Analyzing a simple current mirror circuit involves understanding its operating principle and characteristics. A current mirror circuit is a common building block in electronic circuits used to copy or replicate the current flowing through one transistor (the reference transistor) into another transistor (the mirror transistor). This can be achieved by properly biasing the transistors in the circuit.
Let's look at a basic NPN BJT (Bipolar Junction Transistor) current mirror circuit as an example:
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Vcc
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R
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\|/
--- Qref (NPN) Iout
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|<--Iin (Reference current)
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--- Qmirror (NPN)
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GND
Here, we have two NPN BJTs - Qref (the reference transistor) and Qmirror (the mirror transistor). The goal is to make the current flowing through Qmirror, labeled as Iout, approximately equal to the current flowing through Qref, labeled as Iin.
To analyze this circuit, follow these steps:
Biasing: The key to a current mirror circuit is proper biasing. Qref needs to be in the active mode (also called the saturation region) to act as a constant current source. This is achieved by applying an appropriate base-emitter voltage (Vbe) to Qref and a resistor (R) in series with Vcc to set the reference current, Iin. This resistor can be calculated using Ohm's law: R = (Vcc - Vbe) / Iin.
Transistor characteristics: Ensure that both transistors have the same characteristics or similar parameters. For example, the transistors should ideally be from the same batch or have matching parameters to achieve better current matching.
Current equation: The current flowing through Qref (Iin) can be approximated using the equation for a BJT in the active mode:
Iin ≈ (β * Iref) / (1 + β)
where β is the current gain (hFE) of the transistor.
Current replication: The current through Qmirror (Iout) will approximately be equal to Iin if both transistors are matched, and their parameters (such as β) are similar.
Errors: In reality, there can be some errors in current replication due to mismatched transistor characteristics and other parasitic effects. These errors can be minimized through careful transistor selection, temperature compensation, and other techniques.
Practical considerations: Consider the voltage headroom requirements (Vcc and Vbe) and power dissipation in the circuit while designing and selecting components.
Remember that the accuracy of a simple current mirror circuit depends on how closely the two transistors match and the accuracy of the biasing conditions. For more precise current replication or in applications requiring high accuracy, more complex current mirror circuits with additional compensation techniques may be used.