“Exciting project alert! I’ve implemented a Wilson Current Mirror on a breadboard, showcasing precision current mirroring in analog circuits.
Current mirrors are fascinating analog circuits that can replicate or mirror currents in 2 different parts of an electrical system. There are different types of current mirrors and they are used in various applications like PLL(phase-locked loops), Sense Amplifiers, Cascode Amplifiers, etc. as reference current elements. In this project, I have used TinkerCad to design and implement a Wilson Current mirror circuit on a breadboard.
Previously, I was working on an article related to the mitigation of input offset and input bias currents in an Op-Amp Integrator. While studying the simplified Op-Amp model, I realized that if these currents were to be made equal via some procedure, then they would not be integrated under zero signal conditions. There were multiple ways to do so and, one of them is by using a current mirror between the inverting and non-inverting input terminals. I found this application of current mirrors quite mind-blowing and hence decided to do this project.
Working of the circuit:
1) The circuit consists of two pairs of matched NPN transistors (Q1/Q2 and Q3/Q4) arranged in a ladder-like fashion.
2) The input current (I-ref) flows into the base of Q1. Q1 mirrors this current, and Q2 mimics the behavior of Q1, resulting in a mirrored current at the collector of Q2.
3) Q3 and Q4 form an intermediate pair. The collector of Q2 is connected to the base of Q3. This connection ensures that Q3 mirrors the current from Q2, producing a mirrored current at the collector of Q3.
4) The final output current (Iout) is drawn from the collector of Q4. Since Q4 mirrors the current from Q3, the output current closely replicates the input current I-ref, with high precision.
Key Advantages:
- High Accuracy: Minimizes static error (Iin - Iout) compared to simpler current mirrors.
- High Output Impedance: Negative feedback boosts output impedance, improving stability and reducing noise sensitivity.
- Minimal Resource Usage: Requires only three transistors and one resistor.
Limitations and Considerations:
- Higher Headroom Requirements: Needs larger minimum voltage difference between input/output and common rail than simpler mirrors.
- Potential Noise Increase: The feedback loop can amplify collector current fluctuations from Q3, contributing to noise at the output.
- High-Frequency Considerations: High-frequency biasing can introduce peaking in the frequency response due to the loop.
We are done with the theory part of the project. Now, let’s move on to the connection part -
Set the potentiometer to a level between 3.7 to 4.7 Kilohms. Now, the circuit is ready to be used. Turn on the voltage supply and adjust the potentiometer to observe the currents in the reference terminal and output terminal. The currents should be the same with a variation of 5 to 10 percent.”