How to Design a Precision Current Pump with Op-Amps

In circuit theory,电压来源和当前来源are equally ideal and equally easy to implement. You just draw a circle, and then you add plus and minus signs for voltage or an arrow for current. Now you have a circuit element that generates a specified voltage under all conditions or that drives a specified current under all conditions.

在现实生活中,来源不理想,和furthermore, approximating a theoretical voltage source is significantly easier than approximating a theoretical current source. Voltage sources are as simple as a battery, a齐纳二极管，或电压分隔器与缓冲液结合。

Current sources, on the other hand, usually require some clever circuit design and greater attention to operational details.

电流源体系结构

There are various ways to design a current source. Before we look at a two-op-amp topology, let’s briefly review some other options. You can learn more about all these topics by clicking on the respective links.

One interesting approach is to use avoltage regulator as a current regulator：

LT3085的应用图。图像线性设备（模拟设备）

另一个选择是我在上一篇文章中讨论的基于放大器的电路如何设计简单的，电压控制的双向电流源。An amplifier-based circuit is vaguely reminiscent of the two-op-amp topology, but one of the amplifiers is an instrumentation amp rather than an op-amp.

电压可编程电流源的图。图像线性设备（模拟设备）

最后，我们有霍兰当前泵，这是在Sergio Franco博士撰写的AAC文章中进行了彻底分析的。

两次拓扑拓扑

我发现该电路被描述为“精确电流泵”，在模拟设备的旧应用程序中。它产生的双向输出电流与输入电压成正比。

这是原始电路图：

精确电流泵的图。图像类似物Devices

我喜欢这个电路的一些事情。首先，仅需要两种组件：操作放大器和电阻。

其次，运算放大器具有相同的零件号。It’s true that this circuit uses two op-amps whereas the Howland pump uses only one, but the fact that both op-amps can be the same exact part is advantageous because you can use a dual-op-amp IC package and thereby minimize whatever additional cost or board space is required for the second op-amp.

Third, four of the five resistors (R2, R3, R4, R5) can have the same value, and then the voltage-to-current gain is controlled by one resistor (R1). The value of R2–R5 isn’t critical, so you can adapt the circuit to components that you already have in the lab or to an existing BOM. Keep in mind, though, that higher-precision resistors will produce a higher-precision current source.

第四，输入电压是差异的。这为您提供了如何提供控制电压的灵活性，并且它使您可以利用电路的双向输出电流功能，而无需生成延伸下方以下的控制电压。

两次AMP电流源的基本操作

We’ll use an LTspice implementation to help us analyze the two-op-amp current source.

在这里，我正在使用LTSpice“理想的单孔操作放大器”。我最初使用OP-77尝试了它，但是模拟运行未正确。OP-77 Macromodel可能存在问题，因为我还有另一个使用LT1001A OP-AMP的电路版本，并且可以正确模拟。

恒流源电路通常依赖于某种反馈，该反馈会导致电压源产生指定的电流，而不管负载电阻如何。（您可以在我为A的电压控制的LED驱动器中看到一个直接的示例color-sensor project。)

在两次启动电流泵中，U1放大了差分控制电压，U2被配置为电压跟随器，该电压跟随器感应到负载上的电压并将其馈回输入阶段。

上面显示的电压源配置产生的差分输入电压从+250 mV到–250 mV。根据应用注释中提供的方程式，输出电流应从2.5 mA到–2.5 mA，因为_v= 1和r1 =100Ω，这正是我们观察到的：

One thing that you need to watch out for with this circuit is the U1 output voltage. All of the load current comes from U1. If we neglect the very small currents that flow through the feedback resistor R4 and into the positive input terminal of U2, the voltage at U1’s output terminal will be equal to I_出去multiplied by the sum of the load resistance and the resistance of R1.

\[V_{OUT,U1}\approx \left(R_{LOAD}+R1\right)I_{OUT}\]

This voltage can easily exceed what the op-amp’s output stage can actually generate, especially if you’re using ±3 V or ±5 V rails rather than the ±12 V or ±15 V analog supply voltages that were, I believe, more common in the past.

由于这一限制，我想说的是，对于具有低负载电阻和/或小输出电流的应用，两次AMP电流泵是一个不错的选择。

Conclusion

We’ve taken a quick look at a bidirectional current-source circuit that has reasonable BOM requirements and incorporates a differential control-voltage input stage. In the next article, we’ll use LTspice to analyze the circuit’s performance in greater detail.