Current Mirror Calculator
Design a BJT or MOSFET current mirror. Solves reference resistor, output impedance, and Wilson/cascode improvements.
How to Use
- Pick BJT or MOSFET mirror.
- Enter target output current and supply voltage.
- Tool computes reference resistor and estimated matching error.
Show Work
Formulas
History of the Current Mirror
Howard Bobrow at RCA described the basic BJT current mirror in 1956, but Bob Widlar at Fairchild popularized it in the mid-1960s as the foundational building block of bipolar integrated circuits. The 1963 uA702 op-amp was one of the first commercial ICs to use cascaded current mirrors for biasing. Widlar's 1967 LM709 and Jim Solomon's 1968 LM741 established the mirror-plus-diff-pair topology that every analog op-amp since 1970 has inherited. Modern CMOS current mirrors appear in every analog IC design.
About This Calculator
Pick BJT (two matched NPN/PNP) or MOSFET mirror. Enter target output current, supply voltage, β (for BJT), and Vref (diode-connected transistor drop — typically 0.7 V BJT, 1-2 V MOSFET). The tool computes R_ref = (Vcc − V_ref)/I_ref, resistor power dissipation, and expected matching error (≈ 2/β for simple BJT mirror).
For tighter matching, use Wilson (error ≈ 2/β²) or cascode mirrors. For MOSFET mirrors, match transistor area ratio (W/L) to set output-to-reference current ratio. Everything runs client-side.
About the Current Mirror Calculator
The Current Mirror Calculator is a free tool for electronics and circuit design. It runs right in your web browser, so there is nothing to download. Design a BJT or MOSFET current mirror. Solves reference resistor, output impedance, and Wilson/cascode improvements.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What is a current mirror?
Two matched transistors where the output transistor mirrors the reference current. Reference current is set by a resistor; output tracks it regardless of load impedance (within the transistor\'s compliance range).
Why use?
Stable bias currents for analog ICs (op-amp tail currents, bandgap references), current-source loads for high-gain amp stages, and converting single-ended to differential current.
Matching?
BJT mirror: ~1-5% matching (β mismatch). Wilson mirror: ~0.1%. Cascode mirror: high Zout but needs more headroom. MOSFET: depends on Vt matching, typically 2-10% unless layout-tuned.
How do I use the Current Mirror Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Op-Amp Tail
Set differential-pair bias current. Typically 1-100 µA from bandgap + mirror chain.
Active Load
Replace collector resistor with current mirror — boosts amp gain to thousands.
LED Driver
Constant-current LED drive with simple mirror + PNP for 10-100 mA channels.
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