双极性三极管开关特性Word格式.docx

1、为单片机可接收的TTL信号，为了与输入控制信号一致，加入反相器74LS14。 TransistorsA Transistor is a solid-state device designed to control DC current. Transistors are most commonly found in low DC powered sensors as the output switch. There are two types of transistors - NPN and PNP. The figure below shows a NPN （Current Sink） Ope

2、n Collector Transistor Figure 1: Sensor NPN OutputOutput StyleDepending on model, incremental encoders are available with several different electrical output styles. Choice of signal depends on receiving instrument and cable distance. Line driver outputs with complimentary outputs can be used with l

3、onger cables as noise spikes can be cancelled.NPNUses an NPN type transistor and an internal resistor pulling up to the power supply rail.The output is an active voltage.NPN Open CollectorUses an NPN type transistor but without an internal pull up resistor to the supply rail. The output is passive s

4、o a separate power supply can be used.PNPUses a PNP type transistor and an internal resistor pulling down to zero volts.PNP Open CollectorUses a PNP type transistor but without an internal pull down resistor to zero volts.Push PullA problem with NPN and PNP type outputs is the high output impedance.

5、 This can be solved by a complementary output allowing better switching to zero and positive supply rails.Line DriverThis output style has two complimentary outputs per channel allowing better transmission in noisy environments and long cable lengths. The receiver can process the signal, eliminating

6、 noise spikes.PTC protectionA positive temperature coefficient resistor can be added to the output of a NPN or PNP encoder, protecting it from output short circuits.Sinking/SourcingSinking sensors allow current to flow into the sensor to the voltage common, while sourcing sensors allow current to fl

7、ow out of the sensor from a positive source. For both of these methods the emphasis is on current flow, not voltage. By using current flow, instead of voltage, many of the electrical noise problems are reduced.When discussing sourcing and sinking we are referring to the output of the sensor that is

8、acting like a switch. In fact the output of the sensor is normally a transistor, that will act like a switch （with some voltage loss）. A PNP transistor is used for the sourcing output, and an NPN transistor is used for the sinking input. When discussing these sensors the term sourcing is often inter

9、changed with PNP, and sinking with NPN. A simplified example of a sinking output sensor is shown in See A Simplified NPN/Sinking Sensor. The sensor will have some part that deals with detection, this is on the left. The sensor needs a voltage supply to operate, so a voltage supply is needed for the

10、sensor. If the sensor has detected some phenomenon then it will trigger the active line. The active line is directly connected to an NPN transistor. （Note: for an NPN transistor the arrow always points away from the center.） If the voltage to the transistor on the active line is 0V, then the transis

11、tor will not allow current to flow into the sensor. If the voltage on the active line becomes larger （say 12V） then the transistor will switch on and allow current to flow into the sensor to the common. A Simplified NPN/Sinking SensorSourcing sensors are the complement to sinking sensors. The sourci

12、ng sensors use a PNP transistor, as shown in See A Simplified Sourcing/PNP Sensor. （Note: PNP transistors are always drawn with the arrow pointing to the center.） When the sensor is inactive the active line stays at the V+ value, and the transistor stays switched off. When the sensor becomes active

13、the active line will be made 0V, and the transistor will allow current to flow out of the sensor.A Simplified Sourcing/PNP SensorMost NPN/PNP sensors are capable of handling currents up to a few amps, and they can be used to switch loads directly. （Note: always check the documentation for rated volt

14、ages and currents.） An example using sourcing and sinking sensors to control lights is shown in See Direct Control Using NPN/PNP Sensors. （Note: This example could be for a motion detector that turns on lights in dark hallways.）Direct Control Using NPN/PNP SensorsIn the sinking system in See Direct

15、Control Using NPN/PNP Sensors the light has V+ applied to one side. The other side is connected to the NPN output of the sensor. When the sensor turns on the current will be able to flow through the light, into the output to V- common. （Note: Yes, the current will be allowed to flow into the output

16、for an NPN sensor.） In the sourcing arrangement the light will turn on when the output becomes active, allowing current to flow from the V+, thought the sensor, the light and to V- （the common）.At this point it is worth stating the obvious - The output of a sensor will be an input for a PLC. And, as

17、 we saw with the NPN sensor, this does not necessarily indicate where current is flowing. There are two viable approaches for connecting sensors to PLCs. The first is to always use PNP sensors and normal voltage input cards. The second option is to purchase input cards specifically designed for sour

18、cing or sinking sensors. An example of a PLC card for sinking sensors is shown in See A PLC Input Card for Sinking Sensors.A PLC Input Card for Sinking Sensors“采用集电极开路的输出方式，有什么好处？”A：集电极开路输出大概有以下几个好处：1.可以实现线与功能，即两个或多个输出端可并联在一起，然后接一上拉电阻至高电平。这样，只要有一个输出是低，那么结果就是低，即实现了与的功能。2.跟上面的有点类似，那就是多个门输出端接在一起时，不会导致损

19、坏。3.可以用来控制较高的电平。典型应用可以看看ULN2003。4.跟3类似，当输出断开时，为高阻态，这样就可以做输入口使用了。典型应用请看8951单片机的IO口结构，置1时即为输入口。对于集电极（OC）或者漏极开路（OD）输出的，如果要输出高电平，必须接上拉电阻，因为OC或OD门，置1时输出相当于悬空。其中第4点有些不太明白，问：“当输出断开时，为高阻态，这样就可以做输入口使用了”，那当OC门输入端输入什么信号时，输出呈高阻态？还有“如果要输出高电平，必须接上拉电阻，因为OC或OD门，置1时输出相当于悬空。”输出高电平，必须接上拉电阻，是不是为了提高带负载能力，（相當”灌電流”？）还有其他的

