是在运放的反馈环路里面的，运放输出会增大来补偿 Riso 的压 降，所以负载电压 Vload 会等于 Vin。 在 AC 频率上时，Cf 是短路的。当这个发生时，Rf 可以被认为是开路的因为 Cf 的阻抗 XCF会远远小于 Rf 的阻抗。因此在 AC 时，这个电路看起来会和标 准的 Riso 电路一样。The first design step in this circuit is to select Riso.The same method that was used to select Ris
8、o in “Method 1: Riso” is used here, and Riso is selected to produce a zero in the Aol curve at f(Aol = 20dB). Then, Rf can be selected to any value greater than 100*Riso in order to prevent interactions with Riso. The last step is to select a value of Cf in the range shown. Using this range ensures
9、that the two feedback paths, Rf and Cf, will never create a resonance that would cause instability. Smaller values for Cf will result in faster settling time, at the expense of overshoot for certain load ranges.这个电路的第一个设计步骤是选择 Riso。这里使用与“方法 1：Riso”中选 择 Riso 相同的方法，使得 Aol 曲线上等于 20dB 的频率点上产生一个零点。 然后，Rf
10、 可以选择为任意一个大于 100*Riso 的值，以防止其与 Riso 相互作 用。 最后一步是在幻灯片左下方所示的范围内选择 Cf 的值。使用这个范围可以 保证两个反馈路径，Rf 和 Cf，永远不会产生谐振而导致不稳定。更小的 Cf 值会有更快的 settling time（建立时间） ，但以一定负载范围内的过冲为代 价。The results show that the output and load voltage arrive at the final level without excessive overshoot or ringing, indicating a stable s
11、ystem.The increase in Vo to overcome the voltage drop from Riso can also be clearly seen here.从结果可见，运放输出和负载电压到达稳定值而没有过大的过冲与振铃，说 明这是一个稳定的系统。为克服 Riso 压降而增大的 Vo 在这里也可以清楚地 看到。While the Riso + Dual Feedback circuit solves the dc accuracy issue with the Riso circuit, it has some disadvantages as well. As
12、shown here an Riso circuit will generally remain stable with reasonable variation in the transient response over a wide range of capacitive loads.The Riso+Dual Feedback circuit is not as tolerant to changes in the output capacitance and can quickly become unstable. Therefore, the Riso + Dual Feedbac
13、k circuit is best for situations where the output capacitance is known and will not vary significantly. The Riso+Dual Feedback method generally results in a slower settling time than the Riso circuit as well.当 Riso+双反馈解决 Riso 电路 DC 精度问题的同时，它也有一些缺点。如这 里所示，一个 Riso 电路在合理的大范围容性负载的瞬态响应变化下通常会 保持稳定。Riso+双反
14、馈电路对输出电容的变化容忍度没有那么大，电路易 受到容性负载的变化而变得不稳定。因此，Riso+双反馈电路适用于输出电 容确定并且不会显著变化的场合。 Riso+双反馈方法通常会导致建立时间比 Riso 电路更慢。Stability part 3 showed how to perform open-loop analysis on many common op amp circuits. However, those circuits all had only one feedback loop. If we want to perform a simulated open-loop ana
15、lysis on multiple-feedback circuits, like the circuit with Riso + dual feedback compensation, a different method is required which well now discuss.稳定性第三部分展示了在多种常见的运放电路上如何进行开环分析。然而这 些电路都是只有一条反馈路径。如果我们想在多反馈电路上进行开环分析仿 真，像 Riso+双反馈补偿电路那样，需要一个不同的方法。我们将会在这里 讨论。Opening either feedback path still leaves a
16、closed-loop path around the other loop.If FB1 is opened, FB2 remains as a closed-loop feedback path.If FB2 is opened, FB1 remains as a closed-loop feedback path.The circuit will not properly report open-loop curves unless both feedback loops are open.断开任意一条反馈路径，另一条路径都会是闭回路，维持电路闭回路特性。 如果 FB1 断开，FB2 作为闭环反馈路径保持闭合。如果 FB2 断开，FB1 作为 闭环反馈路径保持闭合。这个电路不会正确反映开环曲线，除非两条反馈环 路都断开。Breaking the loop directly at the output will remove the connections between the output and both feedback loop
