Wilson Current Mirror

An improved current mirror configuration that uses three transistors to provide highly accurate current replication.

• It significantly reduces base current error and increases output resistance compared to the basic two-transistor current mirror.
• Because of its improved accuracy and high output impedance, the Wilson current mirror is widely used in precision analog integrated circuits.

Need

In a basic current mirror:

• Base current causes output current error.
• Output resistance is limited due to the Early effect.
• Accuracy depends heavily on \beta

To overcome these limitations, the Wilson current mirror introduces a feedback mechanism using an additional transistor.

Circuit Concept

The Wilson current mirror uses three transistors:

• Q1: Reference transistor
• Q2: Output transistor
• Q3: Feedback transistor

The third transistor provides negative feedback that:

• Compensates base current error
• Increases output resistance
• Improves current matching

Mathematical Analysis

Applying KCL at internal nodes and assuming identical transistors:

V_{BE1} = V_{BE2}

I_{C1} = I_{C2}

After analysis, the output current is obtained as:

I_{out} =\frac{\beta^2 + 2\beta}{\beta^2 + 2\beta + 2} I_{ref}

For large \beta:

I_{out} \approx I_{ref}

The error between I_{out} and I_{ref} is:

I_{out} - I_{ref}\approx\frac{2}{\beta^2 + 2\beta + 2} I_{ref}

This error is extremely small even for moderate values of \beta.

Output Resistance Improvement

The Wilson mirror increases output resistance approximately by a factor of:

\frac{\beta}{2}

This makes it behave much closer to an ideal current source compared to the basic mirror.

Key Features

• Uses three transistors
• Very small static current error
• High output resistance
• Negative feedback improves stability
• Suitable for precision current mirroring

Advantages

• Very High Accuracy : Base current error is greatly reduced.
• High Output Resistance : Improved current source behavior.
• Reduced Dependence on \beta : Works well even with moderate gain.
• Better Stability : Negative feedback enhances performance.

Disadvantages

• Higher Minimum Voltage Requirement : Requires more voltage headroom than basic mirror.
• Increased Circuit Complexity : Uses three transistors.
• Higher Noise Contribution : All three transistors contribute noise.
• Not Suitable for Multiple Independent Outputs : Cannot easily generate many identical outputs from one reference.

Applications

1. Precision Bias Circuits : Used where accurate current matching is required.

2. Operational Amplifiers : Forms high-accuracy internal current mirrors.

3. Active Loads in Amplifiers : Provides high output resistance for higher gain.

4. Analog Integrated Circuits : Used in high-performance analog IC blocks.

5. Low-Error Current Sources : Suitable where minimal input-output error is required.