A current mirror is a fundamental circuit designed to replicate a specific current from one part of a circuit to another, ensuring the output current remains stable despite changes in the load. Essentially, it copies a current from an input active device and uses it to control the current in an output active device, allowing the copied current to be either a steady direct current (DC) or a varying signal current.
Understanding the Core Concept
The primary function of a current mirror is to provide a stable, controlled current that is a copy or multiple of a reference current. This is achieved by exploiting the matched characteristics of two or more transistors.
Here's how the core concept translates into operation:
- Current Replication: The circuit takes an input or "reference" current and reproduces it, or a scaled version, as an output current.
- Control Mechanism: One active device (the input side) establishes a specific voltage or operating point based on the reference current. This voltage is then used to control a second active device (the output side), forcing it to conduct a current that "mirrors" the reference.
- Load Independence: A key advantage is its ability to maintain a constant output current, largely independent of the resistance or voltage of the load connected to its output.
Key Components and Basic Structure
The simplest form of a current mirror typically uses two matched transistors, most commonly Bipolar Junction Transistors (BJTs) or Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), operating in the same conditions.
A common BJT current mirror configuration includes:
- Reference Transistor (Q1): This transistor is often "diode-connected" (its collector and base are shorted together). A reference current (I_REF) flows through Q1, establishing a specific base-emitter voltage (V_BE) across it.
- Output Transistor (Q2): This transistor's base is connected to the base of Q1, and its emitter is connected to the same common rail (ground or V_EE). Its collector provides the mirrored output current (I_OUT) to the load.
- Reference Current Source: An external circuit or resistor that defines the initial I_REF.
Mechanism of Operation
The working principle relies on the matched characteristics of the transistors and their shared control voltage:
1. Establishing the Reference Voltage (V_REF)
- The reference current (I_REF) flows into the collector of Q1. Because Q1 is diode-connected, it essentially acts like a diode.
- The current I_REF flowing through Q1 generates a specific base-emitter voltage (V_BE1) across it. This V_BE1 is the control voltage for the mirror.
- For BJTs, this relationship is governed by the Shockley diode equation, where a specific collector current dictates a unique V_BE.
2. Replicating the Current
- The base of Q1 is directly connected to the base of Q2. Therefore, the same V_BE1 established by I_REF across Q1 is also applied across the base-emitter junction of Q2 (V_BE2 = V_BE1).
- Since Q1 and Q2 are ideally matched (fabricated on the same integrated circuit chip under similar conditions), applying the same V_BE to Q2 causes it to conduct the same amount of collector current as Q1, assuming their current gain (beta) and other parameters are identical.
- Thus, the output current (I_OUT) flowing through the collector of Q2 will ideally be equal to the reference current (I_OUT = I_REF).
This table illustrates the ideal relationship:
| Input (Reference) Current (I_REF) | Control Voltage (V_BE) | Output Current (I_OUT) |
|---|---|---|
| 100 µA | V_BE1 | ~100 µA |
| 1 mA | V_BE2 | ~1 mA |
| 5 mA | V_BE3 | ~5 mA |
Why Use a Current Mirror? Benefits and Applications
Current mirrors are ubiquitous in analog integrated circuits due to their numerous advantages:
- Precise Current Biasing: They provide stable and accurate bias currents for various stages in amplifiers, operational amplifiers, and other linear circuits.
- Active Loads: In many amplifier designs, resistors are replaced by current mirrors acting as "active loads." This significantly increases the gain of the amplifier by presenting a high dynamic impedance without requiring large voltage drops.
- Current Sources: They can act as simple current sources, delivering a constant current to a varying load.
- Reduced Sensitivity to Voltage Changes: The output current is less sensitive to variations in the supply voltage or the load, providing better circuit stability.
- Current Scaling: By using transistors with different emitter areas (BJTs) or W/L ratios (MOSFETs), current mirrors can be designed to produce an output current that is a scaled multiple (or fraction) of the reference current.
Variations and Enhancements
While the basic current mirror is effective, more advanced configurations exist to improve performance, such as:
- Wilson Current Mirror: Offers higher output impedance and better current matching accuracy, especially at low current levels.
- Widlar Current Mirror: Allows for the generation of very small output currents that are significantly less than the reference current.
- Cascode Current Mirror: Provides even higher output impedance and better rejection of supply voltage variations.
These advanced designs overcome limitations of the basic current mirror, such as sensitivity to variations in transistor characteristics or limited output impedance, making them crucial for high-performance analog circuit design.
