This page is part of the N0NJY General Class self-study course for Technician operators upgrading to General.
G6 covers the physical components that make up the circuits in your station equipment — semiconductors, vacuum tubes, passive components, ferrite materials, batteries, and digital logic devices. For most operators, this knowledge serves two practical purposes: understanding what your equipment is doing so you can operate it intelligently, and being able to diagnose and repair faults when something stops working correctly.
This module gives you a thorough working knowledge of semiconductors and ferrite materials, enough vacuum tube knowledge to pass the exam and understand tube amplifier terminology, a factual treatment of how components fail, and battery knowledge grounded in what actually damages them in the field.
Semiconductor devices are built from materials — primarily silicon — whose electrical conductivity falls between conductors (metals) and insulators (glass, ceramic). By introducing controlled impurities into pure silicon (a process called doping), manufacturers create two types of semiconductor material:
N-type silicon has extra electrons available for conduction. The majority carriers are electrons (negative charge carriers).
P-type silicon has a deficit of electrons, creating “holes” that behave as positive charge carriers. The majority carriers are holes.
Virtually all semiconductor devices are built from junctions between N-type and P-type material. Understanding what happens at these junctions is the key to understanding diodes, transistors, and FETs.
Where N-type and P-type silicon meet, electrons from the N side diffuse across the junction and fill holes on the P side. This creates a thin depletion region with no free charge carriers — essentially an insulating barrier. This depletion region is why a semiconductor junction blocks current in one direction.
When you apply a forward bias voltage (positive to the P side, negative to the N side), you push electrons and holes toward the junction, collapse the depletion region, and allow current to flow. When you apply reverse bias (positive to N, negative to P), you pull carriers away from the junction, widen the depletion region, and block current. This one-way current flow is the fundamental property of all diode-based devices.
A rectifier diode passes current in one direction and blocks it in the other. The key specifications:
A zener diode is specifically designed to operate in reverse breakdown at a controlled, precise voltage — the zener voltage. Below the zener voltage in reverse bias, the diode blocks current normally. At the zener voltage, it breaks down in a controlled, non-destructive way and clamps the voltage at that level. This makes zener diodes ideal for voltage regulation and reference circuits. If the zener voltage is 5.1 volts, the voltage across a properly loaded zener will be maintained at 5.1 volts regardless of modest changes in supply voltage or load current, as long as sufficient current flows through the zener.
An LED is a PN junction made from semiconductor materials that emit photons when forward current flows. The forward voltage drop of an LED is higher than a silicon rectifier diode and depends on the color (wavelength) of the emitted light:
LEDs must always be used with a current-limiting resistor in series. Without a series resistor, the LED draws excessive current, overheats, and fails. The resistor value is calculated from the supply voltage, the LED forward voltage, and the desired operating current (typically 10 to 20 milliamps for indicator LEDs).
Diodes fail in two distinct ways, and knowing which has occurred tells you what caused the failure:
A bipolar junction transistor has three terminals — base, collector, and emitter — and two PN junctions. It is a current-controlled device: a small current flowing into the base controls a much larger current flowing from collector to emitter. This current amplification is the basis of transistor amplifier and switching circuits.
In an NPN transistor, the collector and emitter are N-type, the base is P-type. Current flows from collector to emitter when base current flows into the base. This is the more common type in modern circuits. In a PNP transistor, the polarities are reversed — current flows from emitter to collector when base current flows out of the base.
Beta (β or hFE) is the current gain of a BJT: the ratio of collector current to base current. A typical small-signal transistor might have a beta of 100 to 300, meaning 1 milliamp of base current controls 100 to 300 milliamps of collector current. Beta varies significantly between individual transistors of the same part number and changes with temperature — this is one reason amplifier circuits use negative feedback to stabilize gain rather than relying on the transistor's beta alone.
BJTs most commonly fail from thermal overstress:
A field-effect transistor controls current through a channel using an electric field rather than a control current. The three terminals are gate, drain, and source — analogous to base, collector, and emitter in a BJT. FETs are voltage-controlled devices: the voltage applied to the gate controls the current flowing from drain to source. Because the gate is insulated from the channel (in MOSFETs) or reverse-biased (in JFETs), the input impedance is extremely high — often thousands of megaohms. Almost no current flows into the gate under normal operating conditions.
In a JFET, the gate-channel junction is a reverse-biased PN junction. The gate voltage varies the width of the depletion region around the channel, which varies the channel's resistance and thus the drain current. JFETs are normally-on devices — with zero gate voltage, drain current flows. Applying a reverse gate voltage (for an N-channel JFET, a negative voltage on the gate) reduces and eventually cuts off drain current.
