Q.What is a power amplifier?

Where a voltage (small-signal) amplifier only enlarges a voltage, a power amplifier delivers a large amount of power to a load such as a loudspeaker or motor. It works as a DC-to-AC power converter: it draws DC power from the supply and, under the control of the input signal, converts it into an AC output at the load. This conversion is never perfect, so the efficiency — the ratio of AC output power to DC input power, $\eta = P_{out(ac)}/P_{in(dc)}$ — is always less than 100%. A voltage amplifier therefore always precedes a power amplifier, because a large output power needs a large input voltage swing.

Power amplifiers are classified by how much of the input cycle the output (collector) current flows for, which the Q-point position decides:

• Class A — Q-point at the centre of the load line; current flows for the full 360 cycle; lowest distortion but poor efficiency (25-30%). …