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This is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the darlington pair a very high current gain such as 10000. Darlington pair are sold as complete packages containing the two transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor.
You can make up your own darlington pair from two transistors.
- For TR1 use BC548B with hFE1 = 220.
- For TR2 use BC639 with hFE2 = 40.
The overall gain of this pair is hFE1 × hFE2 = 220 × 40 = 8800.
The pair's maximum collector current IC(max) is the same as TR2.