单运放完成双滞后转换特性

　运放结合战略上被放置的齐纳二极管产生一个过程控制电路，电路有正、负向磁滞带。

　　在需要间断控制器的过程控制应用中，大多数初步选择是两个位置模型或开/关控制器。这种控制器的典型例子是空间加热器。如果温度低到设置点以下，加热器打开，而如果温度高过设置点，加热器关闭。在模拟域，双位置控制器的主要部分是模拟比较器或开环运算放大器。然而，为避免开关误触发，典型应用是使用熟知的Schmitt触发器。

　　双位置控制器模型的合理扩展是能够提供若干个（多于两个）控制器输出的中间设定。使用这种间断控制器模型减少双位置模型固有的循环行为、超过目标或未达目标。然而实际上，双位置模型不能达到满意的时候，使用其它模型通常更快。最通用的例子是三位置控制器。图1表示实现这种控制器的一个简单方法。图中，运算放大器周围的Schmitt触发器A₁和A₂分别实现负和正滞后。可以用模拟比较器，如LM311或相近的产品替代A₁和A₂。图2显示了图1中电路的I/O转换特性:

　　V_Z 和V_Y分别为四个齐纳二极管的击穿和正向导通电压。

　　图3显示了实现三位置控制器更有效的方法。电路基础是单运算放大器，其不需要参考电压。输入和输出二极管决定高于高压和低于低压的开关门限和比较器滞后。在中间带放V_IN(t)消除电路的输入二极管，电路本质上是带正反馈的电压跟随器。输出电压跟随V_A(t)，但是正反馈确定V_A(t)，在输出电压的某些片断设定这个电压。所以，两个约束确定电路状态的输出水平：V_OUT(t)=V_A(t), 和

　　满足这两个约束的唯一条件是V_OUT和V_A=0V，所以当输入二极管反向偏置时，输出仍为0V。0V的输出状态持续到输入电压增加正或负值。然后，双输入齐纳二极管的一个传导，驱动放大器以输入电压±V_H的正或负电压输出。在这个条件下，当绝对输入电压下降时，放大器在±V_L的输入电压上输出再次为0V。因此，V_H 和V_L的设计公式为

　　图4显示了电路用图3值的I/O转换特性，而D₁A和D₁B为6.8V 1N4099齐纳二极管和D₂A和D₂B为3V 1N5225齐纳二极管。图5显示当电路输入采用三角波的输出电压。

　　英文原文：

　　Single op amp achieves double-hysteresis-transfer characteristic

　　An op amp combines with strategically placed zener diodes to yield a process-control circuit that has positive- andnegative-going hysteresis bands.

　　Herminio Martínez, Encarna García, and Juan Gámiz, Technical University of Catalonia, Barcelona, Spain; Edited by Charles H Small and Fran Granville -- EDN, 9/27/2007

　　In process-control applications requiring discontinuous controllers, the most elementary choice is a two-position-mode or on/off controller. A typical example of such a controller is a space heater. If the temperature drops below a setpoint, the heater turns on, and, if the temperature rises above the setpoint, it turns off. In the analog domain, the basis for the basic implementation of a two-position controller is an analog comparator or an open-loop operational amplifier. However, to avoid false switching, the typical implementation uses the well-known Schmitt trigger.

　　A logical extension of the two-position control mode is to provide several—rather than two—intermediate settings of the controller’s output. You can use this discontinuous-control mode to reduce the cycling behavior, overshoot, or undershoot inherent in the two-position mode. In fact, however, it is usually speedier to use some other mode when the two-position mode is unsatisfactory. The most common example is the three-position controller. Figure 1 shows one simple way to implement this controller. In this configuration, the Schmitt triggers around the operational amplifiers, A1 and A2, which implement the negative and positive hysteresis, respectively. You can replace A1 and A2 with analog comparators, such as an LM311 or similar. Figure 2 shows the I/O-transfer characteristic of the circuit in Figure 1:andVZ and VY are, respectively, the breakdown and the forward voltages of the four zener diodes.

　　Figure 3 shows a more efficient way to implement a three-position controller. The circuit’s basis is a single operational amplifier, and it needs no reference voltages. The input and output diodes determine the upper high-voltage and lower low-voltage switching-threshold levels and the hysteresis of the comparator. Putting VIN(t) in the middle band eliminates the input diodes from the circuit, and the circuit is essentially a voltage follower with positive feedback. The output voltage follows VA(t), but the positive feedback establishing VA(t) sets this voltage at some fraction of the output voltage. So, two constraints define the output level in this circuit state: VOUT(t)=VA(t), and

　　The only condition satisfying these two constraints is that VOUT and VA="0V"; so, the output remains at 0V when the input diodes are reverse-biased. A 0V output state continues until input voltage increases with positive or negative values. Then, one of the two input zener diodes conducts, driving the amplifier output positive or negative at an input voltage of ±VH. In this condition, when absolute input voltage decreases, the amplifier output again goes to 0V at an input voltage of ±VL. Thus, the design equations for VH and VL are VH="VZ1"+Vγ, and

　　Figure 4 shows the I/O-transfer characteristic of the circuit with the values in Figure 3, where D1A and D1B are 6.8V 1N4099 zener diodes and D2A and D2B are 3V 1N5225 zener diodes. Figure 5 shows the output voltage when you apply a triangular waveform at the circuit’s input.