Power Amplifiers

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Observations bring about the conclusion, efficiency in Class-A is improved through use of Darlington pair configuration.Many of electronic devices contain amplifiers with voltage swings with the expectations of delivering a significant power at their output. For instance, an audio amplifier driving a loudspeaker, the input to the amplifier is typically a low-amplitude low-power signal such as the LINE output of a CD player or an IPOD. This signal is typically less than 1Vpp containing the venin impedance in the kΩ range. A standard loudspeaker may have 8Ω impedance and require a driving voltage having a magnitude of several volts. Therefore, the power delivered to the loudspeaker is several watts. In conclusion, both voltage amplification and power amplification is vital.Major considerations in a power amplifier include their efficiency and heat dissipation. An example of a typical output (buffer) amplifier configuration in Figure 1 explains it all. This is a push-pull emitter-follower configuration utilizing an npn and a pnp transistor. These types of amplifiers operate in different modes depending on transistor biasing.Class-A type contain transistors which are both in the active side at all times. Therefore, for a sine-wave input the conduction angle is 2π. For this to be successful, a bias current larger than the output current is vital. This large bias current causes significant losses. It can be shown that the maximum possible efficiency is η=25%.Here, only one transistor is in the active region at any time charged with providing current to the load. Therefore, for a sine-wave input the conduction angle is π. No bias current is required (Vbias = 0) and the maximum possible efficiency is η=78.5%. In reality given the non-zero base-emitter voltage the conduction angle will be somewhat less than π and the output voltage