# NPN base resistor common collector [duplicate]

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simulate this circuit – Schematic created using CircuitLab

Hi, usual apologies here, total enthusiastic amateur noob, etc.

The situation is: a Common Collector NPN transistor.

Question: Is it okay to omit the resistor R2? (that is, R2 = 0 Ohms)?

The way I understand it, when the switch is closed, the voltage at the Emitter (as indicated) would be 5V – 0.7V = 4.3V. The reason for that is that both the Collector and the Base are connected directly to 5V, and that VBE must be 0.7 V for silicon-based transistors. Therefore the voltage drop from the Collector to the Emitter will 0.7 V rather than the normal 0.05 to 0.10 V.

But if that is the case, if I want to calculate the value of R2, the total voltage drop is the same, irrespective of whether I calculate it through the loop containing the switch, or going through the transistor form the Collector to the Emitter. Therefore IR = 0 for the Base resistor so that R2 must be 0 Ohms.

Elsewhere I came across the injunction: “Do NOT omit the Base Resistor!” so now I’m uncertain.

(I’m really trying to design a circuit to drive an PWM input to an H-bridge from an Intel Edison, so the above is not my complete circuit – it just crystallises one of the many current conceptual difficulties I have. This particular part has to do with an AND gate, so there will at least be another transistor and a pull-down resistor, and so on.)

## marked as duplicate by CL., pipe, Community♦Oct 14 '16 at 15:48

• You can find your answer here (from a similar question): electronics.stackexchange.com/a/134064/103420 – Rohat Kılıç Oct 14 '16 at 14:20
• You're welcome. But you'd be quicker if you made a search ;) – Rohat Kılıç Oct 14 '16 at 14:30
• I did, I did. But I am not yet at the stage where I know what to search for. "Do I need a base resistor" is still a bit generic for me. – Pieter Beneke Oct 14 '16 at 14:33
• @RohatKılıç that is for linear load R's in series.... – Tony Stewart Sunnyskyguy EE75 Oct 14 '16 at 14:37
• @pipe not same load – Tony Stewart Sunnyskyguy EE75 Oct 14 '16 at 14:39