圖3顯示了實現(xiàn)三位置控制器更有效的方法。電路基礎(chǔ)是單運算放大器，其不需要參考電壓。輸入和輸出二極管決定高于高壓和低于低壓的開關(guān)門限和比較器滯后。在中間帶放V_IN(t)消除電路的輸入二極管，電路本質(zhì)上是帶正反饋的電壓跟隨器。輸出電壓跟隨V_A(t)，但是正反饋確定V_A(t)，在輸出電壓的某些片斷設(shè)定這個電壓。所以，兩個約束確定電路狀態(tài)的輸出水平：V_OUT(t)=V_A(t), 和

　　滿足這兩個約束的唯一條件是V_OUT和V_A=0V，所以當輸入二極管反向偏置時，輸出仍為0V。0V的輸出狀態(tài)持續(xù)到輸入電壓增加正或負值。然后，雙輸入齊納二極管的一個傳導，驅(qū)動放大器以輸入電壓±V_H的正或負電壓輸出。在這個條件下，當絕對輸入電壓下降時，放大器在±V_L的輸入電壓上輸出再次為0V。因此，V_H 和V_L的設(shè)計公式為

　　圖4顯示了電路用圖3值的I/O轉(zhuǎn)換特性，而D₁A和D₁B為6.8V 1N4099齊納二極管和D₂A和D₂B為3V 1N5225齊納二極管。圖5顯示當電路輸入采用三角波的輸出電壓。

　　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- and

negative-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.