# Transistor problem!

1. Sep 14, 2006

### Pseudo Statistic

http://www.brokendream.net/xh4/circDiag.png
We're given this diagram with a question and asked: (NPN transistor)
a) Describe what would happen when the circuit is switched on.
b) Explain why this happens.
c) What is the time constant for the capacitor in this circuit?
d) What is the purpose of the 50k ohm resistor?

I'm having problems answering a.
If the voltage across the base and emitter >= 0.6A, THEN the lamp would be lit..
However, I have ABSOLUTELY NO CLUE how to find the voltage across the base and emitter! Can someone show me how? (I never learnt this in class, with respect to transistors :()
Thanks.

Last edited by a moderator: Apr 22, 2017
2. Sep 14, 2006

### Staff: Mentor

When they say "switched on", they mean that the top rail goes from 0V to 6V pretty much instantaneously. Now, the voltage across the 2200uF cap can't change instantaneously, so there will be an RC ramp-up of the voltage across it until the voltage gets high enough to start to turn on the transistor through the 50k resistor. At that point, some of the RC charging current gets diverted over to the base of the transistor, so the RC charging time constant changes (slows down). The circuit will stabilize at a point where some current is flowing into the transistor base, and some collector current is being pulled down through the load.

Now, use the Beta of the transistor as one of the variables, call the voltage across the cap V1, the voltage at the collector Vc, and the voltage at the base Vb. Take a first cut at the equations for the initial RC charge-up period, then the Vbe turn-on period, and the final stabilized circuit voltages and currents. Show us what you get!