A BJT current mirror is an active circuit configuration that uses matched bipolar junction transistors to copy a precise reference current from one branch to another, forcing the output to maintain that exact current regardless of load voltage changes. In a real circuit, this topology fundamentally changes how we handle biasing and loads: it replaces bulky, heat-wasting passive current-limiting resistors with high-impedance active loads. This substitution is the foundational trick that gives operational amplifiers their massive open-loop gain and stabilizes bias points against supply voltage fluctuations.
The Core Mechanism and a Worked Numeric Example
The magic of the basic two-transistor mirror relies entirely on the base-emitter voltage (Vbe) matching between Q1 and Q2. Because Q1 is diode-connected (base shorted to collector), it develops a specific Vbe to pass the reference current. Since Q2 shares the exact same base and emitter nodes, it is forced to adopt the same Vbe, and therefore passes the same collector current—assuming the transistors are perfectly matched and at the same temperature.
Worked Example: Sizing a 5mA Mirror
Let’s design a basic NPN current mirror to source exactly 5mA into a load, using standard 2N3904 transistors and a 12V DC supply.
Vcc = 12V
Target I_out = 5mA
Vbe (at 5mA, 25°C) ≈ 0.65V (from the 2N3904 datasheet Vbe(sat) and on-characteristics curves)
2. Calculate the Reference Resistor (R_ref):
The current through Q1 is determined by R_ref. The voltage across R_ref is Vcc minus the Vbe of Q1.
V_Rref = 12V - 0.65V = 11.35V
R_ref = V_Rref / I_ref = 11.35V / 0.005A = 2270Ω
3. Select Standard Component:
Choose the nearest standard E24 resistor value: 2.2kΩ.
Recalculating actual I_ref: 11.35V / 2200Ω = 5.15mA.
Therefore, I_out will mirror at approximately 5.15mA.
The Early Effect Reality Check: In the real world, Q1 and Q2 will only mirror perfectly if their Collector-Emitter voltages (Vce) are identical. Because Q1's Vce is clamped to ~0.65V, but Q2's Vce will vary depending on the load, the Early Effect (base-width modulation) will cause Q2's current to drift slightly. For hobbyist LED driving, this 2-5% error is irrelevant. For precision analog design, we must upgrade the topology.
Where You Meet the BJT Current Mirror in Practice
You rarely build discrete current mirrors for simple switching, but you are constantly using them inside integrated circuits and specific analog subsystems.
- Op-Amp Input Stages: Open up the datasheet schematic for a classic LM358 or LM741. The tail current source biasing the differential input pair is almost always a BJT current mirror. It provides the high impedance necessary for high common-mode rejection.
- Audio Amplifier Biasing: In Class-AB audio amplifiers, current mirrors are used in the voltage amplifier stage (VAS) as active loads, allowing the stage to achieve massive voltage gain without requiring a physically massive, high-wattage collector resistor.
- Constant Current LED Drivers: When driving high-power LED strips where supply voltage might sag from 14.4V down to 11V, a discrete BJT mirror ensures the LEDs draw exactly 350mA the entire time, preventing thermal runaway and color shifting.
Common Pitfalls: What People Confuse It With
The most frequent mistake among junior engineers and hobbyists is confusing an active current mirror with a passive current divider. They look similar on paper but behave completely differently under load.
| Feature | Passive Current Divider | Active BJT Current Mirror |
|---|---|---|
| Core Components | Two or more resistors in parallel | Matched transistors + one reference resistor |
| Output Impedance | Low (dictated by parallel resistance) | Very High (dictated by transistor Early voltage) |
| Load Dependence | Current changes drastically if load resistance changes | Current remains constant regardless of load resistance (within compliance voltage) |
| Voltage Headroom | Requires large voltage drops to maintain ratio | Only requires Vce(sat) (~0.2V) to maintain regulation |
Another common confusion: Assuming any two random NPN transistors from a bin will mirror perfectly. They won't. Vbe can vary by ±50mV between random parts, which translates to a massive current mismatch. Always use transistors from the same batch, or better yet, a monolithic dual transistor like the BC847BPDW1 where the silicon is physically identical and thermally coupled.
Decision Tree: Choosing the Right Mirror Topology
Not all mirrors are created equal. Use this decision matrix to select the exact topology or part number for your workbench or PCB layout.
| If your requirement is... | Then choose this topology... | Why it wins |
|---|---|---|
| Quick, robust discrete biasing for an LED or relay coil where 5% accuracy is fine. | Basic 2-Transistor Mirror with Emitter Degeneration | Adding 100Ω resistors to the emitters of Q1 and Q2 swamps out Vbe mismatches and prevents thermal runaway. |
| High accuracy (<1% error) and high output impedance for an analog signal path. | Wilson Current Mirror (3 transistors) | The third transistor creates a feedback loop that cancels out base current errors (Beta dependence) and shields Q2 from the Early Effect. |
| Microamp-level bias currents (e.g., 10µA) without using impractical 1MΩ+ resistors. | Widlar Current Source | Places a small resistor only on Q2's emitter, exploiting the logarithmic Vbe/current relationship to generate tiny currents from a standard reference. |
| You need a guaranteed 1mA to 10mA source on a PCB and don't want to calculate discrete matching. | Integrated Current Regulator (LM334) | The LM334Z is a 3-terminal IC that acts as a temperature-compensated, laser-trimmed current mirror. Set it with a single resistor. |
FAQ: Troubleshooting and Matching
Q: My mirrored current is drifting upward as the circuit runs. What is happening?
A: You are experiencing thermal runaway. As Q2 dissipates power (Vce × Ic), its silicon die heats up. For every 1°C increase, Vbe drops by roughly 2mV. Because the base voltage is fixed by Q1, this drop in Vbe causes Q2 to draw exponentially more current, which creates more heat, creating a death spiral. Fix: Add 47Ω to 100Ω emitter degeneration resistors to both transistors to introduce negative feedback, or physically glue Q1 and Q2 together with thermal epoxy.
Q: Why is my output current 20% lower than my reference current?
A: You are likely suffering from Beta (β) base-current theft. In a basic mirror, the reference current must supply the base currents for both Q1 and Q2. If you are using low-gain transistors (β < 50), the base current steals a significant chunk of your reference current. Fix: Switch to a Wilson mirror topology, or use high-beta transistors like the 2N5089 (β > 400).
Q: What is the 'compliance voltage' of a current mirror?
A: This is the minimum voltage drop required across the output transistor (Q2) for it to maintain regulation. For a standard BJT mirror, Q2 must stay out of saturation, meaning Vce must be at least Vce(sat) + V_emitter_resistor. Typically, you need at least 0.5V to 1.0V of headroom between the load and the supply rail. If your load pulls the collector voltage below this threshold, the mirror breaks down and acts like a simple resistor.
The Bottom Line Recommendation: Stop debating which discrete topology is theoretically perfect for your next prototype. If you are building a one-off analog front-end on a breadboard, default to a basic 2N3904 mirror with 100Ω emitter degeneration resistors; it solves 90% of thermal and mismatch issues for under $0.10. If you are designing a production PCB and need a precision constant current source between 1mA and 15mA, stop designing discrete mirrors entirely and drop in an LM334Z-1.0 or configure an LM334 with a trim pot. It saves board space, eliminates matching headaches, and guarantees datasheet-level accuracy.
