Darlington Pair Calculator
Compute effective gain, input impedance, and Vbe drop of a Darlington pair (two cascaded BJTs). High-gain switching and audio output stage.
How to Use
- Enter β₁ (first BJT) and β₂ (output BJT).
- Tool computes total current gain and Vbe drop (~1.4 V).
Show Work
Formulas
History of the Darlington Pair
Sidney Darlington at Bell Labs patented the compound-transistor configuration in 1953 (US Patent 2,663,806), originally for telephone amplifier applications needing high gain from then-expensive germanium transistors. The TIP series (TIP120/TIP127 NPN/PNP from Texas Instruments, 1970s) brought Darlingtons to every hobbyist's bench for solenoid, relay, and motor driving. Power Darlingtons dominated consumer audio amplifier output stages until MOSFETs took over in the 1990s.
About This Calculator
Enter β₁ and β₂ (current gains of the two BJTs) and desired collector current. The tool computes β_total, required base drive current Ib = Ic/β_total, total Vbe drop (~1.4 V), and power dissipation at saturation (~1 V × Ic).
Darlington pairs have elegant gain but poor switching speed and high Vce(sat). For modern fast switching above 1 A, prefer a single MOSFET with gate driver. Everything runs client-side.
About the Darlington Pair Calculator
Use the Darlington Pair Calculator — a free, easy tool for electronics and circuit design. Nothing is uploaded, and you do not need an account. Compute effective gain, input impedance, and Vbe drop of a Darlington pair (two cascaded BJTs). High-gain switching and audio output stage.
How it works
Put each value in its box and read the answer as you go. Because it recalculates live, you can play with the inputs to see how each one moves the result — handy for checking your own working or planning ahead. Everything happens on your device, so it is fast and private.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
Darlington gain?
β_total = β₁·β₂ + β₁ + β₂ ≈ β₁·β₂ for typical β > 50. Two cascaded BJTs multiply current gains. A pair of β=100 transistors gives β_total = 10,000+.
Drawbacks?
Vbe_sat ≈ 1.4 V (two Vbe drops) — much higher than single-BJT 0.7 V. Slow switching (3-5× single-transistor fall time). MOSFET cascade topologies dominate modern high-current switching.
TIP120?
Classic NPN Darlington with β_min = 1000 up to 5 A. Widely used in hobbyist projects for solenoid, relay, and motor drive. Needs a turn-off resistor from base-2 to ground for reliable turn-off.
How do I use the Darlington Pair Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Solenoid Driver
TIP120 drives 12 V 2 A solenoid from MCU GPIO — no need for gate driver.
Audio Output Stage
Pairs of PNP/NPN Darlingtons in Class-AB audio amp output — high current, simple biasing.
Logic Buffer
High-β Darlington buffers open-collector output from opto-isolator for heavy current.
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