Modified Current Mirror

In a basic current mirror, the output current accuracy depends on the transistor current gain \beta. When the value of \beta is small or when multiple matched output transistors are used, the base current drawn by each transistor increases the total error. As a result, the mirrored current becomes smaller than the reference current.

• To overcome this limitation without changing the transistor current gain \beta, a Modified Current Mirror is used.
• This configuration reduces the error caused by base currents and improves current mirroring accuracy.

Need

In a basic current mirror:

• The reference transistor must supply base currents of all output transistors.
• As the number of output branches increases, total base current increases.
• For lower values of \beta, the error becomes significant.
• The output current reduces from the ideal value.

Therefore, an improved configuration is required to minimize base current error.

Circuit Concept

In the modified current mirror:

• An additional transistor is introduced.
• This transistor supplies the required base currents.
• The reference branch is relieved from feeding multiple base currents.
• Accuracy improves even when \beta is moderate.

This configuration works effectively when the number of output transistors (n+1) is greater than 10.

Mathematical Analysis

Applying KCL at the reference node:

I_{\text{ref}} = I_C + I_{B1} + I_{B2}

Since:

I_B = \frac{I_C}{\beta}

For identical transistors:

I_{C1} = I_{C2} = I_C

Substituting:

I_{\text{ref}} = I_C + \frac{I_C}{\beta} + \frac{I_C}{\beta}

I_{\text{ref}} = I_C\left(1 + \frac{2}{\beta}\right)

In general, for n+1 total transistors:

I_C = \frac{I_{\text{ref}}}{1 + \frac{n+1}{\beta(\beta+1)}}

For large n and practical cases:

I_C \approx \frac{I_{\text{ref}}}{1 + \frac{n+1}{\beta(\beta+1)}}

Thus, the modified circuit significantly reduces the effect of base current error.

Key Features

• Reduces base current error
• Improves accuracy of mirrored current
• Works better for moderate \beta
• Suitable for multiple output branches
• Does not require changing transistor gain

Advantages

• Improved Accuracy : Output current is closer to reference current.
• Less Dependence on \beta : Performs well even when current gain is moderate.
• Suitable for Multiple Outputs : Effective when many output transistors are used.
• Better Bias Stability : Used in precision bias circuits.
• No Need for Very High \beta : Practical for real transistors.

Disadvantages

• Increased Circuit Complexity : More transistors compared to basic mirror.
• Higher Chip Area : Slight increase in silicon area in IC fabrication.
• Still Not Ideal : Some dependence on \beta remains.
• Design Complexity : Requires careful transistor matching.

Applications

1. Bias Networks in Analog ICs : Used to generate stable bias currents.

2. Differential Amplifiers : Provides accurate current replication for active loads.

3. Operational Amplifiers : Used in internal current biasing stages.

4. Current Distribution Circuits : Supplies identical current to multiple branches.

5. Analog Integrated Circuits : Used where high-accuracy current mirroring is required.