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Question

恒流源是电路中广泛使用的一个组件，这里我整理一下比较常见的恒流源的结构和特点。
恒流源分为流出(Current Source)和流入(Current Sink)两种形式。
最简单的恒流源，就是用一只恒流二极管。实际上，恒流二极管的应用是比较少的，除了因为恒流二极管的恒流特性并不是非常好之外，电流规格比较少，价格比较贵也是重要原因。
最常用的简易恒流源用两只同型三极管，利用三极管相对稳定的be电压作为基准，
电流数值为：I = Vbe/R1。
这种恒流源优点是简单易行，而且电流的数值可以自由控制，也没有使用特殊的元件，有利于降低产品的成本。缺点是不同型号的管子，其be电压不是一个固定值，即使是相同型号，也有一定的个体差异。同时不同的工作电流下，这个电压也会有一定的波动。因此不适合精密的恒流需求。
为了能够精确输出电流，通常使用一个运放作为反馈，同时使用场效应管避免三极管的be电流导致的误差。如果电流不需要特别精确，其中的场效应管也可以用三极管代替。
电流计算公式为：
I = Vin/R1
这个电路可以认为是恒流源的标准电路，除了足够的精度和可调性之外，使用的元件也都是很普遍的，易于搭建和调试。只不过其中的Vin还需要用户额外提供。
从以上两个电路可以看出，恒流源有个定式（寒，“定式”好像是围棋术语XD），就是利用一个电压基准，在电阻上形成固定电流。有了这个定式，恒流源的搭建就可以扩展到所有可以提供这个“电压基准”的器件上。
最简单的电压基准，就是稳压二极管，利用稳压二极管和一只三极管，可以搭建一个更简易的恒流源。
电流计算公式为：I = (Vd-Vbe)/R1
TL431是另外一个常用的电压基准，利用TL431搭建的恒流源，其中的三极管替换为场效应管可以得到更好的精度。
TL431的其他信息请参考《TL431的内部结构图》和《TL431的几种基本用法》
电流计算公式为：I = 2.5/R1
事实上，所有的三端稳压，都是很不错的电压源，而且三端稳压的精度已经很高，需要的维持电流也很小。利用三端稳压构成恒流源，也有非常好的性价比。
这种结构的恒流源，不适合太小的电流，因为这个时候，三端稳压自身的维持电流会导致较大的误差。
电流计算公式为：I = V/R1，其中V是三端稳压的稳压数值。
实际的电路中，有一些特殊的结构，也可以提供很好的恒流特性，最典型的就是一个很高的电压通过一个电阻在一个低压设备上形成电流，这个恒流源的精度，取决于高压的精确度和低压设备本身导致的电压波动。在一些开关电源电路中，这个结构用来给三极管提供偏置电流。
电流计算公式为： I = Vin/R1
值得一提的是，以上这些恒流源并不都适合安培以上级别的恒流应用，因为电阻上面太大的电流会导致发热严重。
可以通过使用更小的电阻来降低这个热量，不过在单电源供电模式下，多数运放都不能有效检测和输出接近地或者Vcc的电压，因此必须使用特殊的器件才能达到要求。有个简单的办法是通过一个稳压器件（稳压管，或者TL431等）偏置电阻上面的电压，使得这个电压进入运放的检测范围。
恒流源的实质是利用器件对电流进行反馈，动态调节设备的供电状态，从而使得电流趋于恒定。只要能够得到电流，就可以有效形成反馈，从而建立恒流源。
能够进行电流反馈的器件，还有电流互感器，或者利用霍尔元件对电流回路上某些器件的磁场进行反馈，也可以利用回路上的发光器件（例如光电耦合器，发光管等）进行反馈。这些方式都能够构成有效的恒流源，而且更适合大电流等特殊场合。

Answer 1

Constant-current source is widely used in the circuit of a component, here I tidy common constant-current source of structure and characteristics.
Constant-current Source Current flow (divided into Current (returned) and Sink) two forms.
The simplest constant-current source, is to use a constant current diodes. Actually, the application of constantcurrent diode is less, but because the constant current diodes constant flow characteristics is not very good, current specification is less, the price is more expensive.
The most common use two simple constant-current source with type transistor, using the transistor relatively stable voltage as benchmark,.
Current values for: i. = Vbe/R1.
This constant current source advantage is simple, but the current numerical control, nor can freely use the special components, be helpful for reducing the cost of the product. Faults are different types of pipe, the voltage is not a fixed value now, even the same type, also have individual differences. At the same time different working voltage electrical flow, it will also have certain fluctuations. Therefore not suitable for precision constant-current demand.
In order to accurately output current, often use an op-amp, also used as the feedback of the mosfet avoid triode. In the current error. If the current need not special accurate, one of the mosfet can also use triode.
Current calculation formula for:
I = Vin/R1
This circuit can be considered constant-current source circuit, in addition to the standard of precision and adjustability enough, use of the components are also common, easy to build and debugging. Only one Vin need an additional user.
From the above two circuits can see, constant current source is a formula (cold, "sing" is like chess term XD), is to use a voltage references in resistance, formed on fixed current. With this formula, the constant-current source can be extended to build all can provide the "voltage references in the device".
The simplest voltage regulator, is using diode, a transistor, stabilizing diode and can build a more simple constant-current source.
Current calculation formula is: i. = (Vd - Vbe) / R1
TL431 is another common voltage TL431, using the benchmark of constant current source, the transistor replacement for mosfet can get better precision.
The other TL431 information consult the TL431 internal structure and several basic usage of TL431
Current calculation formula is: i. = 2.5 / R1
In fact, all three of the regulator, is very good, but the voltage source voltage accuracy has three components, the need to maintain high current too small. Using three terminal voltage constitute constant-current source, also has a very good price.
The structure of the constant-current source, and is not suitable for the current, because this is the third time, maintain current voltage itself will lead to greater error.
Current calculation formula is: i. = V/V is three, R1 voltage stabilizing of the numerical.
The actual circuits, some special structure, also can provide good constant flow characteristic, the most typical is a high voltage through a resistor in a low-pressure equipment, formed the constant-current source current of high precision, accuracy and low depends on the device itself in voltage fluctuation. In some switching power supply circuit, the structure is used to provide bias current transistor.
Current calculation formula is: i. = Vin/R1
What is worth mentioning, these constant-current source are not suitable for ampere above level constant-current application, because it is too large current resistance will cause severe fever.
Can use smaller resistance to reduce the heat, but in a single power supply mode, most op-amp are not effective detection and output close or Vcc voltage, therefore must use special device can meet the requirements. There is a simple way is by a voltage regulator (etc), or TL431 bias resistors of voltage, make the above into the test voltage range.
Constant-current source is to use the essence of the current feedback device, dynamic adjusting equipment, thus makes the current state of power tends to a constant. As long as can be current, can effective feedback form, so as to establish the constant-current source.
Can undertake current feedback device, and current transformer, or use of current hall element to some of the magnetic field on the feedback device, also can use to shine on the photoelectric devices (e.g., etc) coupler feedback. These are able to form effective constant-current source, and is more suitable for large current special occasions.