Current mirror

describes the working principle of the current mirror circuit along with simulation.

1. VEGEteK - 006 Current mirror circuit

2. Topics of this tutorial 1- What is a current mirror? 2- Theory of operation. 3- Applications of current mirror. 4- Simulation. 5- Practical example.

3. What is a current mirror? It is a circuit that outputs a constant current that is equal to another current called “reference current”. I _ref I _out Figure 1: Current mirror basic circuit Q1 along with the series resistance before it determines the reference current. While Q2 is responsible of delivering the output current or mirrored current from the voltage source to the load. I = V / R Since resistance is not known and current is to be made constant, then current mirror circuit operates by changing voltage.

4. Q 1 DC LOAD Base current GND NPN transistors works by supplying current to the gate. More current to the gate allows more collector current (which is effectively the output current) to flow according to the following equation:   𝛽 (beta) is the “DC gain” which determines how much can the transistor amplify its input (base) current. This is why BJTs are called “current-driven” devices. The emitter current is determined by this equation:   Collector Current Emitter Current More gate current -> More collector and emitter current

5.           Collector current is given by this equation:   Since the two transistors are identical:   Thus makes the output current:   Since base current is small compared to collector current, we can assume:   Q 1 Q 2 R 1 DC DC R 2 GND 1

6. It is used whenever constant current is required regardless of source voltage. Making a constant minimum load for power supply circuits. Making multiple current sources copied from one source. Current mirror makes a low impedance current source from a poor current source. Stabilize current against temperature change since the circuit changes voltage . Applications of current source