Amplifier classes
The class describes how much of the input cycle the device conducts for, and that single choice sets both efficiency and linearity.
| Class | Conducts for | Efficiency | Linearity |
|---|---|---|---|
| A | 100% of the cycle | lowest | best |
| AB | more than half | moderate | good |
| B | half the cycle | better | fair |
| C | less than half | highest | worst |
In a Class A amplifier, what percentage of the time does the amplifying device conduct?
100%.
Which of these classes of amplifiers has the highest efficiency?
Class C.
The trade-off is direct and unavoidable, and it decides which modes each class can amplify:
Which of the following describes a linear amplifier?
An amplifier in which the output preserves the input waveform.
For which of the following modes is a Class C power stage appropriate for amplifying a modulated signal?
FM.
FM has constant amplitude, so distorting the amplitude does not damage the information — you may use an efficient, non-linear Class C stage. SSB carries its information in amplitude, so it needs a linear amplifier (A or AB) and pays for it in efficiency.
This is why an FM-only VHF amplifier can be small and efficient while an HF SSB amplifier of the same output runs hot.
Efficiency
How is the efficiency of an RF power amplifier determined?
Divide the RF output power by the DC input power.
A 100 W output amplifier drawing 200 W of DC is 50% efficient — and the missing 100 W is heat, which is why the heat sink is the size it is.
Neutralisation
What is the purpose of neutralizing an amplifier?
To eliminate self-oscillations.
Stray capacitance between an amplifier’s output and its input feeds some output back to the input. With enough gain and the right phase, that feedback sustains itself and the amplifier oscillates — producing a signal of its own, on a frequency of its choosing, that you did not intend to transmit.
Neutralisation injects a deliberate, opposite-phase feedback that cancels the accidental one. Note the connection to the previous lesson: a screen grid solves the same problem by construction, reducing the grid-to-plate capacitance that causes it.
Oscillators
Which of the following are basic components of a sine wave oscillator?
A filter and an amplifier operating in a feedback loop.
That is the general recipe, and it is the same mechanism neutralisation exists to prevent: gain plus in-phase feedback. An oscillator is a deliberate self-oscillation, with a filter to decide the frequency.
What determines the frequency of an LC oscillator?
The inductance and capacitance in the tank circuit.
The tank is the filter, and its resonant frequency — from the formula in the electrical principles lesson — is where the loop’s phase condition is satisfied.
Digital building blocks
Which of the following describes the function of a two-input AND gate?
Output is high only when both inputs are high.
How many states does a 3-bit binary counter have?
8.
2³ = 8. An n-bit counter has 2ⁿ states — the same arithmetic as the decibel doublings, in a different setting.
What is a shift register?
A clocked array of circuits that passes data in steps along the array.
Each clock pulse moves every bit one place along. It is how serial data becomes parallel and back, and how digital delay lines work.
Check yourself
- You want to amplify an SSB signal. Which classes may you use, and why not Class C?
- Your amplifier draws 300 W of DC and produces 180 W of RF. Efficiency?
- An amplifier produces a signal with no input. What is happening and what is the cure?
Answers
- Class A or AB — a linear amplifier that preserves the input waveform. SSB carries information in amplitude, and Class C destroys it.
- 180 / 300 = 60%. The remaining 120 W is heat.
- Self-oscillation from stray output-to-input feedback. The cure is neutralisation.