|  Hi Guys  
  
The idle condition of a power amplifier output stage is referred to as its "operating class". It is important to note that the bias condition is signal-dependent and has nothing to do with how the output stage devices are controlled. Again, bias condition is a universally-applicable concept.  
  
Class-A: All of the output devices contribute to the signal over the full audio cycle (360-degrees)  
  
Class-A2: Tubes only, the "2" indicates that grid conduction occurs in the output devices. This is simply class-AB below with a low-impedance drive circuit and very high idle current.  
  
Limiting Class-A: The peak signal current never exceeds the total idle current. This term was common in tube days, but still applies universally although it may be considered redundant to the class-A definition above.  
  
Sliding Class-A: Solid-state, a method of varying the idle condition so that neither half of the circuit ever turns off even though transference of signal control may occur.  
  
Class-B: In push-pull, each half of the output stage contributes exactly half the output signal (180-degrees)  
  
Class-B2: Tubes only, the "2" indicates grid conduction made possible by a low-impedance drive circuit.  
  
Class-AB: In push-pull, each half of the output stage contributes to slightly more than half of the signal output. Most "class-B" output stages are actually biased this way, with a slight overlap of conduction between circuit halves.  
  
Class-C: The output device conducts for only half the signal cycle (180-degrees) with a tuned load providing the remainder. Used in RF.  
  
Class-D: Solid-state only, a method of using a nonlinear output stage where the devices switch 'on' and 'off' in a pulse-width-modulated (PWM) format, and the output signal is integrated using LC filters. This approach is highly load noncompliant inasmuch as the load should be of fixed value versus frequency (resistive rather than inductive or capacitive). Class-D allows cold operation of the output devices but is only suitable for driving subwoofers in audio.  
  
Class-E: Solid-state, where parallel-driven output stages supported by different supply values contribute to the signal. The low-voltage stage amplifies the signal up to its limits with the high-voltage stage contributing higher amplitude signals as required. The low-voltage output stage can be biased class-A,-B or -AB.  
  
Class-G: Solid-state, a multi-tier output stage uses multiple supply voltages,switching between them as the signal requires. The transition shifts the burden of output heat from the low-tier device to the next higher-tier device. Overall dissipation is generally reduced by the number of tiers.  
  
Class-H: Solid-state, a multi-tier output stage supported by multiple supply voltages, switching between them as the signal requires. The supply switches turn 'on' hard and the burden of heat dissipation remains with the lowest-tier devices. Overall dissipation is reduced by the number of tiers.  
  
Class-I: Similar to sliding class-A.  
  
Class-T: A variation of class-D, with all of the same inherent issues.  
  
Class-Z: A method of power transfer using saturable coils "steered" by tubes with output stage power provided by a switching supply. designed by Lundahl (SE) in the 1960s, then revised and patented by Berning in the 1990s.   
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