Current Mirror

Overview

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A current mirror is a fundamental analog circuit that replicates (mirrors) a reference current to create a controlled output current.

Basic Operation

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    VDD                    VDD
     |                      |
   [M1]                   [M2]
     |                      |
     +------- Vg -----------+
     |                      |
   I_ref                  I_out
     |                      |
    GND                   Load

Principle:

• Left side (M1): Reference - establishes gate voltage through biasing
• Right side (M2): Output - mirrors the reference current

Current Relationship

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For matched transistors:

If \((W/L)_1 = (W/L)_2\):

Cascode Current Mirror

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Problem: Basic mirror has finite output impedance.

Solution: Cascode configuration using M1-M2 to absorb drain voltage variations.

      VDD
       |
     [M3] ← Cascode device
       |
     [M1] ← Main mirror
       |
     I_ref

Benefits:

• Higher output impedance: \(R_{out} = g_m \cdot r_{o1} \cdot r_{o2}\)
• Better current accuracy
• Reduced channel length modulation effects

Wide-Swing Current Mirror

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Detects \(I_{in}\) and replicates to \(I_{out}\) through self-adjusting gate voltage feedback.

Features:

• Maintains saturation operation
• Precise current matching
• Extended output voltage range

Transistor vs Resistor Bias

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When to Use Resistors

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Advantages:

• Simplified design
• Linear V-I relationship
• Better temperature stability
• Lower noise
• Lower cost

Disadvantages:

• Less flexible bias adjustment
• Reduced current control
• Impedance matching difficulties

Design Considerations

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1. Matching - Use common-centroid layout
2. Output impedance - Consider cascode for high \(R_{out}\)
3. Headroom - Wide-swing for low VDD
4. Noise - Larger transistors for lower noise
5. Mismatch - Increase W×L product for better matching

Applications

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• Bias current generation
• Active loads in amplifiers
• Current DACs
• Reference current distribution
• Differential pair biasing