# An Analog Tutorial: Current sources

I was at Burr-Brown in 1988 and I knew Mark Stitt well. He was a mentor to me and I was devastated at his untimely death. He had written an amazing Burr-Brown Current Source app note1 that I have referenced at the end of this tutorial. I will be bringing you a series of Analog and Power basics on Planet Analog with links to EDN with more in-depth articles on these different tutorial topics.

So first, what’s a current source?

A basic current source is a circuit that simply provides current to a load.

Here is a simple single Bipolar Junction Transistor (BJT) transistor current source in Figure 1:

Figure 1

Figure 1: A simple current source made from a single BJT2 (Image courtesy of Reference 2)

James Bryant, retired Analog Devices ‘guru’ says, “…current outputs offer advantages in a number of situations, including analog current-loop signaling (0 mA to 20 mA and 4 mA to 20 mA) in high-noise environments, and level shifting an analog signal across a large potential difference without the use of optical or magnetic isolation techniques.” (See Reference 4)

The Current Mirror)

Now let’s look at a two-transistor current source called a Current Mirror. Here the two transistors, Q1 and Q2 , are matched in Figure 2:

Figure 2

Here Q1 is connected as a diode with the short from collector to base. The two transistors are matched with equal VBE , IB , and IC . R1 > sets the Reference current IREF (Image courtesy of Reference 3)

A “Full Wilson” architecture

The REF200 has a “Full Wilson” architecture which gives high accuracy when designed as an IC with laser trimming of resistors in Figure 3.

Figure 3

The Wilson Current Source is another three-transistor architecture current source. All transistors are identical (this is easy in an IC on a monolithic substrate)(See Reference 3)

Adding another transistor (T4) will improve the accuracy and dynamic range of the current source as seen in Figure 4.

Figure 4

Adding T4 will improve the accuracy and the dynamic range of the Wilson Current Mirror (Image courtesy of Reference 4)

A “Widlar” Current Source3

I never had the pleasure of meeting Bob Widlar, but I did learn about his Widlar Current Source at NYU in my undergraduate studies in 1969 (See Bob Widlar cherry-bombs the intercom speaker on EDN. I also me his brother at the Analog Aficionado dinner. (See Analog Aficionados Dinner 2018: Unique analog moments)

This current source uses lower value resistors in the low kOhm range which is good for IC designs since 1MOhm resistors occupy a pretty large area on an IC. See Figure 5.

Figure 5

The Widlar Current Source is commonly used in the front end of the differential transistor pair in many Op Amps. (Image courtesy of Reference 3)

A “Howland” Current Source4

The Howland Pump has a bipolar output, where the previous architectures above were unipolar. See Figure 6.

Figure 6

The Howland Pump: Beware that this architecture needs precisely matched resistors which is easy with laser trimming in an IC but not as easy in a discrete circuit design. (Image courtesy of Reference 4)

A low cost bipolar current source with discrete amplifiers and resistor4 .

We can also design a Bipolar-current circuit using an Op Amp, An Instrumentation Amp, and a resistor to sense the output current level in a feedback configuration. See Figure 7.

Figure 7

A Bipolar-current Op Amp architecture for a discrete circuit design. (Reference 4)

I have provided a small sampling of current source circuitry and architectures in this article. More tutorials will abound here on Planet Analog.

References

1 Implementation and applications of current sources and current receivers, Burr-Brown application guide AN-165-A, by R. Mark Stitt

3 EEEB273 – Electronics Analysis & Design II by Dr. Jamaludin Bin Omar

4 Current-Output Circuit Techniques Add Versatility to Your Analog Toolbox, by James Bryant

## 1 comment on “An Analog Tutorial: Current sources”

1. Steve Taranovich
March 29, 2019

You can get some matched resistor arrays like those from Vishay. See this article on Discrete resistors vs. network resistors http://www.vishaypg.com/docs/49675/apps105.pdf

These can give you good temperature tracking and accurate resistor matches if you want to build a discrete current source design