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How can diodes be used in "current mirror" circuits to match currents accurately?

1 Answer

Diodes can be utilized in "current mirror" circuits to achieve accurate current matching between two or more branches. A current mirror is a circuit configuration that is commonly used to replicate or mirror the current flowing through one branch to another branch. The goal is to make the current in the mirrored branches equal or very close to the reference current.

The basic concept of a diode-based current mirror is straightforward. The circuit typically consists of a diode-connected transistor (where the gate and drain are shorted) as the active device and a load resistor. Here's a step-by-step explanation of how this works:

Reference Current Source: The current mirror circuit has a "reference" branch that generates the desired or reference current. This branch can be implemented using a current source, another current mirror, or any other circuit that generates a stable and known current.

Diode-Connected Transistor: The diode-connected transistor is a MOSFET or BJT transistor (often a MOSFET is used due to its superior performance in integrated circuits) where the gate and drain are shorted together. This configuration effectively turns the transistor into a two-terminal device like a diode. It is essential to ensure that the transistor operates in the saturation region, as this allows it to behave as a good current source.

Load Resistor: The diode-connected transistor is connected to a load resistor, and the mirrored current flows through this resistor.

Current Matching: The key to achieving accurate current matching lies in the fact that the voltage across a diode-connected transistor is logarithmically related to the current flowing through it. This means that small changes in voltage lead to significant changes in current. By using the diode-connected transistor and adjusting its gate voltage appropriately, the current flowing through it can be made equal (or very close) to the reference current.

Controlling the Gate Voltage: To control the gate voltage of the diode-connected transistor, a feedback mechanism can be employed. For example, an operational amplifier (op-amp) can be used as a feedback controller. The op-amp measures the voltage across a sensing resistor in the reference branch (representing the reference current) and adjusts the gate voltage of the diode-connected transistor until the voltage across the load resistor (representing the mirrored current) matches the reference voltage. The op-amp will drive the gate voltage to achieve this matching, ensuring that the currents closely match.

Temperature Compensation (Optional): In practical designs, temperature variations can affect the performance of the current mirror. Temperature-compensation techniques can be incorporated to mitigate the impact of temperature changes on current matching.

By utilizing diode-connected transistors and suitable feedback mechanisms, current mirrors can achieve accurate current matching, making them valuable tools in various analog and mixed-signal integrated circuits. They find applications in voltage references, biasing circuits, and current amplifiers, among others.
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