What is a current mirror circuit and how does it copy a reference current?
1 Answer
A current mirror circuit is an electronic circuit used in analog integrated circuits to replicate or copy a reference current with high precision. It's commonly used in applications where maintaining a consistent current is important, such as in biasing circuits for amplifiers, voltage regulators, and other analog components.
The basic idea behind a current mirror circuit is to use a transistor to regulate the flow of current through another transistor in such a way that the output current of the second transistor matches the reference current provided by the first transistor.
Here's a simple explanation of how a basic current mirror circuit works:
Reference Transistor (Q1): This transistor is used to generate the reference current (I_ref). It operates in the active region, meaning it's in saturation mode for BJTs (Bipolar Junction Transistors) or in the ohmic region for MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors).
Output Transistor (Q2): This transistor is the one whose current needs to be mirrored to match the reference current. Its base (for BJTs) or gate (for MOSFETs) is connected to the collector (for BJTs) or drain (for MOSFETs) of the reference transistor (Q1). The output current (I_out) flows through this transistor.
Biasing: The base-emitter voltage of the output transistor (Q2) is set by the reference transistor (Q1). Since the base-emitter voltage of Q1 and Q2 are approximately equal (due to their connected terminals), and because Q1 is designed to be in saturation or ohmic region, Q2 is also driven into a similar operating region.
Matching: By properly sizing the transistors and designing the circuit, the output transistor (Q2) will replicate the collector (or drain) current of the reference transistor (Q1). This is achieved because the base (or gate) voltage and the emitter (or source) voltage of Q2 are forced to be similar to those of Q1.
The key principle behind the current mirror is that the base-emitter voltage of transistors of the same type tends to be relatively constant, and their collector (or drain) currents are directly proportional to their base (or gate) currents in active operation. This allows the current flowing through the output transistor (Q2) to closely match the reference current generated by the reference transistor (Q1).
It's worth noting that in real-world scenarios, there might be some variations and imperfections due to manufacturing tolerances, temperature effects, and other factors. Design techniques and circuit modifications are often employed to improve the accuracy and stability of the current mirror circuit.
The basic idea behind a current mirror circuit is to use a transistor to regulate the flow of current through another transistor in such a way that the output current of the second transistor matches the reference current provided by the first transistor.
Here's a simple explanation of how a basic current mirror circuit works:
Reference Transistor (Q1): This transistor is used to generate the reference current (I_ref). It operates in the active region, meaning it's in saturation mode for BJTs (Bipolar Junction Transistors) or in the ohmic region for MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors).
Output Transistor (Q2): This transistor is the one whose current needs to be mirrored to match the reference current. Its base (for BJTs) or gate (for MOSFETs) is connected to the collector (for BJTs) or drain (for MOSFETs) of the reference transistor (Q1). The output current (I_out) flows through this transistor.
Biasing: The base-emitter voltage of the output transistor (Q2) is set by the reference transistor (Q1). Since the base-emitter voltage of Q1 and Q2 are approximately equal (due to their connected terminals), and because Q1 is designed to be in saturation or ohmic region, Q2 is also driven into a similar operating region.
Matching: By properly sizing the transistors and designing the circuit, the output transistor (Q2) will replicate the collector (or drain) current of the reference transistor (Q1). This is achieved because the base (or gate) voltage and the emitter (or source) voltage of Q2 are forced to be similar to those of Q1.
The key principle behind the current mirror is that the base-emitter voltage of transistors of the same type tends to be relatively constant, and their collector (or drain) currents are directly proportional to their base (or gate) currents in active operation. This allows the current flowing through the output transistor (Q2) to closely match the reference current generated by the reference transistor (Q1).
It's worth noting that in real-world scenarios, there might be some variations and imperfections due to manufacturing tolerances, temperature effects, and other factors. Design techniques and circuit modifications are often employed to improve the accuracy and stability of the current mirror circuit.