二极管开关电路

在网上搜到的这篇英文，简单的翻译了一下，它简单的介绍了开关二极管作为开关的用法，包括用在数字电路中和模拟电路中，简单易懂，适合初学者，呵呵，分享，在百度里搜中文的开关二极管资料，很难找到这样的文章。（本文原创——百度空间，发现电子设计之美，译者：Flyaswing）。

Diodes can perform switching and digital logic operations. Forward and reverse bias switch a diode between the low and high impedance states, respectively. Thus, it serves as a switch.

二极管有开关等其他数字逻辑功能，加在二极管上的正向和反向偏执分别使使二极管导通和截止。因此，它就像一个开关一样。

Logic

逻辑功能

Diodes can perform digital logic functions: AND, and OR. Diode logic was used in early digital computers. It only finds limited application today. Sometimes it is convenient to fashion a single logic gate from a few diodes.

二极管可以实现“与”和“或”的数字逻辑。早期的计算机中就使用二极管来实现这些逻辑。但是当今这种用法已经很少见。但是，有时候用少量的二极管实现来做单个逻辑门也是很方便的。

上图为一个“与”门，逻辑门都有输入和输出，其逻辑功能是针对输入而言。高电平代表逻辑1，低电平代表逻辑0。上图中，逻辑电平通过开关来产生。如果开关弹起，则输入为高电平（逻辑1）。如果开关闭合，则将二极管的负极接地，所以输入为低电平。输出端的逻辑取决于A、B两个输入。逻辑门的逻辑功能通常用真值表描述。此“与门”的逻辑功能真值表如图（c）所示。“与门”的逻辑功能即：输入有一个为0，则输出即为0。

A two input OR gate composed of a pair of diodes is shown in Figure below. If both inputs are logic low at (a) as simulated by both switches “downward,” the output Y is pulled low by the resistor. This logic zero is recorded in the first line of the truth table at (c). If one of the inputs is high as at (b), or the other input is high, or both inputs high, the diode(s) conduct(s), pulling the output Y high. These results are reordered in the second through fourth lines of the truth table. Summary: any input “high” is a high out at Y.

下图是由一对二极管组成的“或门”。其中有一个开关按下时，输出即为高电平，即有一个输入为高，输出即为高电平。

A backup battery may be OR-wired with a line operated DC power supply in Figure above (d) to power a load, even during a power failure. With AC power present, the line supply powers the load, assuming that it is a higher voltage than the battery. In the event of a power failure, the line supply voltage drops to 0 V; the battery powers the load. The diodes must be in series with the power sources to prevent a failed line supply from draining the battery, and to prevent it from over charging the battery when line power is available. Does your PC computer retain its BIOS setting when powered off? Does your VCR (video cassette recorder) retain the clock setting after a power failure? (PC Yes, old VCR no, new VCR yes.)

图（d）为二极管或门的一个应用实例，其中一个备用电池与直流电源线或，所以当电源断电时，备用电池便可以为负载供电。备用电池与二极管串联，在电源可用时，也不会对电池反向充电。PC断电时，BIOS也能保持它原来的设置，就是采用了这种结果。

Analog switch

模拟开关

Diodes can switch analog signals. A reverse biased diode appears to be an open circuit. A forward biased diode is a low resistance conductor. The only problem is isolating the AC signal being switched from the DC control signal. The circuit in Figure below is a parallel resonant network: resonant tuning inductor paralleled by one (or more) of the switched resonator capacitors. This parallel LC resonant circuit could be a preselector filter for a radio receiver. It could be the frequency determining network of an oscillator (not shown). The digital control lines may be driven by a microprocessor interface.