Question

What is thermal Runaway? How do you avoid it in amplifier circuits using BJT? Derive suitable expression to avoid it

Answer 1

We know that in a transistor , power is dissipated in the collector and hence it is made physically larger than the emitter and base region. As the power is dissipated , there is a chance for the collector base junction temperature to be raised. As the temperature at collector base junction increases, the reverse leakage current ICBO increases. This is because ICBO arises due to the flow of minority carriers which are thermally generated across reverse biased collector-base junction (reverse biased pn junction) . As the temperature increases, thermal generation increases, ICBO increases..

IC = αIE + ICBO

So, as ICBO increases, IC increases. Power dissipated =

I2 * R.

So, as collector current increases, power dissipated increases which in turn increases the collector base junction temperature. So the process is cumulative leading eventually to the destruction of the transistor.

Thermal runaway can be prevented by using aheat sink.

Answer 2

Thermal runaway is a process of positive feedback. It can be controlled by controlling thermal impedance.

Explanation:

• Thermal runaway is a process of positive feedback. In this process, the system gets hot and dissipated the heat in the surrounding and further gets hotter causing destructive results.
• It is a process of uncontrolled feedback.
• it can be generally expressed as ΔT = ΔW x R (thermal)
• It can be prevented by creating a small change in temperature to a big change in power dissipation.
• In order to control it, you need to control the thermal impedance.