Now let's look at a two-transistor current source called a Current Mirror. A small sampling of current source circuitry and architectures are also provides in this article...
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: 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 Source
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 Source
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.