运算放大器稳定性分析3

Therefore, here is the recommended standard open-loop SPICE circuit configuration for op amp circuits. The feedback loop is broken between the op amp output and the feedback elements. The ac signal source is injected into the feedback network and measurements are taken at the output, Vo, and feedback node, Vfb. With the feedback loop broken as shown, the equations for generating the desired curves are as follows: Aol_loaded = Vo/Vfb 1/ß = 1/Vfb Aol*ß = Vo 因而此处我们推荐使用标准的开环 SPICE 电路设置，反馈环路在运放的 输出与反馈网络之间断开，交流信号从反馈网络中注入，在运放的输出 端接测量得到 Vo，在反馈点测试得到 Vfb。 由于断开了回路，可以得到所需的曲线： Aol_loaded = Vo/Vfb 1/ß = 1/Vfb Aol*ß = Vo


Several example circuits ready for open-loop simulation are shown here. They can be used for review if there is confusion regarding where to break the loop in many standard circuit configurations. Note that for proper stability analysis, any output loading must remain directly on the output of the op amp and should not be placed on the other side of the inductor. Doing so would remove the effects the output loads have on the op amp output. 很多电路可以运用开环 SPICE 电路仿真，在实际电路中不知在何处断开 环路而感到困惑时可以用这些例子作为参考。 注意为了得到正确的稳定性分析结论，运放输出端所接的负载必须直接 体现在电路中，且不应该放置在电感的另外一端，否则就体现不同的负 载效应。


Once you’ve verified the dc operating point of the circuit, perform an ac transfer characteristic analysis over the op-amp bandwidth. Click Analysis, AC Analysis, AC Transfer Characteristic to do this. Set the start and end frequencies and then press “OK”. 检查完静态工作点之后，需要在分析的带宽范围内得到交流传输特性， 点击分析 Analysis 交流分析 AC Analysis 交流传输特性 AC Transfer Characteristic 设置起始与终止频率，然后点击“OK”


Before checking the ac behavior of the circuit, a quick check of the dc operating point should be performed. Simply click Analysis, DC Analysis, Calculate Nodal Voltages to do this. Vfb should show the input offset voltage, or Vos, of the op amp, while Vo will show Vos multiplied by the closed-loop gain. 在分析交流响应之前，应该快速的检查一下直流的静态工作点，点击分 析 Analysis 直流分析 DC Analysis 计算结点电压 Calculate Nodal Voltages。 Vfb 会显示出输入失调电压 Vos， 输出电压 Vo 会显示为 Vos 和闭环增益 之积。

Hale Waihona Puke To properly generate the open-loop curves in SPICE, the circuit being simulated must have a closed loop feedback path at dc while being open for all ac frequencies. The circuit at the top left shows the desired dc circuit with the L1 switch closed and C1 switch open. A closed loop circuit at dc allows the output to be properly biased to a recommended dc operating point, commonly mid-supply. The circuit at the bottom left shows the desired ac circuit with the L1 switch open and C1 switch closed. With the loop open for ac frequencies, the ac stimulus can be applied to generate the open-loop curves. Thankfully, there’s a straightforward way to create a circuit that meets both the dc and ac criteria using the ideal properties of SPICE components. Switch L1 is replaced with a 1Tera-Henry inductor, and switch C1 is replaced with a 1Tera-Farad capacitor. At dc, L1 is a short and C1 is an open-circuit, providing a proper dc operating point. For all ac frequencies, L1 is an open-circuit and C1 is a short resulting in the proper open-loop ac connections. 为了在 SPICE 中得到正确的开环曲线，电路必须要对直流建立反馈但是 对交流是开路的。 左上角的图中，通过 L1 开关对直流闭合，C1 开关对直流断开，直流的闭 合使得输出正确地偏置，通常是在供电的中心点。 左下角的电路，通过 L1 对交流信号开路，C1 对交流信号短路，环路对交 流信号是断开的，因而交流 AC 仿真可以得到开环曲线。 幸好，通过 SPICE 的理想模型这种方法可以同时满足直流和交流的要求， L1 是 1 Tera-Henry 的电感，C1 是 1 Tera-Farad 的电容。 对于直流信号，L1 为短路而 C1 为开路，提供合适的直流偏置，对于交流 信号，L1 为开路而 C1 为短路，从而提供了合适的交流通路。


However, simply breaking the feedback loop of a circuit will not produce correct simulation results! Without a proper dc bias, the output will saturate to one rail or the other, reducing the performance of the output stage. As shown here, the op-amp output is near the positive rail, resulting in erroneous Aol and Aolß curves. 然而，单纯地断开反馈回路是无法得到正确的仿真结果的。没有合适的 直流偏置，输出会直接饱和到任意一个电源轨，导致不正确的输出。 如图所示，运放输出接近正向电源，得到错误的 Aol 与 Aolß 曲线。


The Aol, 1/ß, and Aolß curves required for rate of closure and phase margin measurements can not be obtained from a circuit in a standard closed-loop configuration. To generate these curves, the feedback loop of the amplifier needs to be opened up, or “broken”. Then, a small signal source is used to excite the high-impedance side of where the loop was broken. Measurements can then be taken at the op amp inverting input (Vfb) and output (Vo), which will be used to derive the desired curves. 为了获得闭合速率与相位裕量时，我们需要 Aol, 1/ß, 以及 Aolß 曲线， 但是这些曲线无法从一个标准的闭合回路架构中得到。 为了获得这几组曲线，电路的反馈回路需要断开，然后用一个小信号在 断开处进行激励。 然后在运放的反相端得到 Vfb，在输出端得到 Vo，通过这两项可以推导 得到我们所需要的曲线。