G6A: Basic Components – Ham Radio General License Study Guide

G6A covers the discrete electronic components that form the building blocks of radio circuits: batteries, diodes, capacitors, resistors, bipolar transistors, MOSFETs, vacuum tubes, and inductors. Understanding each component's key characteristics prepares you both for the exam and for practical work on amateur radio equipment.

The exam draws from topics including the minimum safe discharge voltage for a lead-acid battery, what makes a low internal resistance battery useful, the forward threshold voltages of germanium and silicon diodes, the distinguishing characteristics of electrolytic and ceramic capacitors, why wire-wound resistors are unsuitable for RF circuits, the operating points of a bipolar transistor used as a switch, how a MOSFET gate is constructed, which vacuum tube element controls electron flow and what the screen grid does, and what happens to an inductor operated above its self-resonant frequency.

Batteries

A standard 12-volt lead-acid battery should never be discharged below 10.5 volts for maximum service life. Discharging deeper than this causes lead sulfate crystals to form on the plates in a way that cannot be fully reversed during recharging — a process called sulfation — which permanently reduces the battery's capacity.

Batteries with low internal resistance can deliver high discharge current. All batteries have some internal resistance; when current flows, a voltage drop occurs across that internal resistance. A battery with low internal resistance loses less voltage under load and can supply more current before its terminal voltage collapses. This is why premium batteries used in transceivers and emergency power systems are selected partly for low internal resistance — it allows the transceiver to draw the large peak currents needed during transmit without a significant voltage drop.

Diodes

A diode allows current to flow freely in one direction (forward biased) and blocks current in the other direction (reverse biased). However, a small forward voltage must be applied before the diode begins conducting — this is called the forward threshold voltage.

Capacitors

Electrolytic capacitors provide high capacitance in a small physical volume — their defining characteristic. They achieve this through a thin oxide layer acting as the dielectric between plates. The tradeoff is that they are polarized (must be installed with the correct polarity), have relatively high leakage current, and are not suitable for AC or RF applications. They are used extensively in power supply filters where large capacitance at low cost and small size is needed.

Low-voltage ceramic capacitors are comparatively low in cost. They are inexpensive to manufacture and widely used. However, ceramic capacitors at low voltage ratings are less stable over temperature and voltage than film types, and they offer less capacitance density than electrolytics.

Resistors

Wire-wound resistors are constructed by winding resistance wire around a ceramic or fiberglass core. This winding gives the resistor significant inductance. In DC and low-frequency circuits this inductance is negligible, but in RF circuits the inductance can make circuit performance unpredictable — the resistor no longer acts as a simple resistance and can interact with the surrounding circuit in unintended ways. For RF circuits, carbon-composition, metal-film, or surface-mount chip resistors are used instead.

Bipolar Transistors and MOSFETs

A bipolar transistor used as a switch operates at two points on its characteristic curve:

• Saturation — the transistor is fully on; maximum collector current flows and the collector-emitter voltage is very low
• Cutoff — the transistor is fully off; no collector current flows

The region between saturation and cutoff (the active region) is where the transistor amplifies. A switch toggles rapidly between saturation and cutoff, avoiding the active region during steady-state operation.

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) has its gate electrode separated from the semiconductor channel by a thin insulating layer — typically silicon dioxide. This insulating layer gives the MOSFET an extremely high input impedance because essentially no DC current flows into the gate. The gate voltage controls the conductivity of the channel through capacitive coupling through the insulating layer.

Vacuum Tubes

In a vacuum tube, electrons emitted from the heated cathode travel toward the positive plate (anode). The control grid, placed between cathode and plate, regulates this electron flow. A negative voltage on the control grid repels electrons; a less-negative or positive voltage allows more electrons through. This is how the tube amplifies — small voltage changes on the control grid produce large changes in plate current.

The screen grid, placed between the control grid and the plate, has a different purpose: it reduces the capacitance between the control grid and the plate. Without the screen grid, this capacitance would allow RF energy to feed back from the plate to the grid, causing instability. By shielding the control grid from the plate, the screen grid allows the tube to amplify at high frequencies without oscillating.

Inductor Self-Resonance

Every real inductor has a small amount of distributed capacitance between its windings. This capacitance increases as turns are packed more closely together. At a specific frequency called the self-resonant frequency (SRF), this winding capacitance resonates with the inductance of the coil.

Below the SRF, the component behaves as an inductor (reactance increases with frequency). At the SRF, the impedance peaks. Above the SRF, the component becomes capacitive — the capacitance of the winding dominates and the reactance decreases with increasing frequency. An inductor used above its SRF will not perform as intended and may cause circuit problems. This limits the usable frequency range of wound inductors, especially at VHF and above.

G6A Practice Questions