Amplifier Classes — Getting Power to the Antenna
Why Amplifier Classes Matter
Your transmitter's final stage is a power amplifier (PA). How you bias that amplifier determines two critical things:
- Linearity — does it faithfully reproduce the signal shape? (Critical for SSB and AM)
- Efficiency — how much of the DC power becomes RF? (The rest becomes heat)
You can't have both — it's always a trade-off.
The Classes Explained
| Class | How it works | Efficiency | Linearity | Used for |
|---|---|---|---|---|
| A | Transistor conducts all the time (360°). Always "on." | ~25-50% | Excellent | Low-power stages, driver stages |
| AB | Conducts most of the time (180°-360°). Slightly biased below full-on. | ~50-65% | Very good | SSB power amplifiers — the standard choice |
| B | Conducts exactly half the time (180°). Needs push-pull pairs. | ~65-78% | OK | Push-pull audio amps |
| C | Conducts less than half the time (<180°). Very "punchy." | ~75-85% | Poor | FM and CW only! |
Exam favourite: Class C amplifiers are NOT suitable for SSB or AM. Why? SSB and AM carry information in their amplitude variations. Class C clips the signal, destroying the amplitude information. Class C is fine for FM and CW because those modes have constant amplitude — only the frequency or on/off state carries information.
Why Class AB is King for SSB
Most HF transceivers use Class AB for the final PA. It's the sweet spot:
- Linear enough to handle SSB without excessive distortion
- Efficient enough that a 400W output doesn't need a massive heatsink
- If you overdrive a Class AB amp (turn up the mic gain too high), it starts producing intermodulation distortion — "splatter" that interferes with adjacent frequencies
Decibels — The Radio Person's Shortcut
Decibels (dB) let you work with ratios by adding and subtracting instead of multiplying and dividing. Learn these shortcuts:
| Change in dB | Power ratio | Voltage ratio | Think of it as... |
|---|---|---|---|
| +3 dB | × 2 | × 1.41 | Double the power |
| +6 dB | × 4 | × 2 | Double the voltage |
| +10 dB | × 10 | × 3.16 | Ten times the power |
| +20 dB | × 100 | × 10 | Ten times the voltage |
| −3 dB | × 0.5 | × 0.71 | Half the power |
Example: Your amplifier has 13 dB of gain. That's 10 + 3 = ×10 × ×2 = ×20 power. Feed it 20W, get 400W out. Simple!
Power formula: \( G_{dB} = 10 \log_{10}(P_{out}/P_{in}) \)
Voltage formula: \( G_{dB} = 20 \log_{10}(V_{out}/V_{in}) \)
The factor is 10 for power, 20 for voltage — because power is proportional to voltage squared.
Voltage formula: \( G_{dB} = 20 \log_{10}(V_{out}/V_{in}) \)
The factor is 10 for power, 20 for voltage — because power is proportional to voltage squared.