An amplifier makes a signal bigger. It does not create energy: a small input signal steers a much larger flow of power taken from a DC supply, and the output is a bigger copy of the input. Radios use amplifiers everywhere, from a microvolt-level receiver front end to the final stage that drives the antenna.
How it works
A tiny base current steers a big collector current. That is gain.
Collector current (output)2.0mA
In a transistor (or a FET, or a tube) a small control signal changes how much current the device lets through from the supply. A wiggle at the input becomes a larger wiggle in the output current, and a load such as a resistor, tuned circuit or antenna turns that into voltage and power. Between fully off and fully on, the device acts like a variable resistor steered by the input. That middle region is where amplification happens; a device driven only fully on or off is acting as a switch instead.
The device is first given a steady DC bias so that it sits partway along its range. Where you put the bias, relative to the signal, decides the amplifier's class.
Classes of operation
Bias slides the signal against cutoff. The device only conducts while the signal is above the line.
The class is how much of each input cycle the device conducts. More conduction keeps the output a faithful copy of the input; less conduction wastes less power as heat.
Illustrative ideal figures for a single device with a tuned load. Real amplifiers fall a little short.
Heat per 100 W of output27.3W
Class
Conducts
Linear?
Used for
A
all 360°
best
low-level stages, drivers, receiver preamps
AB
180° to 360°
good
SSB and other linear power amplifiers
B
about 180°
fair, usually two devices in push-pull
linear stages that need better efficiency
C
less than 180°
poor
CW and FM only
Classes D, E and F use the device as a fast switch, with a filter to recover the sine wave. They reach efficiencies well above class C, which is why they appear in compact, low-heat power amplifiers.
Linearity matters when the amplitude carries information. SSB and AM change with the signal's strength, so a non-linear amplifier distorts them and spreads a wide, messy signal called splatter. CW and FM have constant amplitude, so they can use class C: a tuned circuit at the output rebuilds the sine wave from the current pulses. See Harmonics and spurious emissions and Receiver overload and intermodulation.
In practice
Heat is the cost of linearity. Even ideally, a class A stage delivering 100 W turns another 100 W of supply power into heat, and a class B stage about 27 W; real stages are worse. Heatsinks, fans and duty cycle follow from this.
Do not overdrive. The output cannot exceed what the supply allows. Past that point the peaks flatten (compression), making distortion. A linear amplifier needs just enough drive, and an ALC line to the radio helps prevent excess.
Gains multiply, so dB add. A 15 dB preamp, a 10 dB gain stage and 3 dB of loss give 22 dB overall. See Decibels.
Noise is added at every stage. A receive preamp helps weak signals only if the first stage adds little noise; a strong signal can overload it. See Sensitivity, selectivity and dynamic range.
Amplifier plus feedback can oscillate, sometimes by accident. Careful layout, bypassing and sometimes neutralizing prevent it; made on purpose it is an oscillator.