Current Mirrors in Analog Design: Working, Types, and Limitations

One of the most widely used building blocks in analog design is the current mirror. A current mirror copies (or “mirrors”) a reference current into another branch of a circuit. It provides bias currents to amplifiers, sets operating points, and is crucial in integrated circuit design. In this article, we’ll explore how current mirrors work, their variations, and their practical limitations.

1. What is a Current Mirror?

A current mirror is a circuit that forces the current in one transistor (the output transistor) to match the current in another transistor (the reference transistor). The concept is simple: once one branch is biased with a known current, the mirror replicates this current elsewhere in the circuit.

2. Basic Principle of Operation

In MOSFET implementation, if two transistors are matched and share the same gate-source voltage (V_GS), they ideally carry the same drain current. By forcing a reference current through one device and tying its gate and drain, the voltage developed ensures the other device mirrors this current.

Equation for drain current in saturation (ignoring channel length modulation):

I_D = (1/2) μ C_ox (W/L) (V_GS – V_TH)²

Thus, if both devices have equal W/L ratios and identical parameters, their drain currents are equal.

3. Types of Current Mirrors

a) Simple Current Mirror

Consists of two identical MOSFETs or BJTs. One is diode-connected to set V_GS, the other mirrors the current. It is compact but suffers from limited output resistance.

b) Widlar Current Mirror

Introduces an emitter or source resistor in the reference leg, reducing current without needing very large resistor values. Useful for generating small currents in ICs.

c) Wilson Current Mirror

Uses three transistors to improve output resistance and reduce error from base or channel current. Offers much higher accuracy than the simple mirror.

d) Cascode Current Mirror

Improves output resistance further by stacking transistors, making the output current less dependent on voltage variations. Very common in high-performance analog ICs.

4. Important Parameters

• Output Resistance: Higher output resistance means better current replication independent of output voltage.
• Compliance Voltage: The minimum voltage at the output node required for proper mirroring.
• Accuracy: Affected by transistor mismatch, channel length modulation, and base/gate currents.

5. Limitations of Current Mirrors

• Mismatch: Device mismatches due to process variations degrade accuracy.
• Finite Output Resistance: Channel length modulation reduces mirror accuracy.
• Voltage Headroom: Cascode mirrors require higher supply voltages, which may not be feasible in low-voltage technologies.
• Temperature Sensitivity: Device parameters vary with temperature, affecting current ratios.

6. Applications of Current Mirrors

• Biasing: Provide stable bias currents to analog amplifiers.
• Active Loads: Replace resistive loads in amplifiers to achieve high gain.
• Reference Circuits: Used in bandgap references and voltage regulators.
• Analog IC Building Blocks: Used extensively in op-amps, comparators, and ADCs.

7. Interview Questions on Current Mirrors

• Explain the principle of operation of a current mirror.
• What are the limitations of a simple current mirror?
• Why is cascode current mirror preferred in high-gain circuits?
• How does channel length modulation affect current mirrors?
• Compare Wilson and Widlar current mirrors.

Conclusion

Current mirrors are indispensable in analog IC design, providing stable biasing and enabling high-gain amplifier stages. Their variations, such as Wilson and cascode mirrors, address real-world limitations like finite output resistance and mismatch. For interviews and practical circuit design, mastering current mirrors is essential.