![]() The total power P tot supplied to the amplifier comes from the DC power supply : P tot=V supply×I C0. It can be shown through an integral calculus over one period of the signal that the average absorbed power P L can be decomposed into one power due to the biasing P L,DC and one useful power due to the signal variations across the load P L,AC : eq 2 : Average DC and AC power absorbed by the load The instant power absorbed by the load P L(t) comes from the contribution of the biasing signal and the alternative signal : P L(t)=(V supply-V C(t))×I C(t). The symbol ω represents the angular frequency : ω=2πf. The very visual decomposition of the output signal shown in Figure 2 into one DC component (V C0,I C0) and one AC component (v C(t),i C(t)) allows us to write for the total output signals V C(t) and I C(t) : eq 1 : Decomposition of the output signalsįor a pure resistive load such as in Figure 1, v C(t)=V AC×sin(ωt) and i C(t)=-I AC×sin(ωt). In Figure 2 is presented how the output signal can be decomposed into two components : the biasing DC signal and the AC amplified signal : fig 2 : DC and AC component of the output signals In the following, we will refer to this configuration as basic CEAA for Common Emitter Amplifier Class A. This is the most basic configuration for a Class A power amplifier, note that the load resistance R L is directly wired to the collector branch. The output stage of the CEA is shown in Figure 1 below : fig 1 : Output stage of a basic CEAA In this section, we present the basic configuration of a Class A amplifier using the CEA as an example. Basic Common Emitter Configuration in Class A Everything that we will mention applies to other configurations such as the Common Collector Configuration or the MOSFET. In the next sections, we present two different configurations of Class A amplifiers based on a CEA structure. Moreover, the output load is fixed, that means that they are optimally designed to be connected to a specific load value : either to another amplifier or to a specific loud speaker.ĭue to these features, Class A amplifiers are mostly found in the most common type of BJT configuration : the Common Emitter Configuration (CEA). We will see in the next section, however, that this last affirmation is valid only for a certain architecture.Ĭlass A configurations are generally low power amplifiers. Their efficiency is very low with a theoretical maximum of 50 %. In the case of a Class A amplification, the biasing is chosen such as the operating point is located in the middle of the load line : V C0=V supply/2 and I C0=V C0/R C with R C being the resistance of the collector branch.Īs seen during the Amplifier Classes tutorial, class A amplifiers have a 360° conduction angle, meaning that 100 % of the input signal is used for the amplification process. The aim of this section is to get a reminder of the general characteristics of a Class A amplifier before talking about new configurations. It is also important to keep in mind that the class of an amplifier is fully determined from the biasing parameters : the power supply V supply, and the resistances of the voltage divider network. The distortion which shows how faithfully the output signal is reproduced.The ratio η=P out/P abs that highlights the efficiency of the amplifier.In this article, we will focus more in detail about the Class A amplifiers.īefore going into the core of the topic by presenting different Class A configurations, it is important to have in mind the selection criteria of an amplifier class : In the introduction tutorial Amplifier Classes, we have presented the different classes of amplification that can be found.
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