Build and Understand Current Mirror Circuits
A current mirror is one of the most important building blocks in analogue electronics. Rather than copying a voltage, it copies (or "mirrors") a current from one branch of a circuit into another. Current mirrors are found inside almost every analogue integrated circuit, including operational amplifiers, voltage regulators, audio amplifiers, instrumentation amplifiers, analogue-to-digital converters and voltage references. Understanding current mirrors is an important step towards learning professional analogue circuit design.
Project Overview
A basic current mirror consists of two closely matched transistors.
One transistor is configured to establish a reference current, while the second transistor reproduces nearly the same current in another branch of the circuit.
Since the mirrored current remains nearly constant over a wide range of load voltages, current mirrors behave like constant-current sources.
Project Difficulty
| Item | Value |
|---|---|
| Difficulty | ⭐⭐⭐⭐ Advanced |
| Build Time | 3–6 Hours |
| Supply Voltage | 5–15 V DC |
| Test Equipment | Multimeter and Oscilloscope |
| Main Components | Matched Transistors |
Types of Current Mirrors
| Type | Description |
|---|---|
| Simple BJT Current Mirror | Uses two matched bipolar transistors. |
| Wilson Current Mirror | Provides improved accuracy and output resistance. |
| Widlar Current Source | Produces very small currents efficiently. |
| Cascode Current Mirror | Offers excellent output resistance and accuracy. |
| MOSFET Current Mirror | Widely used inside CMOS integrated circuits. |
Basic Circuit
+VCC │ R │ ┌──┴──┐ │ │ Q1 Q2 │ │ └──┬──┘ │ GND
Q1 establishes the reference current while Q2 reproduces nearly the same current in the output branch.
Typical Components
| Quantity | Component |
|---|---|
| 2 | Matched NPN Transistors (BC547, 2N3904, etc.) |
| 1 | Resistor (Reference Current) |
| 1 | DC Power Supply |
| 2 | Digital Multimeters |
| 1 | Breadboard |
| Several | Connecting Wires |
How a Current Mirror Works
The collector and base of transistor Q1 are connected together, forcing it to operate as a diode-connected transistor.
The current flowing through Q1 creates a base-emitter voltage (VBE).
Because Q2 shares the same base-emitter voltage and is closely matched to Q1, it conducts nearly the same collector current.
As long as Q2 remains in its active operating region, the output current remains almost constant even if the load voltage changes.
Advantages
- Provides nearly constant current.
- Simple circuit with few components.
- Excellent current stability.
- Widely used inside integrated circuits.
- Suitable for biasing analogue amplifiers.
- Improves amplifier performance.
Applications
- Operational amplifiers.
- Differential amplifiers.
- Current sources.
- Voltage regulators.
- Analogue integrated circuits.
- Current steering circuits.
- Audio amplifier bias circuits.
- Instrumentation amplifiers.
- Oscillators.
- Precision analogue systems.
Testing
- Assemble the current mirror on a breadboard.
- Apply the supply voltage.
- Measure the reference current.
- Measure the mirrored output current.
- Change the output load resistance.
- Observe how the current remains nearly constant.
- Compare theoretical and measured values.
- Repeat using different transistor pairs.
Troubleshooting
| Problem | Possible Cause |
|---|---|
| Output current incorrect | Transistors not well matched. |
| Current changes with load | Output transistor leaving the active region. |
| No output current | Incorrect transistor pin connections. |
| Current unstable | Temperature differences between transistors. |
| Poor accuracy | Low transistor gain or resistor tolerance. |
Project Improvements
- Build a Wilson current mirror.
- Construct a Widlar current source.
- Design a MOSFET current mirror.
- Measure output resistance.
- Investigate temperature compensation techniques.
- Compare matched and unmatched transistor pairs.
- Use precision current mirrors in audio amplifiers.
- Implement a cascode current mirror.
Skills Learned
- BJT transistor operation.
- Constant-current sources.
- Current biasing techniques.
- Analogue integrated circuit design.
- Differential amplifier design.
- Precision analogue electronics.
Safety Notes
- Verify transistor pinouts before wiring.
- Observe maximum voltage and current ratings.
- Use matched transistor pairs for the best performance.
- Avoid overheating the transistors during soldering.
- Disconnect the power before modifying the circuit.
Key Points
- A current mirror copies a reference current into another branch of a circuit.
- It acts as a simple and effective constant-current source.
- Current mirrors are fundamental building blocks in analogue ICs.
- Matched transistors improve accuracy and stability.
- Understanding current mirrors is essential for advanced analogue circuit design.