Current Mirror: Working and Circuit

What is Current Mirror?

A current mirror is an electronic circuit used in analog and mixed-signal applications to replicate or mirror the current flowing through one active device and produce an identical current in another device. The primary purpose of a current mirror is to maintain a consistent current in multiple parts of a circuit, ensuring that different components receive the same current for precise operation.

The basic idea behind a current mirror involves connecting two transistors in such a way that the current flowing through one transistor is mirrored or copied by the other transistor. Bipolar junction transistors (BJTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs) are commonly used as the active devices in current mirrors.

There are various configurations of current mirrors, such as the simple or basic current mirror, the Wilson current mirror, and the cascode current mirror, each with its own advantages and applications. Current mirrors are frequently employed in analog integrated circuits, operational amplifiers, voltage references, and other electronic systems where precise current matching is essential for proper functionality.

Use of Current Mirror

A current mirror circuit is used in electronic circuits to replicate or mirror a current from one part of the circuit and provide an identical current in another part. The primary purpose of a current mirror is to ensure that different components or branches of a circuit receive the same current, maintaining consistency and precision in the operation of the circuit.

Current Mirror Circuit Diagram

1. Power Supply and Biasing:
   • Vdd serves as the power supply voltage.
   • Resistor R is connected to Vdd, indicating its role in setting up a bias current for the current mirror.
2. Master Transistor (Q1):
   • Q1 is configured as a PMOS transistor.
   • The drain of Q1 is connected to Vdd, indicating a common-source configuration.
   • The gate of Q1 is connected to resistor R and is used to establish a bias voltage.
   • The source of Q1 is connected to the ground.
3. Slave Transistor (Q2):
   • Q2 is also a PMOS transistor.
   • The gate of Q2 is connected to the gate of Q1, establishing a current mirror configuration. The gate of Q2 mirrors the gate voltage of Q1, creating a mirrored current.
   • The source of Q2 is connected to the ground.
   • The drain of Q2 serves as the output (Vout) of the current mirror circuit.

Working of Current Mirror

Working:

1. Basic Configuration: The simplest form of a current mirror consists of two transistors – a reference transistor (master) and a matching transistor (slave). These transistors are typically connected in a way that the current flowing through the reference transistor is mirrored or duplicated by the slave transistor.
2. Identical Transistors: For an effective current mirror, the transistors involved (usually MOSFETs or bipolar junction transistors) should be as identical as possible in terms of their characteristics, like threshold voltage, mobility, and size. This ensures that the current mirror operates accurately.
3. Biasing: The reference transistor is biased with a fixed current source, and its collector or drain current is established based on this bias. The goal is to maintain a constant voltage across the reference transistor.
4. Replication: The replicated current flows through the matching transistor (slave) due to the similarity in their configurations. The matching transistor adjusts its collector or drain current to match that of the reference transistor.
5. Output Current Stability: The key to the current mirror’s functionality is the feedback mechanism. As the replicated current flows through the matching transistor, any deviation in its characteristics (due to temperature changes or manufacturing variations) causes the voltage across the transistor to adjust, bringing the current back in line with the reference.

Applications of Current Mirror

1. Biasing Circuits:
   • One of the primary applications of current mirrors is in biasing circuits. They are used to provide a stable and well-defined bias current for other circuit components, such as amplifiers and differential pairs. By mirroring a reference current, a current mirror ensures that the biasing conditions are consistent and can help maintain the operating point of the circuit.
2. Current Sources:
   • Current mirrors are often employed to generate precise current sources. By mirroring a known reference current, they create a stable output current that can be utilized in various analog circuits. This is particularly important in applications where a constant current is required, such as in current-steering digital-to-analog converters (DACs) or voltage regulators.
3. Differential Amplifiers:
   • In differential amplifier configurations, current mirrors play a crucial role in maintaining balanced operation. By providing mirrored currents to the input differential pairs, they contribute to improved common-mode rejection and symmetrical signal handling. This enhances the performance of differential amplifiers, which are commonly used in operational amplifiers and communication systems.
4. Cascode Amplifiers:
   • Cascode amplifiers, which consist of a common-emitter stage cascaded with a common-base stage, often use current mirrors. The current mirror helps maintain a consistent bias current for both stages, improving the overall performance of the cascode amplifier. This is especially beneficial in applications where high gain and a wide bandwidth are desired, such as in radio frequency (RF) circuits.

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