# Homework Help: Mode of operation for transistor

1. Feb 10, 2012

### magnifik

I am trying to determine the mode of operation for the following circuit and find the voltages and currents:

I am given that β = 50

I know that VEC > VEC,sat for a transistor in active mode
I applied KVL on the right loop and got:
VCC = 300Ic + 1000Ic + VEC
VEC = VCC - 1300Ic

i'm not sure if the above is right or where to go from here. what is throwing me off is the 300 Ω resistor connected to the 1kΩ resistor. is the node between the two B or C? or are B and C equal in this case?

2. Feb 10, 2012

### rude man

Assuming the transistor is on, what must be the base voltage without writing down any equations at all?

3. Feb 10, 2012

### magnifik

VBE = VBE,on = 0.7 V (as defined in the text)

4. Feb 10, 2012

### rude man

That's the voltage between the base and the emitter. So what's the voltage at the base?

5. Feb 10, 2012

### magnifik

i'm unsure. is it the voltage at that node between the two resistors?

6. Feb 10, 2012

### rude man

Why yes, the base is the node between the two resistors.

7. Feb 10, 2012

### magnifik

what confuses me about this is the position of Vc. normally, it is the voltage across the bottom resistor, but since there are two resistors with a node in between, is Vc the voltage across the 300-ohm resistor? or does Ic occur in both the 300-ohm and 1k-ohm resistor

8. Feb 11, 2012

### rude man

Vc is the collector voltage to ground. "c" stands for "collector".

The base voltage is Vb = Vcc - Vbe = Vcc - 0.7V.

Ic is the current flowing out of the collector.

9. Feb 11, 2012

### magnifik

Vcc = Vbe,on + Vb
--> Vb = 2.5 - 0.7 = 1.8

Vb/1000 + (Vb-Vc)/300 = 0
1.8/1000 + 1.8-Vc/300 = 0
--> Vc = 2.34

Ic = 1.8/1000 = 1.8 mA

2.5 = Vec + 300Ic + 1000Ic
--> Vec = .16

since Vec = 0.2 in this problem and Vec is less than this, the transistor is not in active mode so i re-solve for saturation mode?

10. Feb 12, 2012

### rude man

Yes that's exactly right, sorry for the delay in answering.

So now what do you do? (Hint - the base current is no longer assumable to be ic/50).

11. Feb 12, 2012

### magnifik

i solve for Ie then do the KVL for the CE loop. then solve for Ic from that equation. then get Ib from the relationship Ib = Ie - Ie. if Ib and Ic are both positive then the saturation mode assumption is correct.

thanks for pointing me in the right direction!

12. Feb 12, 2012

### rude man

Yer' welcome! Post your results if you care to.