Current Mirror Calculator

Design a BJT or MOSFET current mirror. Solves reference resistor, output impedance, and Wilson/cascode improvements.

Calculator Electronics Updated Apr 23, 2026
X Facebook LinkedIn Reddit Email
How to Use
  1. Pick BJT or MOSFET mirror.
  2. Enter target output current and supply voltage.
  3. Tool computes reference resistor and estimated matching error.
Input
mA
V
V (0.7 BJT, 1-2 MOSFET)
Presets
Current Mirror
R_ref
—
I_ref
—mA
Mirror Error
—%
P_ref
—mW

Show Work

Enter values.

Formulas

R_ref
R_ref = (V_cc − V_BE) / I_ref
Set reference.
BJT error
ΔI/I ≈ 2/β
Base-current error.
Wilson mirror
Error ≈ 2/β²
Adds 3rd transistor.
Cascode
Zout ≈ gm·ro²
Very high Zout.
MOSFET
Vref = Vt + √(2·I/k)
Saturation bias.
Scaling
Ratio W/L sets Iout/Iref
MOSFET area ratio.

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.

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.

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.

Last updated: