General Course, Module 4: The Deeper Parts Bin

4.1Choosing Parts Like You Mean It

Batteries first, because your field box runs on them. Two specifications matter beyond amp-hours:

• A standard 12-volt lead-acid battery should never be discharged below 10.5 volts if you want maximum life. Below that, the chemistry starts eating itself.
• Low internal resistance is the specification behind a battery's ability to deliver high discharge current. Internal resistance is a resistor you cannot remove, in series with everything, and Ohm's Law caps the current it will allow.

Diodes now come with numbers attached. That forward voltage drop from Technician has values: approximately 0.3 volts for germanium, 0.7 volts for silicon. Germanium's smaller toll made it the classic detector diode; silicon's ruggedness made it everything else.

Capacitors have personalities, chosen by job:

• Electrolytic: high capacitance for a given volume, the big cans in power supply filters. The tradeoff is leakage, tolerance, and polarity.
• Ceramic: comparatively low cost, the workhorse for bypassing and general duty.

Resistors carry one RF warning: wire-wound resistors do not belong in RF circuits, because a resistor made of coiled wire is secretly an inductor, and its inductance makes circuit performance unpredictable. At DC it is a resistor; at 14 MHz it is a component you did not design.

That same treachery afflicts real inductors: every coil has stray capacitance between its turns, giving it a self-resonant frequency. Operate an inductor above its self-resonance and it becomes capacitive, an inductor in name only. At RF, every part is all three of R, L, and C; good design just decides which one dominates.

Transistors deepen by one layer each:

• A bipolar transistor used as a switch operates at two points: saturation and cutoff, fully on and fully off, sprinting through the wasteful in-between region. (File this: it becomes amplifier class efficiency in Module 5.)
• The MOSFET's defining construction: its gate is separated from the channel by a thin insulating layer. No current flows into the gate at all; the field does the work through the glass, which is also why static electricity kills MOSFETs.

And the vacuum tube refuses to leave the exam, because high-power amplifiers still use them. Two grid facts: the control grid regulates the flow of electrons between cathode and plate (a small grid voltage steering a large plate current, the tube's whole trick), and the screen grid exists to reduce grid-to-plate capacitance, taming the feedback path that makes tube amplifiers oscillate.

4.2Chips, Connectors, and the Ferrite Drawer

The second half of the cabinet holds the parts that connect, contain, and integrate.

Integrated circuits sort into two great families. An operational amplifier is the classic analog IC, a device that works on continuously varying signals. The digital side's exam entry is CMOS, whose advantage over the older TTL family is low power consumption, the property that put a computer in your pocket. And at microwave, the MMIC, the Monolithic Microwave Integrated Circuit, packs an entire amplifier stage into a part the size of a grain of rice.

Connectors are chosen by frequency and duty, extending your Technician ladder:

• RCA phono: the humble connector for low-frequency and DC connections to a transceiver, the PTT and audio jacks on the back panel.
• BNC: the bayonet quick-connect, good for low-SWR operation to about 4 GHz, which is why test equipment wears it.
• SMA: small and threaded, suitable to several GHz, the handheld antenna connector.
• Type N: the moisture-resistant connector useful to 10 GHz, repeating from Technician with its numbers attached.

The ferrite drawer deserves its own paragraph, because ferrites run through this entire hobby. The performance of a ferrite core at different frequencies is determined by the composition, or "mix," of materials in it: type 43 for HF choking, type 61 higher up, and using the wrong mix means the part politely does nothing. Wind an inductor on a ferrite toroid and you get the full package: large inductance values, magnetic properties optimized for a chosen frequency range, and a magnetic field that stays contained in the core instead of coupling into everything nearby. All of these. And the common-mode chokes you have been clipping onto cables since Technician work by a mechanism you can now name: a ferrite bead reduces common-mode current by creating an impedance in the current's path.

One old friend closes the drawer with its bias stated formally: an LED emits light when forward biased, the same forward current from your Technician course wearing its General-class vocabulary.