-61 ohms is not a common standard value. 56 ohms is, and will result in about 20mA LED current.
You got 2.3k by using the base current needed to drive one LED but you're driving 10 LEDs. If you use 56 ohms each, the total LED current and therefore transistor Ic = 200mA. Using the rule-of-thumb Ic/Ib = 10, we need 20mA base current. To supply that current from 5V, R1 should be about 200 ohms. The nearest standard value 220 ohms should be fine.
You have not mentioned the actual base drive source. If it's really a 5V DC supply, then the values given above are OK. But if you're going to drive it from a digital CMOS IC output, you have to consider one more factor. Most CMOS outputs have a limited output current capability. If you try to draw several mAs from it, the output voltage drops and the source as seen by the load is no longer 5V.
Rb = 5-0.75/0.9mA = 4.7k
how 1.5v? we cant neglect R3 because of 1cbo is 0.1μAdc.So,how do we get R1..hmmmIn this case it should be modified as:
Rb = ( 5 - 1.5 ) / 0.9mA = 3.9k
Please note that 3.9k includes the internal resistance of the input voltage source.
Also 0.9mA is not right... do you know why? It is right if R3 is omitted.
Added:
Rb = ( Vcc - 2*Vf ) / ( Ic / G_current + 2*Vf / R3 )
R1 = Rb - Rs (of the input 5V source)
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