Digital Ear Amplifier Classification
Class A (Class A) Amplifier - Good Linear Amplifier
Class A (Class A) amplifiers are amplifiers in which current flows continuously through all of the output devices. This type of amplifier avoids the nonlinearities generated by the switching of the device. As long as the bias and dynamic range are properly controlled, it is considered a good linear amplifier only from a distortion perspective. But its efficiency is low
Class A amplifiers are structurally different and have two different modes of operation.
1.The two emitter followers are connected in parallel, and the bias current is increased until sufficient current flows under normal load without turning off any of the devices. The biggest advantage of this measure is that it does not suddenly drain the output current. If the load impedance is lower than the calibration value, the amplifier will have a short-term cut-off. There may be a slight increase in distortion, but there is no serious defect in the sense of directness.
2.It can be referred to as the Control Current Source Type (VCIS), which is essentially a separate emitter follower with an active emitter load to achieve proper current bleed. When this class is used as an output stage, it is necessary to clarify the impedance of the drive to be driven before starting the design.
These amplifiers are used in applications that require high linearity and available power to meet the requirements.
Class B (Class B) Amplifier - the most popular amplifier
Class B (Class B) amplifiers are a type of operation where the device has an on-time of 50%. This type of amplifier is arguably the most popular type of amplifier, and perhaps 99% of the amplifiers currently produced belong to this category.
Class B amplifiers use a push-pull amplifier structure. The output of the Class B amplifier uses forward and reverse transistors. To reproduce the input signal, each transistor turns on only during the half cycle (180°) of the signal waveform (Figure 2). This allows the amplifier to be zero current when idle and therefore more efficient than a Class A amplifier.
Class B amplifiers are a compromise. Its efficiency is improved and audio quality is degraded. This is because there is a crossover point and both transistors jump from the on state to the off state. It is well known that Class B audio amplifiers have crossover distortion when processing low level signals and are not suitable for low power applications.
Class AB (Class A) amplifier
In fact, it is a combination of Class A (Class A) and Class B (Class B). The on-time of each device is between 50 and 100%, depending on the magnitude of the bias current and the output level. The bias of this type of amplifier is designed in Class B (Class B) and then the bias current is increased to allow the amplifier to enter Class AB (Class A).
When the output of Class AB (Class A) amplifier is below a certain level, both output devices are turned on, and the state works in Class A (Class A); when the level is increased, the two devices will be completely cut off, and the other device will be completely cut off. One device will supply more current. Thus, at the beginning of the AB (Class A) state, the distortion will suddenly rise, and its linearity is inferior to Class A (Class A) or Class B (Class B).
This performance is achieved by biasing the two transistors to a signal output that is close to zero, ie, the point at which the Class B amplifier begins to exhibit nonlinearity (Figure 3). For small signals, both transistors are turned on, acting as a Class A amplifier; for large signal shifts, only one transistor is turned on every half of the waveform period, so it works like a Class B amplifier.
Class AB speaker amplifiers feature high signal-to-noise ratio (SNR), low THD+N, and typical efficiency of up to 65%, making them ideal for high-fidelity speaker drivers.