I'm a beginner in electronics, and started the 2nd chapter of AoE, regarding transistors. I've decided to design my own LED driver, as mentioned on page 76 (3rd ed.)
V1 = +3.3 [V], V2_LOW = 0 [V] , and V2_HIGH = +3.3 [V].
The current (I1) through D1 should be 5 [mA].
Using a green LED (for example), the voltage drop V_D1 is 2 [V] at 5 [mA].
BETA of Q1 is at least 25.
Given the above, I need to figure out the values of R1 and R2. Here's my attempt:
To make sure that I1 = 5 [mA] when there is a signal, I'll need to find an appropriate R1:
V_D1 + V_R1 = V1, so the voltage drop across R1 (V_R1) is 3.3 - 2 = 1.3 [V].
V_R1 = I1*R1, so R1 should be 1.3[V]/5[mA] = 2.6 [k].
Now, I need to find R2 such that there is saturation when there is a signal:
- I1 < BETA*I2 = BETA*(V2/R2), so R2 should be no more than BETA*V2/I1 = 16.5 [k].
In conclusion, R1 = 2.6 [k] and R2 = 16.5 [k].
Is my solution correct? If not, where and why is it faulty?