Multistage amplification is used to increase the overall gain of an amplifier chain. The total gain is the product of each stage’s gain. For example, a microphone can be connected to first a small signal amplifier (voltage amplifier), then a power amplifier before being supplied to a speaker or some other load. The PA (large signal amplifier) is the final stage of the amplifier chain and is the most power hungry. The major features of these PAs are it’s efficiency (usually drain efficiency for FET archetypes or Collector efficiency for BJT amplifiers) and impedance matching to the load. The output power of a PA is typically in tens of watts (small signal amplifiers generally output in mW up to 1 Watt maximum).
“Small signal” transistors are used for small signal amplifiers whereas “power” transistors are used for PAs. Small signal transistors behave linearly whereas power transistors can suffer from nonlinear distortion.
PAs can be classified based on the operating point (Q point) location. Class A amplifiers have a Q point at the center of the active region. For Class B, the Q point is at the cutoff region. For Class AB, the Q point is between that of class A and class B. For Class C, it is below cutoff.
A major parameter of a PA is its efficiency. This is the ratio of AC power to DC input power and is generally expressed in a percentage.
Harmonic distortion of a PA involves the presence of harmonic multiples of the fundamental frequency at the output. A large input signal can cause this type of distortion