ADALM2000 Experiment: Stable Current Source

This article will focus on stable current sources using bipolar junction transistors (BJTs) and NMOS transistors.

Stable current source (BJT)

Target

The purpose of this experiment is to investigate how the zero gain concept can be used to produce a stable (less sensitive to changes in input current level) output current.

Material

► ADALM2000 active learning module

►Solderless Breadboard

► One 2.2 kΩ resistor (or other similar value)

► One 100 Ω resistor

► One 4.7 kΩ resistor

► Two small signal NPN transistors (2N3904 or SSM2212)

illustrate

The circuit corresponding to the BJT stable current source is shown in Figure 1.

Hardware Setup

Figure 1. Steady current source

Hardware Setup

The breadboard connections are shown in Figure 2. The output of W1 drives one end of resistor R1. Resistors R1 and R2 and transistor Q1 are connected as shown in the November 2020 StudentZone article. Since Q2's VBE is always less than Q1's VBE, Q1 and Q2 are selected from stock parts where possible so that Q2's VBE is less than Q1's VBE at the same collector current. The base of transistor Q2 is connected to the zero-gain output of Q1's collector. R3 is connected between the Vp supply and Q2's collector and is used with the 2+ oscilloscope input to measure the collector current.

Figure 2. NMOS zero-gain amplifier breadboard circuit

Procedure

A zero-gain amplifier can be used to create a stable current source. Now, as the input supply voltage represented by W1 varies, the voltage seen by the collector of transistor Q1 is more stable, so it can be used as the base voltage of Q2 to produce a more stable current in transistor Q2.

The waveform generator is configured for a 1 kHz triangle wave with a peak-to-peak amplitude of 3 V and an offset of 1.5 V. The input (2+) of oscilloscope Channel 2 is used to measure the regulated output current at the collector of Q2.

Configure the oscilloscope to capture multiple cycles of the two signals being measured. Make sure the XY function is enabled.

Examples of waveform diagrams using an oscilloscope are shown in Figures 3 and 4.

Figure 3. Relationship between Q2 collector voltage and W1 voltage

Figure 4. Oscilloscope plot of Q2 collector current vs. W1 voltage

Stable current source (NMOS)

Material

► ADALM2000 active learning module

►Solderless Breadboard

► One 2.2 kΩ resistor (or other similar value)

► One 168 Ω resistor (100 Ω and 68 Ω resistors in series)

► One 4.7 kΩ resistor

► Two small signal NMOS transistors (CD4007 or ZVN2110A)

illustrate

The circuit corresponding to the MOS stable current source is shown in Figure 5.

Figure 5. Stable current source

Figure 6. Steady current source breadboard circuit

Hardware Setup

The breadboard connections are shown in Figure 6. The output of waveform generator W1 drives one end of resistor R1. Resistors R1 and R2 and transistor M1 are connected as shown in the November 2020 StudentZone article. Since the VGS of M2 is always less than the VGS of M1, if possible, M1 and M2 are selected from component inventory so that the VGS of M2 is less than the VGS of M1 at the same drain current. The gate of transistor M2 is connected to the zero-gain output of the drain of M1. R3 is connected between the Vp supply and the drain of M2 and is used with the 2+ oscilloscope input to measure the drain current.

Procedure

The waveform generator is configured for a 1 kHz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 2 V. The input (2+) of oscilloscope Channel 2 is used to measure the regulated output current at the drain of M2.

Configure the oscilloscope to capture multiple cycles of the two signals being measured. Make sure the XY function is enabled.

Figure 7 provides an example of an image of an oscilloscope display.

Figure 7. Relationship between M2 drain voltage and W1 voltage

Questions about BJT and NMOS

►This type of circuit is sometimes called a peaking current source. Why do you think this naming convention is used?

►The peak value of the output current is very narrow. How can I adjust the circuit to produce a wider, flatter peak?

You can find the answers to your questions on the StudentZone blog.

About the Author

Doug Mercer graduated from Rensselaer Polytechnic Institute (RPI) in 1977 with a bachelor’s degree in electrical engineering. Since joining Analog Devices in 1977, he has contributed directly or indirectly to more than 30 data converter products and holds 13 patents. He was named an ADI Fellow in 1995. In 2009, he transitioned from full-time employment and continues to serve as a consultant to ADI as a Fellow Emeritus, contributing to the Active Learning Program. In 2016, he was named Engineer-in-Residence for the ECSE Department at RPI. He can be reached at doug.mercer@analog.com.

Antoniu Miclaus is a system applications engineer at Analog Devices, working on ADI educational projects and developing embedded software for Circuits from the Lab®, QA automation, and process management. He joined Analog Devices in February 2017 in Cluj-Napoca, Romania. He is currently an MSc student in the Master of Software Engineering program at Bebis Bolyai University and holds a B.A. in Electronics and Telecommunications Engineering from the Technical University of Cluj-Napoca. He can be reached at antoniu.miclaus@analog.com.