Diodes and transistors
Junctions, forward voltage drops, rectification, Zener regulation, and what the three transistor terminals do.
7 minute read
Semiconductors are where a lot of exam marks live, and the concepts are simpler than the terminology suggests.
The PN junction
Join P-type and N-type semiconductor material and you get a junction that conducts in one direction and blocks in the other. That is a diode.
- Forward biased — positive to the anode, current flows
- Reverse biased — positive to the cathode, current is blocked
Forward voltage drops worth memorising:
| Material | Forward drop |
|---|---|
| Germanium | ~0.3 V |
| Silicon | ~0.7 V |
| Schottky | ~0.2–0.4 V |
| LED | ~1.8–3.3 V, colour dependent |
The silicon figure of 0.7 V is the one that comes up most.
Rectification
Because a diode conducts one way only, it converts AC into pulsating DC.
- Half-wave — one diode, uses one half of each cycle. Ripple frequency equals the mains frequency. Inefficient and harder to filter.
- Full-wave / bridge — uses both halves. Ripple frequency is twice the mains frequency. Smaller gaps to fill, so less ripple and easier filtering.
That doubling of ripple frequency is a favourite exam point.
The Zener diode
A Zener is designed to break down at a precise reverse voltage and hold that voltage steady. Operated reverse biased beyond its breakdown voltage, it makes a simple and reliable voltage regulator or reference.
Note the direction — this catches people out. A normal diode is used forward biased; a Zener is deliberately used in reverse.
Transistors
Bipolar junction transistor (BJT)
Three terminals: emitter, base, collector.
A small current into the base controls a much larger current from collector to emitter. It is a current-controlled device. Two types, NPN and PNP, differing in polarity.
Field-effect transistor (FET)
Three terminals: source, drain, gate.
A voltage on the gate controls current from source to drain. Because the gate draws essentially no current, FETs present very high input impedance — useful in receiver front ends and RF amplifier stages.
Remembering which terminal set belongs to which device is worth a mark on its own:
Emitter-Base-Collector = bipolar. Source-Gate-Drain = field effect.
What transistors do in a radio
- Amplify — small signal in, larger signal out, in RF, IF and audio stages
- Switch — fully on or fully off, in keying and control circuits
- Oscillate — with feedback, generating the carrier and local oscillator signals
- Mix — combining two frequencies to produce sum and difference products in the superheterodyne receiver