![]() ![]() However, for some configurations, this affirmation may not be valid or if one needs a high precision of the value of V B, the full formula should include the base resistance R B : eq 1 : Full expression of the base voltage The base resistance R B is usually not considered in the calculation of V B since it is in a parallel configuration with the bias resistances and its value is most of the time at least superior of one order of magnitude than R 2. In this circuit, the base voltage V B is given by the network divider formula : Considering this fact, the equivalent circuit in DC of Figure 2 is presented in Figure 3 below : fig 3 : CEA equivalent circuit in DC In DC, the coupling and decoupling capacitor act as an open circuit. Since this circuit works with DC and AC signals, both need to be taken into consideration when analyzing the CEA configuration as it is done in the next two sections. With this configuration, the DC is restricted to bias the CEA and it sinks to the ground while the AC can cross the CEA from the input to the output. It is shown later in the tutorial the importance of this capacitance to amplify the voltage signal. The emitter branch is wired to the ground through a “decoupling” or “derivation” capacitance C 2.To summarize, C 1 and C 3 allow the AC signal to cross completely the CEA but blocks the DC signals to enter in the CEA. The coupling capacitance C 3 does the same by blocking any DC component coming from or going to the load R L. The coupling capacitance C 1 lets only the alternative current (AC) signal pass as an input of the CEA configuration while blocking the direct current (DC) to go from the supply to the source. The capacitor C 1 and C 3 are commonly known as “coupling capacitors”.Moreover, three capacitosr have been added : This voltage divider as shown in the tutorial “ Biasing a Bipolar Transistor in Common Emitter Configuration” is the most suitable biasing method since it improves the stability of the amplifier. The resistor R 1 and R 2 are in parallel with the base branch of the bipolar transistor and form what is commonly known as a voltage network divider.The resistance R S represents the internal resistance of the sine source.A resistance R L is in parallel with the resistance R C of the collector, it represents a load, that is to say the next stage of the circuit after the CEA : it can be an antenna, a speaker or simply another amplifier or stage of the electronic circuit.A real circuit of a CEA configuration is given in Figure 2 : fig 2 : Full CEA configurationįirst of all let’s deal with the added resistors: The simplified diagram given in Figure 1 does not include any biasing circuit, coupling and decoupling capacitors etc. The name “Common Emitter” comes from the fact that the emitter branch is directly wired to the ground of the circuit. In this configuration, the input signal is delivered to the base branch, whereas the output is taken to the collector branch of the bipolar transistor. However, some important elements of a real CEA architecture are missing and will be presented more in detail in the next section. The aim of Figure 1 is to purely show the general configuration of a CEA. The first figure below presents the simplified electrical circuitry of a CEA configuration. We begin this series of tutorial by dealing with the most common type of amplifier found in an endless list of applications : the Common Emitter Amplifier that we will refer in the following as “CEA”. In the next three tutorials, including this one, we will present the three elementary topologies of bipolar transistors based amplifiers : the Common Emitter Amplifier, the Common Collector Amplifier and finally, the Common Base Amplifier. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |