Darlington Pair Calculator
Estimate the combined current gain of two cascaded BJTs, or calculate the base current or collector current available from a Darlington pair.
The calculator also reports the combined base-emitter voltage and a VCE(sat)-based conduction-power reference for transistor drivers and switching circuits.
Engineering tool
Darlington Pair Calculator
Calculate combined transistor gain, base or collector current, equivalent VBE, and saturation power reference.
DC current gain of the input transistor.
DC current gain of the output transistor.
Base-emitter drop of Q1.
Base-emitter drop of Q2.
Combined gain
10,000 βtotal
Result console
- Combined current gain
- 10,000βtotal
- Equivalent VBE
- 1.4V
The product-gain model is an engineering estimate. Verify device gain, current sharing, VBE, VCE(sat), temperature, and safe operating area from the datasheet.
Formula reference
Darlington Pair Formulas
The gain-product model is a first-order estimate; use datasheet curves and limits for production design.
Combined gain: βtotal ≈ β1 × β2Base current: Ib = Ic / βtotalCollector current: Ic = Ib × βtotalEquivalent VBE: VBE(total) ≈ VBE1 + VBE2Power reference: P ≈ VCE(sat) × IcVariable definitions
- β1
- input transistor DC current gain
- β2
- output transistor DC current gain
- Ib
- Darlington input base current
- Ic
- estimated output collector current
- VCE(sat)
- saturated pair voltage drop
Worked Example
1 A Darlington Driver
β1 = 100, β2 = 100, Ic = 1 A
βtotal ≈ 100 × 100 = 10,000
Ib = 1 A / 10,000 = 100 µA
VBE(total) ≈ 0.7 V + 0.7 V = 1.4 V
At VCE(sat) = 1 V, the conduction-power reference is approximately 1 W.
Engineering Notes
Very high current gain
Darlington pairs provide very high current gain from a small input base current.
Two junction drops
Total VBE is roughly two base-emitter drops and raises the required input voltage.
Higher saturation loss
Saturation voltage is higher than a single BJT and can create substantial heat at high current.
Slower switching
Switching speed is slower due to stored charge in the cascaded transistor structure.
Compare with MOSFETs
Logic-level MOSFETs are often better for high-efficiency switching and low drive power.
Support reference
FAQ
What is a Darlington pair?
A Darlington pair connects the emitter of one BJT to the base of a second BJT so their current gains multiply approximately. It provides high current gain from a small input current.
How do you calculate Darlington gain?
For a quick engineering estimate, multiply the two transistor gains: βtotal ≈ β1 × β2. Real gain depends on device current, temperature, loading, and topology.
Why is Darlington VBE higher?
The input signal must forward-bias two base-emitter junctions in series, so the total input threshold is approximately VBE1 + VBE2, often around 1.2 to 1.4 V for silicon devices.
Why is Darlington saturation voltage higher?
The cascaded junction arrangement prevents the output transistor from saturating like an isolated BJT. Darlington VCE(sat) is commonly much higher, increasing conduction loss.
When should I use a MOSFET instead?
A logic-level MOSFET is often preferable for efficient switching, low drive current, lower conduction loss, or faster switching. Verify gate threshold, RDS(on), voltage rating, and switching conditions.
Documentation
Related Engineering Guides
Design notes, guides, and engineering articles linked to this tool.
Engineering Guide
How to Choose and Bias a BJT Transistor
Choose and bias BJTs for switching and amplifier circuits with practical checks for base current, base resistor, forced beta, saturation, Q-point, power, thermal limits, and datasheet margin.
18 min · Intermediate
Engineering Guide
Understanding BJTs
Understand BJT fundamentals including NPN and PNP operation, cutoff, active and saturation regions, beta, base current, VBE, VCE, power dissipation, SOA, packages, and practical applications.
16 min · Beginner
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