Calculating Alpha in BJT Common Emitter Circuit

In summary, the conversation discussed the calculation of alpha in a bjt common emitter circuit using two different formulas and why one of them is misleading. The values of alpha were calculated to be approximately 85 and 1.011, with the formula (Ic/Ib)/((Ic/Ib)+1) = Ic/(Ic+Ib) being equal to Ic/Ie. However, there is still a discrepancy in the values, possibly due to factors such as capacitors or resistor configuration. The reason for this misleading result is still unclear.
  • #1
zee824
2
0
i have been asked in my lab report to calculate alpha in a bjt common emmitter circuit using 2 different formulas and explain why one is misleading.β=Ic/Ib= 537.1*10^-6/6.743*10^-6≈85
α= Ic/Ie=573.1*10^-6/566.357*^-6≈1.011
α=β/β +1≈0.988
i have worked out the two values of alpha where our beta is roughly 85 however I am not sure why alpha when calculated from Ic/Ie is missleading.

i have tried to search on the internet however i have found nothing yet and same with my lecture notes.
any help is very much appreciated and i applogise if this is in the wrong section.
thanks
zee
 
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  • #2
welcome to pf!

hi zee! welcome to pf! :smile:
zee824 said:
i have been asked in my lab report to calculate alpha in a bjt common emmitter circuit using 2 different formulas and explain why one is misleading.

β=Ic/Ib= 537.1*10^-6/6.743*10^-6≈85
α= Ic/Ie=573.1*10^-6/566.357*^-6≈1.011
α=β/β +1≈0.988

it's asking why (Ic/Ib)/((Ic/Ib)+1) = Ic/(Ic + Ib) is not equal to Ic/Ie :wink:
 
  • #3


tiny-tim said:
hi zee! welcome to pf! :smile:it's asking why (Ic/Ib)/((Ic/Ib)+1) = Ic/(Ic + Ib) is not equal to Ic/Ie :wink:
hi thanks for the quick reply tiny-tim however in this case they are infact equal since ic+ib=ie but I am still getting different values. i feel it is something to do with the capacitors or the configuration of the resistors that is causing a slight error but I am not sure and I am even less sure about why this is misleading
 
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1. What is the purpose of calculating alpha in a BJT common emitter circuit?

The alpha value, also known as the current gain, is an important parameter used to characterize the amplification capability of a BJT transistor. By calculating alpha, we can determine the amplification factor of the circuit and ensure that it is within the desired range for optimal performance.

2. How do you calculate alpha in a BJT common emitter circuit?

Alpha is calculated by taking the ratio of the collector current (Ic) to the base current (Ib). This can be represented by the equation alpha = Ic/Ib. Alternatively, alpha can also be determined by dividing the collector current by the emitter current (Ie).

3. What is the typical range of alpha values for a BJT common emitter circuit?

The typical range of alpha values for a BJT common emitter circuit is between 50 and 200. This means that for every 1 unit of base current, there will be 50-200 units of collector current. However, the actual alpha value may vary depending on the specific transistor and circuit design.

4. How does changing the value of alpha affect the performance of a BJT common emitter circuit?

Changing the value of alpha can have a significant impact on the performance of a BJT common emitter circuit. A larger alpha value indicates a stronger amplification capability, resulting in a higher output voltage and current. On the other hand, a smaller alpha value may lead to a weaker amplification and therefore lower output.

5. What are some factors that can affect the alpha value in a BJT common emitter circuit?

The alpha value in a BJT common emitter circuit can be influenced by a number of factors, including temperature, biasing voltage, and the physical characteristics of the transistor itself. Additionally, any variations in the transistor's manufacturing process can also result in changes to the alpha value.

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