来源：EDN | 作者：Marián Štofka, Slovak University of Technology, Bratislava, Slovakia

可以使用箝位电路替代Schmitt触发器净化高频输入脉冲。

　　正反馈放大器是信号级脉冲形成电路的基础。这个设备确保了输入信号超过输入极限水平的类触发作用；在大多数情况下，输入信号为电压信号。最著名的触发器为Schmitt触发器（今年将庆祝发现70周年）。英国科学家OH Schmitt在1938年以两级运放带电流反馈的形式，发明了Schmitt触发器。这两个有源器件为电子管。

　　Schmitt触发器具有快速的优势，无论输入信号斜率多大，输出转换时间都恒定。这种作用导致I/O特性上出现磁滞。换句话说，在正输出转换之前，极限移动到较高值，转换到正输出水平之后，极限移动到较低的值。可以设置磁滞的数值——从零到——对用离散器件组成的Schmitt触发器电路。Schmitt电路在逻辑IC中广泛使用，磁滞相当大且确定。

　　另外，可以使用一个电路——快速响应电压限制器或箝位器——作为脉冲形成电路。输入电压范围比Schmitt触发器电路更窄，因为低输入电压时，电压界限不可变，电路为线性放大器。换句话说，由于它的非迟滞特性，输入电压的最终极限与输出电平转换的双边结果精确相等。图1显示这个 电路的例子。图1中电压限制器为高度非线性负反馈的反向放大器。对–0.3到+0.6V范围的输出电压，每个二极管都是不传导的，所以反馈阻抗高。可选的Schottky势垒二极管的前置电压降决定这些电压界限（参考文献1）。因此，反向放大器的电压增益与运放的开环增益近似相等。

　　根据输出电压极性，无论何时输出电压超过这些限制，二极管D₁、D₂或D₃开始导通。然后，放大器的差分增益分别降到–R_I/2R_D和–R_I/R_D的值，在这里，R_D为单二极管的等效串联电阻。动作箝位输出电压到约0.8V，对大输入电压甚至为–0.4V。图中使用Analog Devices公司的AD8045 VHSIC（超高速集成电路）运放，因为其转换速率超过1V/ns（参考文献2）。

　　与中点和地之间带横向电阻RT₁的两并联二极管相比，单反馈二极管，图1的电路为不对称限制结构。由D₁、D₂和RT₁组成的箝位电路，与单二极管D₃相比，为运放的输出和输入之间提供了更高的截止状态隔离度。当D₃打开时，可以在输出波形上观察到约200MHz的微弱阻尼振荡。振荡表明D₁和D₂二极管接通初始阶段自身的削弱。

　　英文原文：

　　High-speed clamp functions as pulse-forming circuit

　　You can use a clamping circuit instead of a Schmitt trigger to clean up high-frequency input pulses.

　　Marián Štofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Charles H Small and Fran Granville -- EDN, 1/10/2008

　　Amplifiers with positive feedback are the bases of signal-grade pulse-forming circuits. This setup ensures a triggerlike operation in which the input signal crosses the input-threshold level; in most cases, the input signal is a voltage signal. The most well-known of these triggers is the Schmitt trigger, which, by the way, will this year celebrate its 70th birthday. British scientist OH Schmitt in 1938 originated the Schmitt trigger in the form of a two-stage amplifier with current feedback. The two active devices were vacuum tubes.

The operation of a Schmitt trigger has the advantage of fast, almost-constant transition times of the output regardless of the slope of the input signal. One consequence of this type of operation is the hysteresis in the I/O characteristic. In other words, the threshold shifts to a higher value before the positive-output transition, and it shifts to a lower value after switching to the positive-output level. You can set the amount of hysteresis—from zero to latch-up—for Schmitt-trigger circuits comprising discrete parts. Schmitt circuits find wide use in logic ICs, in which the hysteresis is rather high and fixed.

　　As an alternative, you can use a circuit—a fast-response voltage limiter, or clamper—as a pulse-forming circuit. The input-voltage range is narrower than that of Schmitt-trigger circuits, because, at low input voltages, the voltage limitation becomes inactive, and the circuit operates as a linear amplifier. On the other hand, because of its nonhysteretic behavior, the decision threshold of the input voltage is precise and equal for both directions of output-level transitions. Figure 1 shows one example of such as circuit. The voltage limiter in Figure 1 is an inverting amplifier with a highly nonlinear negative feedback. For output voltages ranging from –0.3 to +0.6V, the feedback impedance is high because each of the diodes is nonconducting. The forward-voltage drop of the selected Schottky-barrier diodes determines these voltage limits (Reference 1). The voltage gain of the inverting amplifier is thus almost that of the op amp’s open-loop gain.

　　Whenever the output voltage exceeds these limits, diode D1, D2, or D3—depending on the polarity of the output voltage—starts to conduct. The differential gain of the amplifier then drops to the value of –RI/2RD and –RI/RD, respectively, where RD is the equivalent-series resistance of a single diode. The action clamps the output voltage to approximately 0.8V and to –0.4V even for large input voltages. The figure uses an Analog Devices AD8045 VHSIC (very-high-speed integrated-circuit) op amp because its slew rate exceeds the value of 1V/nsec (Reference 2).

　　Figure 1’s circuit has an asymmetrical-limiting configuration to compare the single feedback diode with two series-connected diodes having a transverse resistor, RT1, between their midpoints and ground. The clamping circuitry comprising D1, D2, and RT1 offers higher off-isolation between the output and the input of the op amp than that of the single diode, D3. When D3 is on, you can observe small, weakly damped oscillations at approximately 200 MHz in the output waveform. Oscillations manifest themselves less at the beginning of turn-on of the D1 and D2 diodes.

　　References

　　“Surface Mount RF Schottky Barrier Diodes,” Avago Technologies.

　　"AD8045 3 nV/ Ultralow Distortion Voltage Feedback High Speed Amplifier," Analog Devices, 2004.