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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.)
Best Answer
Yes you can drive it direct in most DC cases, but without a small Series R Emitter followers can become oscillatory with capacitive loads and step pulses.
The value needs to degenerate positive feedback and can be as small as the output impedance, but usually more. Series R's are often placed between bridge drivers to MOSFET Gates to tradeoff between instability and slew rate.
I don't have a short answer, but I references. The Rs loop including (Rb/hFE + Re ) must have a Q<1 thus total gain <1 with positive feedback with ESL and ESR of load, otherwise you have an emitter Follower "Hartley Oscillator". When you add a series R, you reduce the Q of parasitic ESL and load Ciss, it is ok to reduce Rb to 0.
https://www.google.ca/search?q=emitter+follower+oscillation&ie=utf-8&oe=utf-8&gws_rd=cr&ei=Uu4AWIbQEcrcjwT79JLQDA