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G6A

CIRCUIT COMPONENTS

- Resistors; capacitors; inductors; rectifiers; solid-state diodes and transistors; vacuum tubes; batteries

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G6A011 of 12

What is the minimum allowable discharge voltage for maximum life of a standard 12-volt lead-acid battery?

Why A 12-volt lead-acid battery is really six cells in series, and each cell is considered fully discharged at about 1.75 volts. Six times 1.75 gives 10.5 volts, which is the accepted end-of-discharge point. Taking a cell below that drives sulfation of the plates and permanently reduces capacity, so 10.5 volts is the floor for maximum battery life.
Watch out The choice of 12 volts is tempting because that is the nameplate rating, but a healthy lead-acid battery actually rests near 12.6 to 12.7 volts and 12 volts already represents a partly discharged state, not the discharge limit. Values like 6 or 8.5 volts are deep into damaging over-discharge.
Six cells times 1.75 V per cell = 10.5 V. Stop there, or you sulfate the plates.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A022 of 12

What is an advantage of batteries with low internal resistance?

Why A battery behaves like an ideal voltage source in series with its internal resistance, so terminal voltage = EMF minus I times R(internal). The smaller that internal resistance, the less the voltage sags and the less energy is wasted heating the cell when heavy current is drawn. That means a low-resistance cell can deliver large currents, which is why starting and high-power transmit applications call for them.
Watch out Rapid recharge is the closest trap, since charge current also passes through the internal resistance, but the pool credits the discharge side; internal resistance does not set cell voltage (chemistry does) or by itself determine service life.
Low internal R means low voltage sag under load, so it can dump big current.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A033 of 12

What is the approximate forward threshold voltage of a germanium diode?

Why A PN junction does not conduct appreciably until the applied forward voltage overcomes the junction's built-in barrier potential. That barrier depends on the semiconductor material: germanium junctions start conducting at roughly 0.3 volts, while silicon junctions need about 0.7 volts. The lower drop is why germanium diodes were favored for small-signal detection in low-level receiver circuits.
Watch out The 0.7 volt figure is the classic silicon diode threshold, the number the pool asks about in the companion question, so it is easy to swap the two materials.
G comes before S in the alphabet and takes less voltage: Germanium 0.3 V, Silicon 0.7 V.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A044 of 12

Which of the following is characteristic of an electrolytic capacitor?

Why An electrolytic capacitor uses a very thin oxide layer formed on the aluminum (or tantalum) foil as its dielectric, and since capacitance goes up as the dielectric gets thinner, that construction packs a very large capacitance into a small package. That is why values of tens or thousands of microfarads come in parts the size of a thumb, while a film capacitor of the same value would be huge. The tradeoffs are wide tolerance, noticeable leakage current, high internal inductance and a polarity that must be observed.
Watch out Tight tolerance and very low leakage describe good film, mica or ceramic capacitors; electrolytics are among the worst on both counts. They are also poor at RF because of their series inductance and resistance, so they are used for power supply filtering and audio bypassing, not RF tuning.
Electrolytic = big farads in a small can, but sloppy, leaky, polarized and not for RF.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A055 of 12

What is the approximate forward threshold voltage of a silicon junction diode?

Why A PN junction has to be forward biased past its built-in potential before it conducts appreciably. For silicon that knee is right around 0.7 volts, so a conducting silicon diode drops roughly 0.7 V no matter how much current flows. That is why a silicon diode bridge loses about 1.4 V total (two diodes in series conduct on each half cycle).
Watch out The 0.3 volt figure is the threshold for a germanium junction, which is why germanium diodes are preferred in low-level detector circuits where a small signal must still turn the diode on.
Silicon = 0.7 V, germanium = 0.3 V. Si has more letters and a bigger drop.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A066 of 12

Why should wire-wound resistors not be used in RF circuits?

Why A wire-wound resistor is literally a length of resistance wire coiled around a form, which makes it a small inductor as well as a resistor. Inductive reactance rises with frequency (XL = 2*pi*f*L), so at RF that stray inductance adds reactance the designer did not plan for, shifting tuning and impedance and making circuit behavior unpredictable. For RF work use carbon composition, carbon film, or metal film resistors, which have far less inductance.
Watch out Internal capacitance is the wrong mechanism: wire-wound parts do have a little distributed capacitance, but the dominant parasitic by far is the series inductance of the winding. Overheating is not the issue either, since wire-wound resistors are usually chosen precisely for their high power rating.
Wire-wound = wire wound into a coil = an inductor in disguise. Coils and RF do not mix.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A077 of 12

What are the operating points for a bipolar transistor used as a switch?

