Power-Amplifier Classes & Output Stages

A voltage amplifier develops signal amplitude; a power output stage delivers the required load current without violating device, supply, distortion, or thermal limits. Class names describe when devices conduct and how energy is processed, but they do not replace a loss and safe-operating-area calculation.

Linear classes #

A complementary push-pull emitter/source follower lets one device source and the other sink current. Pure class B needs roughly one junction turn-on voltage before either device conducts, creating crossover distortion. Class AB biases both slightly on so their transfer curves overlap.

flowchart TB
  DRIVE["voltage gain / driver"] --> TOP["source device<br/>positive half-cycle"]
  DRIVE --> BOT["sink device<br/>negative half-cycle"]
  TOP --> OUT["load current"]
  BOT --> OUT
  BIAS["bias spreader + thermal tracking"] --> TOP
  BIAS --> BOT

Heat is set by the waveform #

Instantaneous device power is $p_D(t)=v_D(t)i_D(t)$. Average it over the actual signal, load, and clipping conditions. Maximum transistor dissipation may occur below maximum output power because high device voltage and current overlap. Reactive loads rotate current relative to voltage and can push devices into a much harsher safe-operating-area region.

Bias voltage must track output-device temperature. If rising temperature increases quiescent current faster than emitter/source ballast and the bias network reduce it, thermal runaway results. Mounting the bias sensor near the power devices helps, but the complete thermal time constants and worst-case spread still matter.

Switching power stages #

Class D represents the signal with duty cycle or another switching sequence, then recovers the load waveform with the load and/or a low-pass filter. Ideally devices dissipate little because they are either low-voltage on or zero-current off. Real loss comes from on-resistance, switching overlap, gate charge, dead time, body-diode/reverse-recovery behavior, output filtering, and EMI control.

Protection and verification #

Check short-circuit current, inductive flyback, output DC, overtemperature, supply sequencing, current limiting, and recovery. Verify quiescent current after thermal equilibrium, crossover residual, clipping symmetry, load stability, device temperatures, and output spectrum. A clean no-load sine wave proves little about a power stage.