Transistor analysis: find currents and voltages

In summary, the transistor circuit attached can be analysed to find the values of VB, VE, IC, IB and IE using the equations VE = VBB - VBE, IE = (VBB - VBE)/RE, IB = IE/(β+1), IC = β/(β+1) x IE, and VC = VCC - ICRC. By substituting known values of VBB, VBE, RE, β, and VCC, the values of VB, VE, IC, IB and IE can be calculated. However, it is important to note that the previous example provided may be misleading and it is crucial to fully understand the equations in order to correctly analyze the circuit.
  • #1
Steve Collins
46
0

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

VE = VBB – VBE
IE = (VBB – VBE)/RE
IB = IE/β+1
IC = β/(β+1) x IE
VC = VCC – ICRC

β=100
VBE ≈ 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 IE. I'm sure that I am missing something obvious...
 

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  • #2
Steve Collins said:
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 IE. 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.
 
  • #3
Steve Collins said:

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

VE = VBB – VBE
this is the voltage across the 100K, so IB = VE/RB. "VE" is not a good symbol for this voltage.
IE = (VBB – VBE)/RE
what's RE?
IB = IE/(β+1)
right ...
IC = β/(β+1) x IE
right ...

VC = VCC – ICRC
right
β=100
VBE ≈ 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 IE. I'm sure that I am missing something obvious...

See above comments
 
  • #4
I took the relevant equations from a previous example where there was a resistor on the emitter side, RE.

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.
 

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Last edited:
  • #5
Steve Collins said:
I took the relevant equations from a previous example where there was a resistor on the emitter side, RE.

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 RE = 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.
 
  • #6
Steve Collins said:

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 IB.

Set aside your previous example, it is misleading you. The first thing you need to find here is IB. You don't need to know any other parameters before determining IB. (Though you can assume you are dealing with a typical Si junction at B-E.)
 
  • #7
Is this correct?

IB = (VBB - VBE)/RB = (5 - 0.7)/100000 = 43μA

IE = IB(β+ 1) = 43x10-6 x 101 = 4.343mA

IC = β/(β + 1) x IE = 100/101 x 4.343x10-3 = 4.3mA

VE = 0V

VC = VCC - ICRC = 10 - 4.3x10-3 x 2000 = 1.4V

VB = VBB - VBE = 5 - 0.7 = 4.3V
 
  • #8
Steve Collins said:
Is this correct?

IB = (VBB - VBE)/RB = (5 - 0.7)/100000 = 43μA

IE = IB(β+ 1) = 43x10-6 x 101 = 4.343mA

IC = β/(β + 1) x IE = 100/101 x 4.343x10-3 = 4.3mA

VE = 0V

VC = VCC - ICRC = 10 - 4.3x10-3 x 2000 = 1.4V

VB = VBB - VBE = 5 - 0.7 = 4.3V
It's looking better, :smile: though I won't check that your calculator is correctly performing its arithmetic.

But I don't follow this line: VB = VBB - VBE
What is VB if it isn't VBE? VB is usually the voltage difference between base and earth, and here it's 0.7V.
 
  • #9
So VB = VBE + VE

Makes sense, cheers.
 

Related to Transistor analysis: find currents and voltages

1. What is transistor analysis?

Transistor analysis is the process of determining the voltages and currents in a circuit containing transistors. It involves using the principles of circuit analysis and semiconductor physics to analyze the behavior of the transistors and how they affect the overall circuit.

2. Why is transistor analysis important?

Transistor analysis is important because transistors are a fundamental component in electronic circuits and are used in a wide range of devices, from computers to cell phones. By understanding how transistors behave in a circuit, engineers and scientists can design and optimize electronic devices for better performance.

3. What are the different types of transistor analysis techniques?

There are two main types of transistor analysis techniques: DC analysis and AC analysis. DC analysis is used to determine the steady-state behavior of transistors, while AC analysis is used to analyze their behavior when a varying voltage or current is applied.

4. How do you find the currents and voltages in a transistor circuit?

To find the currents and voltages in a transistor circuit, you can use Kirchhoff's laws and Ohm's law to create a system of equations. These equations can then be solved using algebraic or graphical methods, or by using computer software such as SPICE.

5. What are some common challenges in transistor analysis?

Some common challenges in transistor analysis include understanding the complex behavior of transistors, choosing appropriate models for different types of transistors, and taking into account the effects of temperature and frequency on transistor performance. Additionally, transistor circuits can become quite complex, making it difficult to accurately analyze their behavior without the use of simulation tools.

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