Why A transistor used as a switch is driven hard between two extremes: cutoff, where base current is zero so essentially no collector current flows and the device looks like an open switch, and saturation, where enough base current is supplied that the collector-emitter voltage drops to a few tenths of a volt and the device looks like a closed switch. Operating at these endpoints minimizes power dissipation, since either the current or the voltage across the device is nearly zero. Linear operation between the two extremes is deliberately avoided in switching service.
Watch out The active region between cutoff and saturation is where a transistor works as a linear amplifier, not a switch; peak and valley currents describe a tunnel diode, and enhancement and depletion refer to FET types.
Switch = both ends of the curve: fully off (cutoff) or fully on (saturation). The middle is for amplifiers.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A088 of 12

Which of the following is characteristic of low voltage ceramic capacitors?

Why Low voltage ceramic capacitors are mass-produced with a simple high-dielectric-constant ceramic disc or chip, so they are among the cheapest capacitors available. That cheap high-K dielectric comes at a price in performance: typical tolerances are loose (often 20 percent or worse) and the capacitance drifts noticeably with temperature, applied voltage, and age. They are fine for bypassing and coupling, where the exact value does not matter.
Watch out Tight tolerance and high stability describe precision types such as silver mica, NP0/C0G ceramics, or polystyrene film capacitors, not ordinary low voltage ceramics; the highest capacitance per unit volume belongs to electrolytic and tantalum capacitors.
Ceramic = cheap and cheerful. Cheap, but not tight, not stable, not the biggest farads per cubic inch.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A099 of 12

Which of the following describes MOSFET construction?

Why MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor, and the "oxide" in the name is the key: a very thin layer of silicon dioxide insulates the gate electrode from the channel between source and drain. The gate therefore controls channel current purely by the electric field it produces, with essentially no gate current flowing. That is why MOSFETs have extremely high input impedance and why they are easily damaged by static discharge puncturing that thin insulating layer.
Watch out The choice about a back-biased (reverse-biased) junction gate describes a JFET, where the gate really is a PN junction rather than an insulated electrode.
The O in MOSFET is the oxide insulator under the gate: gate is insulated, not junction-coupled.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A1010 of 12

Which element of a vacuum tube regulates the flow of electrons between cathode and plate?

Why In a vacuum tube the heated cathode emits electrons that are attracted to the positively charged plate (anode). The control grid sits between them and its small negative voltage swing steers, or throttles, that electron stream, which is how the tube amplifies: a tiny grid voltage change produces a large plate current change. That is the same role the gate plays in a FET.
Watch out The screen grid (in a tetrode) mainly reduces grid-to-plate capacitance and stabilizes plate current, and the suppressor grid (in a pentode) pushes secondary electrons back to the plate; neither is the main current control. There is no 'trigger electrode' in an ordinary amplifying tube.
Control grid controls the current. Grid = gate of a FET; screen and suppressor are just helpers added later.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A1111 of 12

What happens when an inductor is operated above its self-resonant frequency?

Why Every real inductor has distributed capacitance between its turns, so it behaves like an inductance in parallel with a small capacitor. At the self-resonant frequency the two reactances cancel and the impedance peaks; above that frequency the winding capacitance dominates and the part acts like a capacitor, with impedance falling as frequency rises. This is why RF chokes and coils are only usable well below their self-resonant frequency.
Watch out The choice saying reactance increases describes the inductor below self-resonance, where XL = 2*pi*f*L keeps climbing with frequency; above self-resonance the net impedance drops instead.
Past self-resonance, the stray turn-to-turn capacitance wins: the coil stops acting like a coil and acts like a cap.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G6A1212 of 12

What is the primary purpose of a screen grid in a vacuum tube?

Why A triode has significant capacitance between the control grid and the plate, which feeds signal back from output to input (the Miller effect) and can make an RF amplifier oscillate. The screen grid is a mesh electrode placed between the control grid and plate, held at a steady DC potential and bypassed to ground for RF, so it acts as an electrostatic shield between the two. Adding it drops grid-to-plate capacitance from a few picofarads to a small fraction of a picofarad, which is why tetrodes and pentodes are the standard in RF amplifiers.
Watch out Gain and efficiency do improve as a side effect of the added screen, but the reason the electrode is there is shielding; the screen has no direct effect on control grid resistance, and plate resistance actually goes up, not down.
Screen grid = screening, an electrostatic shield that blocks grid-to-plate capacitance.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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