Current mirror

Posted on Feb 4, 2014

Two NPN transistors - models 2N2222 or 2N3403 recommended. 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

Current mirror
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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. A current mirror may be thought of as an adjustable current regulator, the current limit being easily set by a single resistance. It is a rather crude current regulator circuit, but one that finds wide use due to its simplicity. In this experiment, you will get the opportunity to build one of these circuits, explore its current-regulating properties, and also experience some of its practical limitations firsthand. Build the circuit as shown in the schematic and illustration. You will have one extra 1. 5 k © fixed-value resistor from the parts specified in the parts list. You will be using it in the last part of this experiment. The potentiometer sets the amount of current through transistor Q1. This transistor is connected to act as a simple diode: just a PN junction. Why use a transistor instead of a...

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