Effects of dc beta(hfe) in BJT characteristics

In summary, the variation in dc current gain has an effect on the collector current for different input voltages.
  • #1
iampaul
93
0
I need help in answering the post lab questions given to us by our professor.

1. Is dc beta constant on every point of the characteristics? What is its effect in the linearity of the transistor?

2. What is the effect of a high dc beta on the characteristic curves?

I have attached an image of The bjt configuration.

Our data shows that dc beta isn't constant, but I can't see its effects in the linearity. I don't even know how to define linearity for a three terminal device like the BJT, but I guess that it must be based on Vce and Ic. Even if my guess is right, beta only relates Ib with Ic , and I can't think of any effects on linerarity and the characteristics.Applying KVL to the output side of the configuration produces a linear equation relating Vce and Ic. I can't really find an answer.
Please reply. Thanks in advance!
 
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  • #2
I can't upload the file, I made it with paint but it exceeds the limit for file size.

The bjt configuration has a variable dc source and resistor connected to each of the collector and base
 
  • #3
Reload your file with Paint and save it again as a .PNG file.

It should then be a lot smaller than the .BMP file you probably had before.

However, you should be aware that we can't directly answer coursework questions.

But what effect does the variation in DC current gain have on the collector current for different input voltages? Your input signal will usually be a voltage, not a current.
 
  • #4
In our experiment the base current was considered as the input, and the collector current as the output.The voltage Vbb being fed across the base and emitter was controlled to establish a fixed base current. We set the base current to be equal to 50 micro amps, then 100 and 150. Then A voltage source Vcc was fed across the collector and emitter. We measured the voltage across the 0.33 kilo-ohm resistor to calculate Ic and then the beta.

From our data it seems that, beta increases with Vce and Ic, but I really need to know how and why. I reallt do not know what's next.
 
  • #5
Here's the image
 

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1. What is the significance of dc beta (hfe) in BJT characteristics?

The dc beta (hfe) is an important parameter that describes the amplification capability of a bipolar junction transistor (BJT). It represents the ratio of the collector current (IC) to the base current (IB) in a BJT. This value plays a crucial role in determining the amplification and switching characteristics of a BJT.

2. How does dc beta (hfe) affect the gain of a BJT?

The gain of a BJT is directly proportional to its dc beta (hfe) value. A higher value of dc beta (hfe) results in a higher gain, while a lower value leads to lower gain. This is because a higher dc beta (hfe) indicates a higher amplification capability of the transistor.

3. What factors affect the dc beta (hfe) of a BJT?

The dc beta (hfe) of a BJT is affected by various factors such as temperature, collector current, and the physical properties of the transistor. Generally, the dc beta (hfe) decreases with increasing temperature and increasing collector current. It also varies with the type of BJT and its manufacturing process.

4. Can the dc beta (hfe) of a BJT be controlled?

No, the dc beta (hfe) value of a BJT cannot be controlled as it is an inherent characteristic of the transistor. However, it can be selected by choosing a specific type of BJT with a desired dc beta (hfe) value for a particular application.

5. How can the dc beta (hfe) affect the stability of a BJT circuit?

The dc beta (hfe) value of a BJT can affect the stability of a circuit by causing variations in the amplification and switching characteristics. A change in the dc beta (hfe) value can result in a change in the gain and cause instability in the circuit. Therefore, it is important to choose a BJT with a stable and consistent dc beta (hfe) value for reliable circuit performance.

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