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Thermal runaway in a transistor biased in the active region is due to
1. Heating of the transistor
2. Change in β due to increase in temperature
3. Change in reverse collector saturation current due to rise in temperature
4. Base emitter voltage VBE which decreases with rise in temperature
Which of the above statements is/are correct?
- a)1 and 2
- b)2 and 3
- c)3 only
- d)4 only
Correct answer is option 'B'. Can you explain this answer?
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Thermal runaway in a transistor biased in the active region is due to...
When an input signal is applied, the output signal should not move the transistor either to saturation or to cut-off. However, this unwanted shift still might occur, due to the following reasons
1. Parameters of transistors depend on junction temperature. As junction temperature increases, leakage current due to minority charge carriers (ICBO) increases. As ICBO increases, ICEO also increases, causing an increase in collector current IC. This produces heat the collector junction. This process repeats, and, finally, the Q-point may shift into the saturation region. Sometimes, the excess heat produced at the junction may even burn the transistor. This is known as thermal runaway.
2. When a transistor is replaced by another of the same type, the Q-point may shift, due to changes in parameters of the transistor, such as current gain (β) which varies slightly for each unique transistor and also temperature dependent.
3. Thermal runaway in a transistor biased in the active region is due to change in reverse collector saturation current due to rise in temperature.
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Thermal runaway in a transistor biased in the active region is due to...
Thermal runaway in a transistor biased in the active region refers to the condition when the temperature of the transistor increases uncontrollably, leading to potential damage or failure of the device. Several factors contribute to thermal runaway, and in this case, the correct statements are:
2. Change in β due to increase in temperature
3. Change in reverse collector saturation current due to rise in temperature
Let's discuss each statement in detail:
Change in β due to an increase in temperature:
- The β, or current gain, of a transistor is a crucial parameter that determines the amplification capability of the device.
- In the active region, the β value is relatively constant, but it can vary with temperature changes.
- As the temperature increases, the β value tends to decrease. This can lead to an increase in the collector current, which in turn generates more heat, exacerbating the temperature rise.
Change in reverse collector saturation current due to a rise in temperature:
- The reverse collector saturation current, also known as the leakage current, is the current flowing between the collector and emitter terminals when the transistor is in the off state.
- This leakage current is highly dependent on temperature. As the temperature increases, the reverse collector saturation current also increases.
- The increased leakage current produces more heat, leading to a further temperature rise and potentially causing thermal runaway.
The other two statements are incorrect:
1. Heating of the transistor:
- While heating is a consequence of thermal runaway, it is not the cause. Heating occurs due to the increase in temperature caused by other factors, such as changes in β and reverse collector saturation current.
4. Base-emitter voltage VBE which decreases with the rise in temperature:
- The base-emitter voltage, VBE, typically decreases by about -2mV per degree Celsius with an increase in temperature. However, this decrease in VBE does not directly contribute to thermal runaway in the active region.
- Instead, it affects the biasing of the transistor, potentially leading to a shift in the operating point, which can indirectly impact thermal runaway.
In summary, the correct statements are 2 and 3, as they explain the key factors contributing to thermal runaway in a transistor biased in the active region. The other statements are either consequences or indirectly related to the phenomenon.
2. Change in β due to increase in temperature
3. Change in reverse collector saturation current due to rise in temperature
Let's discuss each statement in detail:
Change in β due to an increase in temperature:
- The β, or current gain, of a transistor is a crucial parameter that determines the amplification capability of the device.
- In the active region, the β value is relatively constant, but it can vary with temperature changes.
- As the temperature increases, the β value tends to decrease. This can lead to an increase in the collector current, which in turn generates more heat, exacerbating the temperature rise.
Change in reverse collector saturation current due to a rise in temperature:
- The reverse collector saturation current, also known as the leakage current, is the current flowing between the collector and emitter terminals when the transistor is in the off state.
- This leakage current is highly dependent on temperature. As the temperature increases, the reverse collector saturation current also increases.
- The increased leakage current produces more heat, leading to a further temperature rise and potentially causing thermal runaway.
The other two statements are incorrect:
1. Heating of the transistor:
- While heating is a consequence of thermal runaway, it is not the cause. Heating occurs due to the increase in temperature caused by other factors, such as changes in β and reverse collector saturation current.
4. Base-emitter voltage VBE which decreases with the rise in temperature:
- The base-emitter voltage, VBE, typically decreases by about -2mV per degree Celsius with an increase in temperature. However, this decrease in VBE does not directly contribute to thermal runaway in the active region.
- Instead, it affects the biasing of the transistor, potentially leading to a shift in the operating point, which can indirectly impact thermal runaway.
In summary, the correct statements are 2 and 3, as they explain the key factors contributing to thermal runaway in a transistor biased in the active region. The other statements are either consequences or indirectly related to the phenomenon.
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