A current mirror is an active circuit block that copies (or "mirrors") a reference current flowing through one branch and forces an identical, highly stable current through a second branch, regardless of the load resistance in that second branch. In a real circuit or installation, it changes how we handle biasing and load driving: it replaces bulky, heat-wasting passive current-limiting resistors with an active, self-adjusting mechanism that maintains precise bias currents in amplifiers or LED drivers even as supply voltage or load impedance fluctuates.
The Core Mechanism and the Water Analogy
At its most basic, a bipolar junction transistor (BJT) current mirror consists of two matched transistors, Q1 and Q2. Q1 is "diode-connected" (its base and collector are tied together), meaning the voltage across its base-emitter junction ($V_{BE}$) is strictly determined by the reference current flowing through it. Because Q2's base is tied to Q1's base, and their emitters share a common ground, Q2 is forced to have the exact same $V_{BE}$. In a BJT, collector current is an exponential function of $V_{BE}$; therefore, identical base-emitter voltages yield identical collector currents.
To visualize this, imagine a main water pipe with a calibrated leak (the reference branch) that sets a specific pressure. A second pipe (the mirror branch) has a pressure-sensitive valve linked directly to the first pipe's leak; as long as the main supply pressure is high enough, the second valve opens just enough to match the flow rate of the calibrated leak, regardless of what nozzle is attached to the end of the second pipe.
The Math: A Worked Numeric Example
Let's build a discrete mirror using two 2N3904 NPN transistors on the bench. We want to mirror a reference current to drive a small indicator LED.
- Define the Supply and Reference Resistor: We have a $V_{CC}$ of 12.0V. We select a reference resistor ($R_{REF}$) of 10kΩ.
- Calculate the Reference Current ($I_{REF}$): The voltage across $R_{REF}$ is $V_{CC}$ minus the $V_{BE}$ of Q1. For a 2N3904 at roughly 1mA, $V_{BE}$ is typically 0.65V. Therefore, $I_{REF} = (12.0V - 0.65V) / 10,000Ω = 1.135 mA.
- Account for Base Current Loss: The reference resistor doesn't just supply Q1's collector; it also supplies the base currents for both Q1 and Q2. If our transistors have a DC current gain ($eta$) of 100, the base current for each is roughly $I_C / 100$. The actual output current ($I_{OUT}$) is $I_{REF} / (1 + 2/eta)$. Plugging in our numbers: $1.135 mA / (1 + 0.02) = 1.112 mA.
This 2% error is the fundamental limitation of the basic two-transistor mirror. In monolithic ICs, $eta$ is often much higher and tightly matched, pushing this error well below 1%. For deeper reading on minimizing these errors in discrete designs, refer to electronics-tutorials.ws for comprehensive breakdowns of emitter degeneration.
Where You Meet Current Mirrors in Practice
You rarely see discrete current mirrors in modern consumer electronics, but they are the backbone of analog integrated circuit design and specific high-reliability bench applications.
- Op-Amp Input Stages: Inside a classic LM358 or TL072, current mirrors act as "active loads" for the differential input pair. They replace passive collector resistors, providing massive AC impedance (high gain) without dropping excessive DC voltage headroom.
- Discrete Audio Amplifiers: In high-end Class AB audio amplifiers, a current mirror is often used to bias the long-tailed pair (LTP) input stage, ensuring the tail current remains rock-solid even when the power supply rails sag during heavy bass transients.
- Parallel LED Strings: When driving multiple high-power LED strings from a single supply, a MOSFET current mirror ensures each string draws the exact same current, preventing one string from hogging current and overheating.
Bench Scenario: When a Discrete Mirror Fails
Warning: Never assume off-the-shelf discrete MOSFETs are matched. Wide manufacturing tolerances in threshold voltage will destroy your current balance unless you use degeneration.
The Setup: A hobbyist is building a 24V grow light and wants to drive two parallel high-power LED strings. To balance the current, they wire two IRF510 N-channel MOSFETs as a current mirror, aiming for 350mA per string. The gates are tied together, the sources are tied directly to ground, and the reference resistor is tuned to set the master branch at 350mA.
The Numbers: The IRF510 datasheet lists the Gate-Threshold Voltage ($V_{GS(th)}$) as a minimum of 2.0V and a maximum of 4.0V. The circuit is designed assuming a nominal $V_{GS(th)}$ of 3.0V.
The Outcome: Upon powering up, String 1 draws 480mA and the LED begins shifting color from cool white to warm yellow due to thermal stress. String 2 draws only 220mA and appears dim. The "mirror" is completely unbalanced.
What Went Wrong: Because the MOSFETs were discrete, their physical silicon dies were not thermally or physically matched. The MOSFET in String 1 happened to have a $V_{GS(th)}$ of 2.4V, while the one in String 2 had a $V_{GS(th)}$ of 3.6V. Since their gates were at the exact same voltage, the one with the lower threshold turned on much harder, hogging the current.
The Fix: Add source degeneration resistors. By placing a 1Ω, 1W resistor in series with the source of each MOSFET, you introduce local negative feedback. If String 1 tries to draw more current, the voltage drop across its 1Ω resistor increases, which effectively reduces its $V_{GS}$, choking off the excess current and forcing the mirror back into balance.
Current Mirror vs. Constant Current Source
The most common point of confusion for beginners is conflating a current mirror with a constant current source (like an LM317 configured as a current regulator). While both deliver a steady current, their fundamental operating principles differ.
| Feature | Current Mirror | Constant Current Source (e.g., LM317) |
|---|---|---|
| Primary Function | Copies an existing reference current to a new branch. | Generates a fixed current from a voltage reference. |
| Input Requirement | Requires a master current to mirror. | Requires a voltage supply and a set resistor. |
| Typical Use Case | IC biasing, differential amplifier active loads, matching parallel branches. | Battery charging, single-string LED driving, power supplies. |
| Component Count | Low (2 transistors + 1 resistor for basic discrete). | Moderate (Regulator IC + set resistor + decoupling caps). |
Frequently Asked Questions
Q: Do I absolutely need matched transistors for a BJT current mirror?
A: For high-precision IC design, yes. For discrete bench work, you can use standard transistors (like two random 2N3904s) if you add emitter degeneration resistors (e.g., 10Ω to 100Ω). The voltage drop across the emitter resistors will swamp out the minor $V_{BE}$ mismatches between the two discrete parts, forcing the currents to match closely.
Q: What is the "Early Effect" and how does it ruin my mirror?
A: The Early effect describes how a BJT's collector current slightly increases as the collector-emitter voltage ($V_{CE}$) increases, due to base-width modulation. In a basic mirror, if the load on Q2 causes its $V_{CE}$ to be much higher than Q1's $V_{CE}$, Q2 will output slightly more current than Q1. To fix this, designers use a Wilson current mirror or cascode configuration to lock the $V_{CE}$ of both transistors to the same value.
Q: Can a current mirror handle AC signals?
A: Yes. While we usually think of them as DC biasing tools, current mirrors are heavily used in AC signal paths as "active loads." Because a transistor configured as a current source has a very high dynamic (AC) impedance but a low DC voltage drop, it allows an amplifier stage to achieve massive voltage gain without requiring a high-voltage power supply.
