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Current Mirror Calculator

Calculate current mirror output current, beta error, reference resistor, power consumption, and Widlar mirror resistor for analog IC and bias circuit design

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Formula

Iout=N×Iref×β/(β+2),Rref=(VCC−VBE)/IrefI_out = N × I_ref × β/(β+2), R_ref = (V_CC − V_BE) / I_ref
I_ref— Reference current (A)
N— Mirror ratio
β— Transistor current gain
V_T— Thermal voltage (V)
R_W— Widlar emitter resistor (Ω)

How It Works

Current mirror calculator computes reference and output currents for precision bias networks — essential for analog IC design, current sources, and active loads. IC designers, analog engineers, and instrumentation specialists use current mirrors to replicate a reference current with high accuracy. Per Horowitz & Hill 'Art of Electronics' (3rd ed., Ch.2), a basic BJT current mirror achieves 1-5% matching between reference and output currents when transistors are thermally coupled. The simple mirror has output impedance r_o = VA/Ic (Early voltage / collector current), typically 50kΩ-500kΩ. Wilson and cascode mirrors improve output impedance by 10-100×. MOSFET mirrors achieve similar performance with zero input current, but require matched threshold voltages (ΔVth < 10mV for 1% accuracy).

Worked Example

Design a 1mA current mirror using 2N3904 pair for analog circuit biasing. Reference current I_ref = 1mA, supply Vcc = 12V. Reference resistor: R_ref = (Vcc - Vbe)/I_ref = (12V - 0.7V)/1mA = 11.3kΩ — select 11kΩ (E24 series). With β = 200, base currents steal I_b = 2×1mA/200 = 10μA, causing 1% mirror error. For improved accuracy, add emitter degeneration resistors Re = 100Ω (100mV drop): this reduces sensitivity to Vbe mismatch by factor of (1 + gm×Re) = 5×, improving matching from 2% to 0.4%. Output impedance: r_o = VA/Ic = 100V/1mA = 100kΩ. Compliance range: Vce > 0.3V (saturation) to Vcc - 0.7V.

Practical Tips

  • ✓Use matched transistor pairs (MAT12: 0.5mV Vbe match, 0.5% β match) for current mirrors requiring <1% accuracy — discrete 2N3904 pairs achieve only 2-5% matching
  • ✓Add emitter degeneration Re = 0.1V/Ic for 5× improvement in matching — the voltage drop reduces sensitivity to Vbe variations
  • ✓For high output impedance, use cascode mirror (10MΩ+) or Wilson mirror (5MΩ+) instead of simple mirror (100kΩ); essential for high-gain amplifier biasing

Common Mistakes

  • ✗Using discrete transistors without thermal coupling — 2mV/°C Vbe difference causes 8%/°C mirror error; mount transistors adjacent on PCB or use matched pair (MAT12, SSM2212)
  • ✗Ignoring Early effect — simple mirror has r_o = VA/Ic; load voltage changes cause 1-5% current variation. Use Wilson mirror for 100× higher output impedance
  • ✗Forgetting base current error — simple BJT mirror has error = 2/β; at β = 100 this is 2% error. Wilson mirror reduces error to 2/β² = 0.02%

Frequently Asked Questions

Replicate a reference current to other circuit branches with high accuracy and high output impedance. Applications: bias current distribution in op-amps, active loads for differential pairs, reference current sources for DACs. Per Gray & Meyer 'Analysis and Design of Analog Integrated Circuits', 90% of analog ICs use current mirrors.
Best results require matched transistors — identical type on same die (IC) or matched pair package. Discrete transistors must be same part number, same manufacturer lot, and thermally coupled. MOSFET mirrors require ΔVth < 10mV for 1% accuracy; BJT mirrors require ΔVbe < 2mV.
Vbe/Vth mismatch (±2mV = ±8% for BJT), β mismatch (±20% = ±1% error for Wilson), Early effect (1-5% load-dependent error), temperature gradient (2mV/°C = 8%/°C for BJT). Integrated matched pairs achieve <0.5% matching; discrete circuits achieve 2-5% without trimming.
For BJTs: use emitter area ratio or multiple transistors in parallel. Two parallel output transistors give 2× current. For MOSFETs: use W/L ratio — output W/L = 2× reference W/L gives 2× current. Emitter/source degeneration resistors must scale inversely with current ratio to maintain matching.

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