Table of Contents
Bipolar Junction Transistors Current Source circuits pass a certain amount of collector current, even if the load on the collector side changes.
The NPN BJT base voltage and emitter resistor set the current. The emitter voltage is 0.6V less than the base voltage. The emitter voltage is held across the emitter resistor.
The collector will pass the whatever current is needed to hold the voltage across the emitter resistor.
Using Ohms law, we can see how much current is flowing through the emitter resistor.
- 1.6V @ Base = 1V @ emitter/220Ω = 0.0045A (4.5mA) though emitter resistor and anything on collector side of the NPN BJT.
- 2.6V @ Base = 2V @ emitter/220Ω = 0.0091A (9.1mA) though emitter resistor and anything on collector side of the NPN BJT.
- Blue LEDs need about 3V to light up. When using a 5V supply, only about 2V will be able to make it to the emitter.
- 3.6V @ Base = 3V @ emitter/220Ω = 0.0136A (13.6mA) though emitter resistor and anything on collector side of the NPN BJT.
- A red LED load on the collector side needs about 2V to light the LED. So when using a 5V supply, only about 3V will be able to make it to the emitter.
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