Thermionic emission
A heated cathode releases electrons into the vacuum inside a valve. A positive anode attracts these electrons. This is the basis of thermionic valve operation.
Valve construction
A diode valve has a cathode and anode. A triode adds a control grid. A tetrode and pentode add further grids to improve operation. The heater or directly heated cathode provides the heat needed for emission.
Valve characteristic curves
Characteristic curves show how anode current changes with grid voltage or anode voltage. They help explain valve gain, operating point and distortion. The slope of a suitable curve gives useful information about transconductance.
Valve applications
Valves have been used as rectifiers, amplifiers, oscillators, detectors and frequency changers. A detector extracts information from a modulated signal. A frequency changer converts a signal to another frequency.
Semiconductor construction
Semiconductors use materials such as silicon. Doping creates P-type and N-type regions. Joining these regions forms a PN junction. Device behaviour depends on the junction and its bias.
Diodes
A diode conducts readily when forward biased. It blocks most current when reverse biased, until its rated breakdown limit is reached. Diodes are used for rectification, detection, switching and protection.
Zener and other special diodes
A Zener diode is designed to operate in reverse breakdown for voltage reference or regulation. Varactor diodes use voltage-dependent capacitance for electronic tuning. Light-emitting diodes convert electrical energy into light.
Transistors
A bipolar transistor has emitter, base and collector terminals. NPN and PNP are the two main types. A small base current can control a larger collector current. Field-effect transistors use an electric field to control channel current.
Biasing
Biasing sets the DC operating point of a transistor. Correct bias keeps an amplifier in its intended region. Poor bias can cause distortion, excess heat or unstable operation.
Amplifier classes
Class A conducts for the full cycle and offers good linearity but lower efficiency. Class B conducts for half a cycle per device. Class AB reduces crossover distortion. Class C conducts for less than half a cycle and is used in suitable tuned RF stages.
Oscillators
An oscillator produces a repeating signal. It needs an active device and a feedback network that meets the conditions for sustained oscillation. LC, RC and crystal oscillators are common types.
Crystal oscillators
Quartz crystals use the piezoelectric effect. They provide a stable resonant frequency. Temperature, ageing, mechanical stress and circuit loading can still affect their frequency.