Analyzing a simple current mirror circuit involves understanding the behavior of the circuit and calculating the mirrored current based on its operating principles. A current mirror is a fundamental circuit used to replicate or mirror the current flowing through one transistor (the reference transistor) to another transistor (the mirror transistor). This circuit is widely used in integrated circuits to ensure consistent and accurate current flows.
Let's analyze a basic NPN current mirror circuit. The circuit consists of two NPN transistors (Q1 and Q2) with their respective base-emitter junctions connected, and a load resistor (R_load) connected to the collector of Q2. The current through Q1 (reference transistor) is mirrored onto Q2 (mirror transistor). The circuit diagram looks like this:
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Vcc
|
R_load
|
+-- Collector (Q2)
|
+-- Base (Q2)
|
| Collector (Q1)
| +-- Base (Q1)
| |
R R
| |
--- ---
| Q1| | Q2|
--- ---
| |
| |
=== ===
| | | |
| V | | V |
|cc | |cc |
| | | |
Here's how you can analyze the circuit:
Assumptions: For this simple analysis, we assume the transistors are well-matched, and the Early effect can be neglected.
Operating Point: To ensure the circuit is in the active region, make sure the collector current of Q1 (Ic1) is larger than the expected collector current of Q2 (Ic2).
Current Mirror Principle: The principle behind the current mirror operation is that the base-emitter voltage of Q1 (Vbe1) is approximately equal to the base-emitter voltage of Q2 (Vbe2), assuming similar transistors.
Base Current Approximation: Since the base-emitter voltage is the same for both transistors, the base currents (Ib1 and Ib2) can be approximated as:
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Ib1 ≈ Ib2 ≈ (Vcc - Vbe1) / R
Collector Current Calculation: The collector current of Q1 (Ic1) and Q2 (Ic2) can be calculated using the following formulas:
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Ic1 ≈ β * Ib1
Ic2 ≈ β * Ib2
Where β (beta) is the transistor's current gain (commonly known as hfe).
Mirrored Current: Since we assume the transistors are well-matched and have the same β, the mirrored current (I_out) through the load resistor (R_load) is approximately equal to Ic1:
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I_out ≈ Ic1
Load Voltage: Finally, you can calculate the voltage drop across the load resistor (V_load) using Ohm's law:
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V_load = I_out * R_load
It's important to note that in practice, for better accuracy, you may need to consider additional factors such as Early effect, temperature variations, and variations due to manufacturing process. But for a basic analysis, the above steps should give you a good understanding of the circuit's behavior as a simple current mirror.