Current Repeater

A type of current mirror circuit used to drive multiple loads using a single reference current. It replicates the reference current into several output branches.

• When identical transistors are used, each output current is approximately equal to the reference current.
• The current repeater is especially useful when the same bias current must be supplied to multiple stages of a circuit.

Need

In practical analog circuits:

• A single bias current is often required in multiple branches.
• Using separate current sources increases circuit complexity.
• Direct connection of multiple loads to a basic mirror reduces accuracy.

To overcome this, a current repeater is used to distribute the same current to multiple outputs efficiently.

Circuit Concept

In a current repeater:

• One transistor acts as the reference transistor.
• Multiple identical transistors act as output transistors.
• All bases are connected together.

If transistors are identical:

I_{C1} = I_{C2} = \cdots = I_{Cn} \approx I_{\text{ref}}

Each branch receives approximately the same current.

Mathematical Analysis

Applying KCL at the collector node of the reference transistor:

I_{\text{ref}} = I_C + I_B + nI_B

Assuming identical transistors:

I_B = \frac{I_C}{\beta}

Substituting:

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

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

Thus,

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

For large \beta:

I_C \approx I_{\text{ref}}

This shows that as the number of output branches increases, base current error increases.

Current Scaling

Different output current values can be achieved by:

• Changing transistor emitter area
• Adding emitter resistors (similar to Widlar configuration)
• Thus, unequal but controlled current distribution is possible.

Key Features

• Provides multiple output currents
• Simple extension of basic current mirror
• Useful for bias distribution
• Accuracy depends on \beta
• Suitable for identical current replication

Advantages

• Multiple Outputs from Single Reference : One reference current can drive several loads.
• Simple Design : Only additional matched transistors are required.
• Compact Implementation : Easily fabricated in ICs.
• Uniform Current Distribution : Identical branches receive nearly equal current.

Disadvantages

• Base Current Error Increases with n : More output branches increase error.
• Depends on Transistor Matching : Mismatch causes unequal currents.
• Finite \beta Reduces Accuracy : Output current slightly less than reference.
• Limited Output Resistance : Affected by Early effect.

Applications

1. Bias Distribution Networks : Supplies identical bias currents to multiple amplifier stages.

2. Differential Amplifier Arrays : Provides equal tail currents in multi-stage systems.

3. Operational Amplifiers : Distributes internal bias current to different internal blocks.

4. Analog IC Design : Used where repeated identical currents are required.

5. Current Steering Circuits : Directs controlled current to multiple branches.