How to Build a Current Mirror Circuit

Chapter 5 - Discrete Semiconductor Circuits

PARTS AND MATERIALS
•   Two NPN transistors—models 2N2222 or 2N3403 recommended (Radio Shack catalog # 276-1617 is a package of fifteen NPN transistors ideal for this and other experiments)
•   Two 6-volt batteries
•   One 10 kΩ potentiometer, single-turn, linear taper (Radio Shack catalog # 271-1715)
•   Two 10 kΩ resistors
•   Four 1.5 kΩ resistors
Small signal transistors are recommended so as to be able to experience “thermal runaway” in the latter portion of the experiment. Larger “power” transistors may not exhibit the same behavior at these low current levels. However, any pair of identical NPN transistors may be used to build a current mirror. Beware that not all transistors share the same terminal designations, or pinouts, even if they share the same physical appearance. This will dictate how you connect the transistors together and to other components, so be sure to check the manufacturer’s specifications (component datasheet), easily obtained from the manufacturer’s website. Beware that it is possible for the transistor’s package and even the manufacturer’s datasheet to show incorrect terminal identification diagrams! Double-checking pin identities with your multimeter’s “diode check” function is highly recommended. For details on how to identify bipolar transistor terminals using a multimeter, consult chapter 4 of the Semiconductor volume (volume III) of this book series. CROSS-REFERENCES Lessons In Electric Circuits, Volume 3, chapter 4: “Bipolar Junction TransistorsLEARNING OBJECTIVES
•   How to build a current mirror circuit
•   Current limitations of a current mirror circuit
•   Temperature dependence of BJTs
•   Experience a controlled “thermal runaway” situation
Current mirror v1 1 0 vammeter 1 3 dc 0 rlimit 1 2 10k rload 3 4 3k q1 2 2 0 mod1 q2 4 2 0 mod1 .model mod1 npn bf=100 .dc v1 12 12 1 .print dc i(vammeter) .end