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Activity 8. Stabilized current source (BJT)


The objective of this activity is to investigate the use of the zero gain concept to produce an output current which is stabilized (less sensitive) to variations of the input current level.


Analog Discovery Lab hardware
Solder-less breadboard
1 - 2.2KΩ Resistor ( or any similar value )
1 - 100Ω Resistor
1 - 4.7KΩ Resistor
2 - small signal NPN transistors (2N3904 or SSM2212)


The breadboard connections are as shown in figure 1 below. The output of the AWG1 drives one end of resistor R1. Resistors R1, R2 and transistor Q1 are connected as in previous zero gain amplifier section. Since the VBE of Q2 is always smaller than the VBE of Q1, you should, if possible, select Q1 and Q2 from your inventory of devices such that (at the same collector current) Q2's VBE is less than Q1's VBE. The base of transistor Q2 is connected to the zero gain output at the collector of Q1. R3, connected between the Vp supply and the collector of Q2, is used along with the 2+ (Single Ended) scope input to measure the collector current.

Figure 1 Stabilized current source

Hardware Setup:

The waveform generator should be configured for a 1 KHz triangle wave with 1.5 volt amplitude and 1.5V offset. The input of scope channel 2 (2+) is used to measure the stabilized output current at the collector of Q2.


The zero gain amplifier can be used to create a stabilized current source. Because the voltage seen at the collector of transistor Q1 is now more constant with changes in the input supply voltage as represented by AWG1, it can be used as the base voltage of Q2 to produce a much more constant current in transistor Q2.

Figure 2 Q2 collector current vs. W1 voltage


This circuit is sometimes referred to as a peaking current source. Why do you think?

Based on the delta VBE of Q1 and Q2, at what input and output current would the gain be zero for different values of R2?

An exercise for the reader is to plot the “stabilized” output current for all the various combinations of Q1 and Q2 from the available inventory of transistors. Why does it vary and by how much? The output of the simple peaking current source is always less than the input current at the peak by a substantial fraction. What is that fraction and why?

How can the circuit be changed to make the output a larger fraction of, or even equal to or larger than, the input?

The output current has a narrow peak. How could multiple copies of the peaking current source be combined to produce a much wider, flatter peak?

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university/courses/electronics/electronics-lab-8.1355516983.txt.gz · Last modified: 14 Dec 2012 21:29 by Doug Mercer