Transistor analysis: find currents and voltages

[Steve Collins]

Homework Statement

I’m having trouble with analysing the transistor circuit attached to find VC, VB, VE, IC, IB and IE.

Homework Equations

These equations are taken from a previous example

V_E = V_BB – V_BE
I_E = (V_BB – V_BE)/R_E
I_B = I_E/β+1
I_C = β/(β+1) x I_E
V_C = V_CC – I_CR_C

β=100
V_BE ≈ 0.7V

The Attempt at a Solution

VE = 5 - 0.7 = 4.3V

My attempt falls down at this stage as there is no resistor on the emmiter side to work out I_E. I'm sure that I am missing something obvious...

 

[aralbrec]

VE = 5 - 0.7 = 4.3V

My attempt falls down at this stage as there is no resistor on the emmiter side to work out I_E. I'm sure that I am missing something obvious...

VE will be zero since it is connected directly to ground :)

As you've mentioned, VBE ≈ 0.7 which means you should be able to calculate the base current.

 

[rude man]

Homework Statement

I’m having trouble with analysing the transistor circuit attached to find VC, VB, VE, IC, IB and IE.

Homework Equations

These equations are taken from a previous example

V_E = V_BB – V_BE
this is the voltage across the 100K, so I_B = V_E/R_B. "V_E" is not a good symbol for this voltage. [/color]
I_E = (V_BB – V_BE)/R_E
what's R_E?
I_B = I_E/(β+1)[/color]
right ...
I_C = β/(β+1) x I_E
right ...

V_C = V_CC – I_CR_C
right
β=100
V_BE ≈ 0.7V right

The Attempt at a Solution

VE = 5 - 0.7 = 4.3V

My attempt falls down at this stage as there is no resistor on the emmiter side to work out I_E. I'm sure that I am missing something obvious...

See above comments

 

[Steve Collins]

I took the relevant equations from a previous example where there was a resistor on the emitter side, R_E.

The previous example was from my first and only lecture on this, so far!, and I am trying to use this previous example to work through the above problem. I have attached the previous example in this thread so that you guys can see the extent of my knowledge.

The help is much appreciated as I would like to get my head around this before the subject becomes confusing.

 

[rude man]

I took the relevant equations from a previous example where there was a resistor on the emitter side, R_E.

The previous example was from my first and only lecture on this, so far!, and I am trying to use this previous example to work through the above problem. I have attached the previous example in this thread so that you guys can see the extent of my knowledge.

The help is much appreciated as I would like to get my head around this before the subject becomes confusing.

OK, but here R_E = 0 so you don't want to divide by zero ..

You need to understand the equations rather than just go by a previous example. The equations I gave you, plus the ones you got right, should be fully understood, and they are all you need. Then you can tackle more complex circuits too.

 

[NascentOxygen]

Homework Statement

I’m having trouble with analysing the transistor circuit attached to find VC, VB, VE, IC, IB and IE.

Don't concern yourself with finding any of these, first off, except for I_B.

Set aside your previous example, it is misleading you. The first thing you need to find here is I_B. You don't need to know any other parameters before determining I_B. (Though you can assume you are dealing with a typical Si junction at B-E.)

 

[Steve Collins]

Is this correct?

I_B = (V_BB - V_BE)/R_B = (5 - 0.7)/100000 = 43μA

I_E = I_B(β+ 1) = 43x10^-6 x 101 = 4.343mA

I_C = β/(β + 1) x I_E = 100/101 x 4.343x10^-3 = 4.3mA

V_E = 0V

V_C = V_CC - I_CR_C = 10 - 4.3x10-3 x 2000 = 1.4V

V_B = V_BB - V_BE = 5 - 0.7 = 4.3V

 

[NascentOxygen]

Is this correct?

I_B = (V_BB - V_BE)/R_B = (5 - 0.7)/100000 = 43μA

I_E = I_B(β+ 1) = 43x10^-6 x 101 = 4.343mA

I_C = β/(β + 1) x I_E = 100/101 x 4.343x10^-3 = 4.3mA

V_E = 0V

V_C = V_CC - I_CR_C = 10 - 4.3x10-3 x 2000 = 1.4V

V_B = V_BB - V_BE = 5 - 0.7 = 4.3V

It's looking better, though I won't check that your calculator is correctly performing its arithmetic.

But I don't follow this line: V_B = V_BB - V_BE
What is V_B if it isn't V_BE? V_B is usually the voltage difference between base and earth, and here it's 0.7V.

 

[Steve Collins]

So V_B = V_BE + V_E

Makes sense, cheers.