Using transistors

[kingofjong]

Hello

I am trying to use a npn transistor, but the circuit simulation program says I have one bad connection. I am not sure what I am doing wrong. I am providing a week signal to the transistor base (0.7v) and I am providing 9 volts to the collector. My load is the resistor. Based on the definition of transistors 9 volts should flow through my resistor. But that is not happening and the current is 11pA. Not sure what I am doing wrong. I have pasted my schematic bellow.



Thanks

 

[BobK]

The circuit you show is called an emitter follower. In that circuit, the voltage at the emitter is approximately the voltage at the base minus 0.7V. Which is, in this case, 0V. So no (or very little current flows). The small current that flows is the amount needed to drop the voltage across the 100Ω resistor enough that the emitter base voltage drops below 0.7V and the transistor is cut off.

Take out the 100Ω resistor and connect the emitter directly to ground.

By the way. It is not a good idea to supply a voltage to the base. The base to emitter is a diode and will have the current through it will be exponential in the voltage, and thus hard to control, and easy to get out of hand. To make things worse, once current starts flowing, it heats up and the current is increased, causing what is known as thermal runaway.

Edited to add: This last paragraph applies equally to LEDs.

Bob
Ω

 

[Harald Kapp]

Take out the 100Ω resistor and connect the emitter directly to ground.

... put the resistor between collector and the 9 V battery instead.

It is not a good idea to supply a voltage to the base.

Use a series resistor to limit base current. In your circuit e.g. 10 kΩ between base and the base voltage source could be used.

Bipolar transistor basics.