Now we can recalculate the circuit and check if we meet our requirements. The anode of the external LED will be connected to VCC1 and R3 is connected to the switch J1 instead of ground. But before I do it, I like to point out that if there is a mechanical switch, the transistor is not needed. Let \$V_X\$ be the voltage at the midpoint of voltage divider R3 and R4.ĬASE 1 - The Transistor's Base Is Connected ( \$V_X=V_B\$) It is not hard calculating the values of the biasing resistors. The following shows the current and voltages using the resistor values calculated above in Multisim electronics circuit simulation software. What I don't know is how Vb can be calculated. To calculate the resistor values we can also use the online BJT biasing calculator. So here is the problem, I know I want the base current to be 80 uA, VBE 0.7 V and VE 1 V. Now lets take the resitive divider biasing scheme and create its thevenin equivalent: Step 1: Step 2: Step 3: V TH V 1 × R 3 R 4 + R 3. Here is my question As you can see that here the transistor is configured to be an amplifier in common emitter mode. Lets also say I want 1 volt across the emitter resistor R2. (EDIT: I've completely rewritten my answer to hopefully provide a better answer.)Ĭonsider the application of Kirchoff's Current Law for the two cases where the transistor's base is and is not connected to the midpoint of voltage divider R3 and R4. I just get to know about transistor a little.
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