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I come across with a transistor circuit as shown below. Does anyone know what is the purpose of R1 and R2? enter image description here

Davide Andrea
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user42423
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1 Answers1

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The resistors between bases and emitters in a Darlington Pair serve to shunt capacitative charges at the bases of the transistors quickly to the emitters. Without these resistors, any capacitative charge at the base would need to pass through the base-emitter junction. This would slow down these transistors turning off. Many commercial Darlington Pairs have such resistors built in. For example TIP120 family of Darlingtons.

enter image description here

Math Keeps Me Busy
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  • Hi, thank you for your reply. Any formula to calculate the value of R1 and R2? – user42423 Jun 15 '23 at 14:01
  • I don't have a formula, but I don't think in most cases the values are that critical. – Math Keeps Me Busy Jun 15 '23 at 14:28
  • R1,R2 pick according the freq. you are controlling the transistor. Lower R1,R2 causes quicker discharge but drains more and it also requires using lower Rb to reach 1.4V at BE because of RB/(R1+R2) divider. So then you need a "strong" Ib signal. In contrast, Darlington pair is mostly used because of weak Ib signal, so you are loosing the Darlington "advantage". – Michal Podmanický Jun 15 '23 at 17:40