In a MOSFET, the gate is insulated from the channel by a thin layer of silicon dioxide (the oxide in the name). This insulation gives the MOSFET its extremely high input impedance and also makes it vulnerable to static electricity — the insulating oxide layer can be permanently punctured by a static discharge of only a few hundred volts. Always handle MOSFETs with ESD (electrostatic discharge) precautions: use a grounded wrist strap, avoid handling the leads unnecessarily, and store them in conductive foam or anti-static bags.
MOSFETs are the dominant device in modern RF power amplifiers because they offer:
An operational amplifier is a high-gain, differential input voltage amplifier in an integrated circuit package. The op-amp has two inputs — inverting (−) and non-inverting (+) — and one output. The output voltage equals the differential input voltage multiplied by the open-loop gain, which is typically 100,000 or more. In practice, op-amps are almost never used open-loop; external feedback resistors set a precise, controlled gain.
The standard schematic symbol is a triangle with the two inputs on the left and the output on the right. The inverting input is marked with a minus sign (−), the non-inverting input with a plus sign (+). Power supply connections (V+ and V−) are usually shown on the top and bottom of the triangle but are sometimes omitted from simplified schematics to reduce clutter.
When you see an op-amp in a circuit with a resistor from the output back to the inverting input, that is a feedback resistor setting the closed-loop gain. The basic closed-loop gain formula for an inverting amplifier is:
Gain = −(Rfeedback / Rinput)
The negative sign indicates that the output is inverted relative to the input. For a non-inverting amplifier (input at the + terminal), the gain is positive and equals 1 + (Rfeedback / Rinput).
Vacuum tubes predate transistors and were the only active amplifying devices available in amateur radio equipment until the late 1950s. Many operators still use tube-based power amplifiers, and tube-based equipment from the 1950s through 1970s remains in active use. The General Class exam covers the basic types.
A triode has three elements: a cathode (heated to emit electrons), a control grid (a mesh or wire between cathode and plate), and a plate (anode). Electrons emitted from the heated cathode travel through the grid to the plate. The voltage on the control grid controls the electron flow — a small negative voltage on the grid reduces plate current; making the grid less negative increases plate current. This voltage control of plate current makes the triode an amplifier.
Adding a second grid (screen grid) between the control grid and plate produces a tetrode. The screen grid, held at a positive voltage, accelerates electrons toward the plate and reduces the capacitance between control grid and plate, improving high-frequency performance. The pentode adds a third grid (suppressor grid) between the screen and plate, connected to the cathode, to suppress secondary electron emission from the plate. Pentodes are more efficient and have higher gain than triodes and are widely used in RF power amplifiers.
Understanding how passive components fail is directly useful for troubleshooting. The failure modes described here are factual and documented — not speculation.
Resistors most commonly fail open. When a resistor is subjected to power dissipation beyond its rated wattage, the resistive element burns through, producing an open circuit. A carbon composition resistor that has been overloaded may also show a significant increase in resistance value (drift) before failing open. Wire-wound resistors can also open from thermal failure of the wire element or from mechanical fatigue in high-vibration environments.
Resistors almost never fail shorted. If you measure near-zero resistance across a resistor that should read several kilohms, suspect contamination (flux, moisture) on the circuit board bridging the resistor terminals, not a shorted resistor itself.
Capacitors fail in two distinct ways depending on their type:
Ceramic and film capacitors most commonly fail open — the dielectric or electrode structure breaks down and the capacitor stops conducting AC. Less commonly, ceramic capacitors fail shorted, which is more damaging to the surrounding circuit.
Electrolytic capacitors (aluminum electrolytic, tantalum) have three common failure modes:
Inductors most commonly fail open — the wire breaks due to mechanical stress, corrosion, or thermal failure at a solder joint or at the point where the wire enters the form. Wire-wound coils in RF circuits can also develop inter-winding shorts if the winding insulation breaks down, which shifts the resonant frequency and reduces Q without completely stopping operation.
Ferrite is a ceramic magnetic material used in inductors, transformers, and RF chokes throughout amateur radio equipment. Different ferrite formulations (called mix numbers or material types) have very different properties at different frequencies. Using the wrong ferrite material for a given application produces a component that either does not work at all or performs far below expectations.
