Current Mirror
Category: Analog Circuits
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This is a simple current mirror circuit. The schematic and simulation is made in Proteus 7.10. Therefore, if you want to do modification or perform some more experiment using this proteus schematic file (.dsn) you should have Proteus 7.10. In this circuit you will learn or get the idea how current mirror circuit works. Following are the key points. 1. The basic principle relies on the fact that matched transistors will have the same collector current, provided that their base-emitter junctions have the same voltage across them. 2. By Connecting all the bases and all the emitters together, every base-emitter juction voltage must be equal and therefore every collector current must be equal. 3. The primary LED string (second string with transistor Q1) is the one that controls the current through the other strings. Since the collector and base of the transistor Q1 are connected. 4. The transistor will be fully conducting until the voltage falls low enough for the base-emitter current to limit. 5. Other transistors Q2 to Q6 have their base connected to Q1. 6. Therefore, Q2 to Q6 will conduct exactly same collector curren (transistors are matched). 7. While the left most string is out of current mirror circuit, therefore its current is different. 8. The voltage drop across the LEDs of the primary string (second string with transistor Q1) must be higher than other strings in order for the current mirror to work properly. In the slave strings (Q2 to Q6), some voltage will be dropped across the collector-emitter juction of the transistors Q2 to Q6. 9. The slave circuits adjust the current by raising or lowering this surplus voltage drop across the transistor. Application: This circuit can be used to drive parallel strings of LEDs.