How Do You Calculate Ib, Ic, and Rb in a Transistor Amplifier Circuit?

[TyErd]

Homework Statement

Find Ib, Ic and Rb of the following diagram where the current gain is 150

Homework Equations

V=IR, and Ai = Ic / Ib

The Attempt at a Solution

I am having more trouble knowing how to divide the voltages and currents up in this circuit. I can't really show any solutions without knowing how to divide it all up.

 

[willem2]

You can calculate I_c with ohms law.
You can calculate I_b if you know I_c and the current gain

 

[TyErd]

but how do I figure out how much voltage goes through Rc?

 

[willem2]

One side of Rc is at Vcc, which is 12V, the other side is at Vout which is 5V

 

[TyErd]

so is Vout the remaining voltage?

 

[willem2]

so is Vout the remaining voltage?

I don't know what you mean by that. One Side of Rc is at 12V and the other side at 5V.
What is the potential difference across the resistor?

 

[Mindscrape]

Nah man, V=IR, which means that I=V/R, and one thing that you may not realize about Ohm's law is that it applies to any voltage drop. Across that 350 ohm resistor you know both the voltages, so you can easily figure out that current. Once you know the collector current, you can get the base current because you know the gain. From there you can once again use the same principle to figure out the resistance of the resistor you want.

This is a good design problem. It shows you a good way to get 5V from a transistor.

 

[TyErd]

What exactly is the output voltage of 5 of? the resistor or the transistor?

 

[Mindscrape]

It's a node that is shared by both the resistor and transistor.

 

[TyErd]

okay, but I am still having trouble understanding how much voltage is across Rc. 12V is the supply voltage and the output voltage is 5V, does that mean the voltage across the resistor is 5V because that's the output voltage? But even then because the 5V is shared between the resistor and transistor, how do you know exactly much voltage is across the resistor. Sorry for making this all confusing

 

[Mindscrape]

It's the voltage drop, or change in voltage.

Ic=(12V-5V)/350Ohms

 

[TyErd]

ohhh so 7V went through that resistor?

 

[Mindscrape]

Yep. :)

 

[TyErd]

okay for finding Ib, because 7V goes through Rc that must mean 5 goes through Rb correct? then because it is a series circuit i can use the current found from the previous question which is 0.02A and then use I=V/R?

 

[willem2]

okay for finding Ib, because 7V goes through Rc that must mean 5 goes through Rb correct? then because it is a series circuit i can use the current found from the previous question which is 0.02A and then use I=V/R?

It's not correct. Note that the bottom of the circuit is connected to the earth, so it's at 0V. The base of the transistor is at one diode drop above 0.

Rb is NOT in series with Rc, so the same current does not go through them. You have to find the current through Rb using the fact that Ic = (current gain) * Ib