Ham Radio General Class Practice Test

Which of the following solid state devices is most like a vacuum tube in its general operating characteristics?

1. A bipolar transistor

2. A Field Effect Transistor

3. A tunnel diode

4. A varistor

The correct answer is: A Field Effect Transistor

The Field Effect Transistor (FET) is most similar to a vacuum tube in its operating characteristics due to its control mechanism. In vacuum tubes, the flow of electrons is controlled by an electric field, similar to how FETs control current flow. Specifically, in a FET, the gate voltage creates an electric field that alters the conductivity of a semiconductor channel between the source and drain terminals, much like how the grid in a vacuum tube controls the current flow between the anode and cathode. This characteristic of voltage control is akin to the way vacuum tubes operate, where the application of a voltage to the grid influences the flow of electrons. In contrast, bipolar transistors operate based on current control, which diverges from the fundamental operation of vacuum tubes. Tunnel diodes and varistors are designed for specific functions that do not mimic the electron flow control characteristic of vacuum tubes, being more focused on non-linear and voltage-dependent operations rather than amplifying signals like vacuum tubes and FETs.