The permeability and loss characteristics of ferrite change dramatically with frequency. A ferrite material optimized for audio-frequency transformers is nearly useless as an HF choke. A material optimized for HF chokes may saturate and overheat when used as a power transformer core. The two properties that determine whether a ferrite material is appropriate for a given application are:
| Mix / Material | Color Code | Frequency Range | Best Use |
|---|---|---|---|
| Type 31 (Fair-Rite) | No standard color | 1–300 MHz | HF common-mode chokes; excellent for 1–30 MHz suppression |
| Type 43 (Fair-Rite) | No standard color | 10 MHz–1 GHz | HF/VHF chokes and ferrite beads; less effective below 10 MHz than Type 31 |
| Type 61 (Fair-Rite) | No standard color | 200 MHz–2 GHz | VHF/UHF applications; poor choice for HF |
| Mix 2 (Amidon/Micrometals) | Red | 1–30 MHz | HF inductors; low loss at HF; good for transmitting coils |
| Mix 6 (Amidon/Micrometals) | Yellow | 10–50 MHz | Upper HF and low VHF inductors |
| Mix 12 (Amidon/Micrometals) | Green/white | 50 MHz+ | VHF inductors |
| Mix 26 (Amidon/Micrometals) | Yellow/white | DC–1 MHz | Power transformers and audio chokes; not for RF |
For the most common amateur radio application — building a common-mode choke to prevent RF from traveling back into the shack on coaxial feedline — Type 31 ferrite (Fair-Rite) is the material of choice for HF. Wind eight to twelve turns of coaxial cable through a large Type 31 toroid. Type 43 also works but is less effective at the low end of HF (below 10 MHz).
Battery technology is relevant to amateur radio for portable operation, emergency communication, and backup power. The two most important types for operators are lead-acid (the most common 12V station and portable battery) and lithium iron phosphate (LiFePO4, increasingly popular for portable and field operation). Each type has specific damage mechanisms that operators in the field often encounter.
A fully charged 12V lead-acid battery has an open-circuit voltage of approximately 12.6 to 12.7 volts. The damage mechanisms in order of frequency:
Deep discharge (sulfation): When a lead-acid battery is discharged below approximately 10.5 volts, lead sulfate crystals form on the plates. Light sulfation can be reversed with a desulfation charger. Severe sulfation permanently reduces capacity and internal resistance. This is the most common cause of premature lead-acid battery failure. The 10.5-volt limit applies to the battery under load — not open-circuit voltage. A battery that reads 11.5 volts open-circuit may drop below 10.5 volts under the load of a 100-watt transceiver.
Overcharging: Sustained overcharging causes electrolysis of water in the electrolyte, producing hydrogen and oxygen gas. The battery loses water, the electrolyte concentration increases, and the plates corrode. In a sealed (AGM or gel) battery, overcharging can rupture the case. In a flooded battery, water can be added, but repeated overcharging degrades the plates regardless. The correct float charge voltage for a 12V lead-acid battery is 13.5 to 13.8 volts.
Temperature extremes: Lead-acid batteries lose capacity significantly at low temperatures. A battery that provides 100% capacity at 77°F (25°C) may provide only 50% capacity at 0°F (−18°C). In cold weather field operation, keep batteries warm — insulated containers, body heat, or vehicle interiors. High temperatures accelerate self-discharge and reduce battery lifespan.
Vibration and physical shock: In mobile installations, vibration can shed active material from the plates, which settles at the bottom of the case and eventually short-circuits the plates. Use batteries designed for deep-cycle or marine applications (which have thicker, more robust plates) in mobile installations rather than automotive starting batteries.
LiFePO4 batteries are increasingly used for portable amateur radio operation because they are significantly lighter than lead-acid batteries for the same capacity, tolerate deeper discharge, and have longer cycle life. The nominal voltage of a LiFePO4 cell is 3.2 volts (versus 2.0V for lead-acid), giving a 12V system four cells in series at 12.8V nominal. Damage mechanisms:
Charging with the wrong charger: A LiFePO4 battery must be charged with a charger specifically designed for that chemistry. A lead-acid charger will overcharge a LiFePO4 battery. The correct charge voltage for a 4-cell (12V nominal) LiFePO4 pack is 14.4 to 14.6 volts. Lead-acid chargers typically charge to 14.4 to 14.8 volts, which may seem similar but the charge algorithm (constant current/constant voltage profile) is different. Always use a lithium- compatible charger and verify the voltage settings before connecting.
Deep discharge without BMS protection: Discharging a LiFePO4 cell below approximately 2.5 volts permanently reduces capacity and may cause internal damage. Most commercial LiFePO4 battery packs include a Battery Management System (BMS) that disconnects the output before the cells reach the minimum voltage. However, the BMS protects the cells from the outside — it cannot prevent damage from internal self-discharge over a long storage period. Store LiFePO4 batteries at approximately 50 to 80% state of charge if they will not be used for several months.
