G6A

Batteries, diodes, transistors and tubes

Explain what limits real components: battery resistance, diode thresholds, parasitic L and C, transistor regions and tube grids.

12 pool questions1 on the exam

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

Low internal resistance means more current and less sag

A battery is an ideal source in series with a small internal resistance. Load current drops voltage across it, so the terminal voltage sags and heat is wasted. A low internal resistance sags less, so the battery can deliver a high discharge current.

Terminal voltage = 12 V minus (current × internal resistance). Low resistance means little sag.

Load current30 A

Same load, same current30A

RememberLow internal resistance = high discharge current. For maximum life, never run a 12 V lead-acid battery below 10.5 V.

A diode conducts only above its threshold voltage

Forward voltage below the threshold passes almost no current. Just above it, current climbs steeply. The threshold depends on the material.

Below its threshold a diode is nearly off. Above it, current rises steeply.

Diode	Forward threshold
Germanium	0.3 V
Silicon	0.7 V

Memory hookSilicon is the common one: 0.7. Germanium is lower, about half: 0.3.

Real parts have hidden extras, and capacitor types differ

A wire-wound resistor is a coil, so it has inductance: unpredictable at RF. Every coil also has a little turn-to-turn capacitance. Above its self-resonant frequency an inductor stops acting like an inductor and becomes capacitive.

Real parts have hidden extras. At RF they start to matter.

Capacitor types are chosen for what they do best:

Type	Known for
Electrolytic	high capacitance for a given volume (not tight tolerance or low leakage)
Low-voltage ceramic	low cost (not tight tolerance or high stability)

RememberWire-wound resistor: inductance, so avoid at RF. Inductor above self-resonance: capacitive.

A transistor switch uses cutoff and saturation, a MOSFET gate is insulated

A bipolar transistor has three operating regions. Fully off is cutoff, fully on is saturation, and in between is the active region where it amplifies. A switch uses only the two ends. Drag the base current to see.

As a switch, a transistor lives at the two ends of the load line: fully off or fully on.

Base current0 µA

Collector current0.0mA

A MOSFET controls its channel with an electric field. Its gate is separated from the channel by a thin insulating layer.

MOSFET: the gate is insulated from the channel by a thin layer.

RememberSwitch = saturation and cutoff. MOSFET gate = insulated by a thin layer.

Vacuum tube: grids steer the electron flow

A heated cathode boils off electrons toward the positive plate. The control grid between them regulates how many get through. The screen grid sits between control grid and plate and reduces grid-to-plate capacitance.

A small voltage on the control grid regulates a large electron flow to the plate.

Memory hookControl grid = the control. Screen grid = the screen (shield) between input and output.

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Go deeper

Compendium articles on the ideas in this lesson, beyond what the exam asks.

Check what stuck

Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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