The Problem :
A MOSFET current mirror is a circuit used to copy (mirror) a current from one branch to another, ensuring consistent current flow in different sections of a circuit. This is crucial in amplifiers and sensor circuits where stable current is essential. However, beginners often face issues with incorrect resistor values or mismatched MOSFETs, leading to unstable performance.
The Solution :
To fix current instability, use matched MOSFETs (such as 2N7000 or IRF540) and calculate the resistor values precisely. A simple rule is to choose a resistor that ensures the voltage drop across it matches the desired current. For example, if you need 1mA and have a 5V source, use a resistor of R = V/I = 5V/1mA = 5kΩ.
Practical Example :
Imagine designing a temperature sensor circuit using a MOSFET current mirror. Without proper matching, the sensor might give fluctuating readings. By using matched MOSFETs and correctly sized resistors, the current through the sensor stabilizes, improving the accuracy of readings.
Sample Calculation :
To mirror a 2mA current using a 5V supply:
Desired current: 2mA
Resistor: R = 5V / 2mA = 2.5kΩ
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