Charging below freezing: Charging a lithium battery at temperatures below 0°C (32°F) causes lithium plating on the anode, which permanently reduces capacity and can create internal short-circuit paths that pose a safety risk. Unlike lead-acid batteries (which just lose capacity in the cold), charging a lithium battery below freezing causes structural damage. Do not charge LiFePO4 batteries in freezing conditions.
Modern amateur radio station equipment — microcontroller-based antenna tuners, digital mode interfaces, SDR hardware, logging computers, and remote control systems — all use digital logic at some level. Understanding the basics allows you to read a block diagram, understand what a microcontroller is doing, and diagnose digital circuit behavior.
All digital logic is built from combinations of a small number of fundamental gate types. Each gate has one or more inputs and one output. The output state (HIGH = 1 or LOW = 0) is determined by the input states according to the gate's truth table.
| Gate | Symbol | Rule | Application |
|---|---|---|---|
| AND | & | Output HIGH only when ALL inputs are HIGH | Enable/disable circuits; all conditions must be met |
| OR | ≥1 | Output HIGH when ANY input is HIGH | Either condition triggers output |
| NOT (Inverter) | Triangle + bubble | Output is opposite of input | Signal inversion; polarity conversion |
| NAND | & + bubble | Output LOW only when ALL inputs are HIGH (NOT-AND) | Universal gate; can implement any logic function |
| NOR | ≥1 + bubble | Output HIGH only when ALL inputs are LOW (NOT-OR) | Universal gate; detector for all-zero condition |
| XOR | =1 | Output HIGH when inputs DIFFER | Parity checking; difference detector |
A microcontroller is a complete computer on a single integrated circuit: processor, memory, and input/output peripherals in one package. In amateur radio equipment, microcontrollers handle:
Understanding that a microcontroller executes software and communicates through defined interfaces (serial port, USB, I²C, SPI) explains why transceiver CAT control requires matching baud rate, parity, and stop bit settings — the microcontroller in the radio is following a specific protocol, and your software must match it exactly.
Beyond op-amps and logic gates, amateur radio equipment uses several other IC types worth recognizing on a schematic:
The G6 subelement covers circuit components as tested in the 2023–2027 FCC General Class question pool. All pool questions are covered below.
Q1 (G6A01) — What is the minimum allowable discharge voltage for maximum life of a standard 12-volt lead-acid battery?
Q2 (G6A02) — What is an advantage of the low internal resistance of nickel-cadmium batteries?
Q3 (G6A03) — What is the approximate junction threshold voltage of a germanium diode?
Q4 (G6A04) — Which of the following is an advantage of a MOSFET compared to a bipolar transistor?
Q5 (G6A05) — What is the approximate forward junction threshold voltage of a silicon diode?
Q6 (G6A06) — Which of the following is a minimum rating that must be considered when selecting a diode for a power supply?
Q7 (G6A07) — What are the stable operating points for a bipolar transistor used as a switch in a logic circuit?
Q8 (G6A08) — Why must the square wave peak-to-peak voltage be much less than the breakdown voltage of a bipolar transistor used in a mixer?
Q9 (G6A09) — Which of the following describes the construction of a MOSFET?
Q10 (G6A10) — Which of the following is a characteristic of a liquid-crystal display?
Q11 (G6A11) — What is the primary advantage of a toroidal core versus a solenoidal core?
Q12 (G6A12) — What is the term for the reduction in an inductor's inductance with increasing current?
Q13 (G6A13) — What do the letters "FET" stand for?
Q14 (G6B01) — In what application is gallium arsenide used as a semiconductor material in preference to germanium or silicon?
Q15 (G6B02) — Which of the following devices is used as a stable reference voltage in a linear voltage regulator circuit?
Q16 (G6B03) — Which of the following is an advantage of BiCMOS logic?
Q17 (G6B04) — Which is the most common type of logic device that combines MOSFET and bipolar transistors in a single device?
Q18 (G6B05) — What is a characteristic of a ferrite bead RF choke when placed on the lead of a component?
Q19 (G6B06) — What is a common use for a ferrite core toroid?
Q20 (G6B07) — What is a characteristic of an ideal operational amplifier?
Q21 (G6B08) — How does a ferrite bead or core at the base of a VHF/UHF linear amplifier help prevent oscillations?
Q22 (G6B09) — What is a common use for junction field-effect transistors?
Q23 (G6B10) — Which element of a triode vacuum tube has the most influence over plate current?
Q24 (G6B11) — Which of the following is the primary advantage of using a Schottky diode in an RF switching circuit compared to a standard silicon diode?
Q25 (G6B12) — What is the peak inverse voltage of a rectifier in a full-wave bridge power supply?