Ham Radio General Class Practice Test

Why must the cases of some large power transistors be insulated from ground?

1. To increase the beta of the transistor

2. To improve the power dissipation capability

3. To reduce stray capacitance

4. To avoid shorting the collector or drain voltage to ground

The correct answer is: To avoid shorting the collector or drain voltage to ground

The insulating of the cases of some large power transistors from ground is essential primarily to avoid shorting the collector or drain voltage to ground. In power transistors, the collector (or drain, depending on whether you're dealing with a bipolar junction transistor or a field-effect transistor) is typically at a high voltage. If the transistor's case is not insulated and is instead grounded, it would create a direct electrical connection that could potentially lead to a short circuit. Such an event can damage the transistor and the entire circuit, leading to failure of the device and safety hazards. The design of many power transistors inherently involves high voltages and currents, making proper insulation critical. Insulating the case ensures that any high voltages present within the transistor do not find an unintended pathway to ground, hence maintaining the integrity and safe operation of the device.