20、好处吗？输出低电平的時候也要加上拉嗎？為什么？（相當”拉電流”？）那拉電流又是不是从电源和负载抽取電流到地，这在实际应用中有什么好处？我们先来说说集电极开路输出的结构。集电极开路输出的结构如图1所示，右边的那个三极管集电极什么都不接，所以叫做集电极开路（左边的三极管为反相之用，使输入为“0”时，输出也为“0”）。对于图1，当左端的输入为“0”时，前面的三极管截止（即集电极C跟发射极E之间相当于断开），所以5V电源通过1K电阻加到右边的三极管上，右边的三极管导通（即相当于一个开关闭合）；当左端的输入为“1”时，前面的三极管导通，而后面的三极管截止（相当于开关断开）。我们将图1简化成图2的样子。图

21、2中的开关受软件控制，“1”时断开，“0”时闭合。很明显可以看出，当开关闭合时，输出直接接地，所以输出电平为0。而当开关断开时，则输出端悬空了，即高阻态。这时电平状态未知，如果后面一个电阻负载（即使很轻的负载）到地，那么输出端的电平就被这个负载拉到低电平了，所以这个电路是不能输出高电平的。 再看图三。图三中那个1K的电阻即是上拉电阻。如果开关闭合，则有电流从1K电阻及开关上流过，但由于开关闭合时电阻为0（方便我们的讨论，实际情况中开关电阻不为0，另外对于三极管还存在饱和压降），所以在开关上的电压为0，即输出电平为0。如果开关断开，则由于开关电阻为无穷大（同上，不考虑实际中的漏电流），所以流过的

22、电流为0，因此在1K电阻上的压降也为0，所以输出端的电压就是5V了，这样就能输出高电平了。但是这个输出的内阻是比较大的（即1K），如果接一个电阻为R的负载，通过分压计算，就可以算得最后的输出电压为5*R/（R+1000）伏，即5/（1+1000/R）伏。所以，如果要达到一定的电压的话，R就不能太小。如果R真的太小，而导致输出电压不够的话，那我们只有通过减小那个1K的上拉电阻来增加驱动能力。但是，上拉电阻又不能取得太小，因为当开关闭合时，将产生电流，由于开关能流过的电流是有限的，因此限制了上拉电阻的取值，另外还需要考虑到，当输出低电平时，负载可能还会提供给一部分电流从开关流过，因此要综合这些电流

23、考虑来选择合适的上拉电阻。 如果我们将一个读数据用的输入端接在输出端，这样就是一个IO口了（51的IO口就是这样的结构，其中P0口内部不带上拉，而其它三个口带内部上拉），当我们要使用输入功能时，只要将输出口设置为1即可，这样就相当于那个开关断开，而对于P0口来说，就是高阻态了。 另一种输出结构是推挽输出。推挽输出的结构就是把上面的上拉电阻也换成一个开关，当要输出高电平时，上面的开关通，下面的开关断；而要输出低电平时，则刚好相反。比起OC或者OD来说，这样的推挽结构高、低电平驱动能力都很强。如果两个输出不同电平的输出口接在一起的话，就会产生很大的电流，有可能将输出口烧坏。而上面说的OC或OD输出

24、则不会有这样的情况，因为上拉电阻提供的电流比较小。如果是推挽输出的要设置为高阻态时，则两个开关必须同时断开（或者在输出口上使用一个传输门），这样可作为输入状态，AVR单片机的一些IO口就是这种结构。集电极输出的负载电阻（转）上拉电阻实际上是集电极输出的负载电阻。不管是在开关应用和模拟放大，此电阻的选择都不是拍脑袋的。工作在线性范围就不多说了，在这里是讨论的是晶体管是开关应用，所以只谈开关方式。找个TTL器件的资料单独看末级就可以了，内部都有负载电阻根据不同驱动能力和速度要求这个电阻值不同，低功耗的电阻值大，速度快的电阻值小。但芯片制造商很难满足应用的需要不可能同种功能芯片做许多种，因此干脆不做

25、这个负载电阻，改由使用者自己自由选择外接，所以就出现OC、OD输出的芯片。由于数字应用时晶体管工作在饱和和截止区，对负载电阻要求不高，电阻值小到只要不小到损坏末级晶体管就可以，大到输出上升时间满足设计要求就可，随便选一个都可以正常工作。但是一个电路设计是否优秀这些细节也是要考虑的。集电极输出的开关电路不管是开还是关对地始终是通的，晶体管导通时电流从负载电阻经导通的晶体管到地，截止时电流从负载电阻经负载的输入电阻到地，如果负载电阻选择小点功耗就会大，这在电池供电和要求功耗小的系统设计中是要尽量避免的，如果电阻选择大又会带来信号上升沿的延时，因为负载的输入电容在上升沿是通过无源的上拉电阻充电，电阻越大上升时间越长，下降沿是通过有源晶体管放电，时间取决于器件本身。因此设计者在选择上拉电阻值时，要根据系统实际情况在功耗和速度上兼顾。