The circuit shown below uses a transistor to turn on an LED. The LED lights when enough forward current flows through it. Assume that this diode has a forward voltage diode drop of 1.8V at 5mA and that 5mA is required to produce light. Also assume that Vcc=5V and that the transistor has a baseemitter diode drop of Vbe=0.6V and VCE-SAT=0.2V. Remember that VCE-SAT is simply the voltage drop across the collector-emitter terminal when the transistor is in saturation.
a) Write down KVL for both the base loop and the collector loop when the transistor is in saturation.
b) If the input voltage Vin is 3V, find the value of Rb that limits the current drawn from Vin to 0.2mA.
c) Find the value of Rc that allow the required current to flow when the transistor is in saturation.
d) What is the minimum β for this transistor that will light the LED?
e) Is the LED on when Vin is high or low?