Diodes
What electronic component allows current to flow in only one direction?
Diode.
That one-way behaviour is the basis of every power supply, every detector, and every protection circuit in a station.
The electrodes are the anode and the cathode. Conventional current flows from anode to cathode inside the diode, and not the other way.
How is the cathode lead of a semiconductor diode often marked on the package?
With a stripe.
A band around the body, at the cathode end. Getting this backwards in a power supply is a memorable experience, and the stripe is the only thing standing between you and it.
Forward voltage drop is the voltage across a conducting diode.
Which is true about forward voltage drop in a diode?
It is lower in some diode types than in others.
Roughly 0.7 V for ordinary silicon, about 0.3 V for germanium and for Schottky diodes, and 1.5 to 3 V for LEDs depending on colour. The differences matter: a low-drop diode wastes less power in a high-current supply, and an LED’s drop sets the resistor you need with it.
LEDs emit light because of forward current — current flowing in the conducting direction. Reverse-bias an LED and it simply does nothing.
Transistors
Which of these components can be used as an electronic switch?
Transistor.
Which of the following can provide power gain?
Transistor.
Switching and amplifying are the same underlying behaviour: a small input controls a larger output. Which one you call it depends on whether you drive it to its extremes or keep it in between.
A transistor can consist of three regions of semiconductor material, which is the physical basis for the two families:
| Family | Electrodes |
|---|---|
| Bipolar junction transistor (BJT) | emitter, base, collector |
| Field-effect transistor (FET) | gate, drain, source |
What does the abbreviation FET stand for?
Field Effect Transistor.
The practical difference: a BJT’s base draws current to control the collector, while a FET’s gate is voltage-controlled and draws essentially none. That makes FETs the natural choice where the driving circuit cannot supply current — which is most of modern electronics, and all of digital logic.
Gain
What does the term gain mean in amplifiers?
All these choices are correct — the ratio of output power to input power, the ratio of output voltage to input voltage, and the ratio of output current to input current.
All three are gain, and which one is meant depends on context. This is exactly the decibel coefficient trap from the foundations track: power ratios use 10 log₁₀, voltage and current ratios use 20 log₁₀. A “20 dB amplifier” is 100 times in power and 10 times in voltage, and the two statements describe the same amplifier.
Check yourself
- You have a diode with a stripe at one end. Which end is that, and which way does current flow?
- A transistor has a gate. Which family, and what is the practical consequence?
- An amplifier turns 1 W into 100 W. What is its gain in dB?
Answers
- The stripe marks the cathode. Conventional current flows anode to cathode inside the diode — into the unmarked end and out of the striped one.
- Field-effect. Its gate is voltage-controlled and draws essentially no current, so the driving stage does not have to supply any.
- A power ratio of 100, so 10 log₁₀(100) = 20 dB.