Current Mirror Circuits

Last Updated : 9 Mar, 2026

Current mirror circuits are widely used in analog electronics to generate a constant current by copying (mirroring) a reference current. They are fundamental building blocks in biasing networks, differential amplifiers, and integrated circuit design.

  • A current mirror takes a known reference current at its input and forces an approximately equal current at its output, independent of load variations (within limits).
  • The concept relies on matched transistors operating in the active region.

Basic Current Mirror Circuit

A basic BJT current mirror consists of two identical transistors:

1
Basic Current Mirror Circuit
  • Q1 is diode-connected (collector tied to base).
  • Q2 acts as the output transistor.
  • A resistor R_1 sets the reference current I_{\text{ref}}

Working Principle

1. The reference current I_{\text{ref}} flows through Q1.

2. This establishes a base-emitter voltage V_{BE1}

3. Since Q1 and Q2 are matched and their bases are connected together:

V_{BE1} = V_{BE2}

4. Therefore, the emitter current of Q2 becomes approximately equal to that of Q1.

5. As long as Q2 remains in the active region:

I_{\text{out}} = I_{C2} \approx I_{\text{ref}}

Thus, the output current is a mirror of the reference current.

Effect of Finite \beta on Current Mirror

The ideal mirror assumes very large \beta. In practice, base currents affect accuracy.

Applying KCL at the collector node of Q1:

I_{\text{ref}} = I_{C1} + I_{B1} + I_{B2}

Since:

I_B = \frac{I_C}{\beta}

For identical transistors:

I_{C1} = I_{C2} = I_C

So,

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

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

Hence,

I_C = \frac{I_{\text{ref}}}{1 + \frac{2}{\beta}}

For large \beta, the denominator approaches 1, and:

I_C \approx I_{\text{ref}}

Output Resistance of Current Mirror

Due to the Early effect, the collector current slightly depends on V_{CE}. Hence, the current mirror does not behave as an ideal current source and has finite output resistance.

The small-signal output resistance is:

r_o = \frac{V_A + V_{CE}}{I_C}

Where:

V_A = Early voltage

V_{CE} = Collector-to-emitter voltage

I_C = Collector current

A larger r_o means better current source behavior.

Compliance Voltage

For proper operation, the output transistor must remain in the active region.

For a BJT current mirror, the minimum collector-emitter voltage required is:

V_{CE(\min)} \approx V_{CE(\text{sat})}

This minimum required output voltage is called the compliance voltage.

If the output voltage falls below this limit, the transistor enters saturation and the current mirror no longer maintains a constant output current.

Current Scaling

If the transistors are not identical and their emitter (or channel) areas are different, the output current scales proportionally:

I_{\text{out}} = \left(\frac{A_2}{A_1}\right) I_{\text{ref}}

where:

A_1 = area of reference transistor

A_2 = area of output transistor

This allows current multiplication or division and is widely used in analog integrated circuit design.

Types

Based on performance improvement and application requirements, current mirrors are classified into the following types:

  1. Modified Current Mirror
  2. Widlar Current Mirror
  3. Current Repeater
  4. Wilson Current Mirror
  5. MOSFET Current Mirror

Advantages

  • Simple Circuit Structure : The basic current mirror uses only two matched transistors, so it is easy to design and implement.
  • Low Input Impedance : The input transistor is diode-connected, which provides low input resistance and helps maintain stable reference current.
  • High Output Impedance : The output transistor behaves like a current source, giving high output resistance and nearly constant current.
  • Easy Integration in IC Fabrication : Both transistors can be fabricated on the same chip, ensuring good matching and compact layout.

Disadvantages

  • Depends on Transistor Matching : Any mismatch in transistor parameters causes error in mirrored current.
  • Finite \beta Reduces Output Current : Because base current is required, the output current is slightly less than the reference current.
  • Early Effect Causes Variation : Changes in collector-emitter voltage V_{CE} affect collector current, reducing accuracy.
  • Higher Noise Without Emitter Resistors : Without emitter degeneration resistors, the circuit becomes more sensitive to noise and temperature variations.

Applications

Current mirrors are extensively used in analog and mixed-signal circuits.

1. Biasing Networks : Provide stable DC bias currents in transistor amplifiers.

2. Differential Amplifiers : Used as active loads to increase voltage gain.

3. Operational Amplifiers : Form internal bias circuits and active loads in IC op-amps.

4. Current Sources and Current Sinks : Used wherever constant current is required independent of load resistance.

5. Analog Integrated Circuits : Essential building blocks in

  • Analog ICs
  • Voltage regulators
  • Current steering circuits
  • DAC and ADC architectures
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